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https://github.com/ashishkumarpalai/interview-preparation
https://github.com/ashishkumarpalai/interview-preparation
Last synced: 16 days ago
JSON representation
- Host: GitHub
- URL: https://github.com/ashishkumarpalai/interview-preparation
- Owner: ashishkumarpalai
- Created: 2023-09-06T13:10:43.000Z (over 1 year ago)
- Default Branch: main
- Last Pushed: 2023-09-06T13:49:29.000Z (over 1 year ago)
- Last Synced: 2023-09-06T14:57:30.644Z (over 1 year ago)
- Size: 1.18 MB
- Stars: 0
- Watchers: 1
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
Awesome Lists containing this project
README
| No. | Topic |
| --- | --------------------------------------------------------------------------------------- |
| 1 | [React Interview Questions & Answers](#react-interview-questions--answers) |
| 2 | [JavaScript Interview Questions](#javascript-interview-questions) |
| 3 | [What is React?](#what-is-react)
## React Interview Questions & Answers
### Table of Contents
| No. | Questions |
| --- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| | **Core React** |
| 1 | [What is React?](#what-is-react) |
| 2 | [What are the major features of React?](#what-are-the-major-features-of-react) |
| 3 | [What is JSX?](#what-is-jsx) |
| 4 | [What is the difference between Element and Component?](#what-is-the-difference-between-element-and-component) |
| 5 | [How to create components in React?](#how-to-create-components-in-react) |
| 6 | [When to use a Class Component over a Function Component?](#when-to-use-a-class-component-over-a-function-component) |
| 7 | [What are Pure Components?](#what-are-pure-components) |
| 8 | [What is state in React?](#what-is-state-in-react) |
| 9 | [What are props in React?](#what-are-props-in-react) |
| 10 | [What is the difference between state and props?](#what-is-the-difference-between-state-and-props) |
| 11 | [Why should we not update the state directly?](#why-should-we-not-update-the-state-directly) |
| 12 | [What is the purpose of callback function as an argument of setState()?](#what-is-the-purpose-of-callback-function-as-an-argument-of-setstate) |
| 13 | [What is the difference between HTML and React event handling?](#what-is-the-difference-between-html-and-react-event-handling) |
| 14 | [How to bind methods or event handlers in JSX callbacks?](#how-to-bind-methods-or-event-handlers-in-jsx-callbacks) |
| 15 | [How to pass a parameter to an event handler or callback?](#how-to-pass-a-parameter-to-an-event-handler-or-callback) |
| 16 | [What are synthetic events in React?](#what-are-synthetic-events-in-react) |
| 17 | [What are inline conditional expressions?](#what-are-inline-conditional-expressions) |
| 18 | [What is "key" prop and what is the benefit of using it in arrays of elements?](#what-is-key-prop-and-what-is-the-benefit-of-using-it-in-arrays-of-elements) |
| 19 | [What is the use of refs?](#what-is-the-use-of-refs) |
| 20 | [How to create refs?](#how-to-create-refs) |
| 21 | [What are forward refs?](#what-are-forward-refs) |
| 22 | [Which is preferred option with in callback refs and findDOMNode()?](#which-is-preferred-option-with-in-callback-refs-and-finddomnode) |
| 23 | [Why are String Refs legacy?](#why-are-string-refs-legacy) |
| 24 | [What is Virtual DOM?](#what-is-virtual-dom) |
| 25 | [How Virtual DOM works?](#how-virtual-dom-works) |
| 26 | [What is the difference between Shadow DOM and Virtual DOM?](#what-is-the-difference-between-shadow-dom-and-virtual-dom) |
| 27 | [What is React Fiber?](#what-is-react-fiber) |
| 28 | [What is the main goal of React Fiber?](#what-is-the-main-goal-of-react-fiber) |
| 29 | [What are controlled components?](#what-are-controlled-components) |
| 30 | [What are uncontrolled components?](#what-are-uncontrolled-components) |
| 31 | [What is the difference between createElement and cloneElement?](#what-is-the-difference-between-createelement-and-cloneelement) |
| 32 | [What is Lifting State Up in React?](#what-is-lifting-state-up-in-react) |
| 33 | [What are the different phases of component lifecycle?](#what-are-the-different-phases-of-component-lifecycle) |
| 34 | [What are the lifecycle methods of React?](#what-are-the-lifecycle-methods-of-react) |
| 35 | [What are Higher-Order components?](#what-are-higher-order-components) |
| 36 | [How to create props proxy for HOC component?](#how-to-create-props-proxy-for-hoc-component) |
| 37 | [What is context?](#what-is-context) |
| 38 | [What is children prop?](#what-is-children-prop) |
| 39 | [How to write comments in React?](#how-to-write-comments-in-react) |
| 40 | [What is the purpose of using super constructor with props argument?](#what-is-the-purpose-of-using-super-constructor-with-props-argument) |
| 41 | [What is reconciliation?](#what-is-reconciliation) |
| 42 | [How to set state with a dynamic key name?](#how-to-set-state-with-a-dynamic-key-name) |
| 43 | [What would be the common mistake of function being called every time the component renders?](#what-would-be-the-common-mistake-of-function-being-called-every-time-the-component-renders) |
| 44 | [Is lazy function supports named exports?](#is-lazy-function-supports-named-exports) |
| 45 | [Why React uses className over class attribute?](#why-react-uses-classname-over-class-attribute) |
| 46 | [What are fragments?](#what-are-fragments) |
| 47 | [Why fragments are better than container divs?](#why-fragments-are-better-than-container-divs) |
| 48 | [What are portals in React?](#what-are-portals-in-react) |
| 49 | [What are stateless components?](#what-are-stateless-components) |
| 50 | [What are stateful components?](#what-are-stateful-components) |
| 51 | [How to apply validation on props in React?](#how-to-apply-validation-on-props-in-react) |
| 52 | [What are the advantages of React?](#what-are-the-advantages-of-react) |
| 53 | [What are the limitations of React?](#what-are-the-limitations-of-react) |
| 54 | [What are error boundaries in React v16](#what-are-error-boundaries-in-react-v16) |
| 55 | [How are error boundaries handled in React v15?](#how-are-error-boundaries-handled-in-react-v15) |
| 56 | [What are the recommended ways for static type checking?](#what-are-the-recommended-ways-for-static-type-checking) |
| 57 | [What is the use of react-dom package?](#what-is-the-use-of-react-dom-package) |
| 58 | [What is the purpose of render method of react-dom?](#what-is-the-purpose-of-render-method-of-react-dom) |
| 59 | [What is ReactDOMServer?](#what-is-reactdomserver) |
| 60 | [How to use InnerHtml in React?](#how-to-use-innerhtml-in-react) |
| 61 | [How to use styles in React?](#how-to-use-styles-in-react) |
| 62 | [How events are different in React?](#how-events-are-different-in-react) |
| 63 | [What will happen if you use setState in constructor?](#what-will-happen-if-you-use-setstate-in-constructor) |
| 64 | [What is the impact of indexes as keys?](#what-is-the-impact-of-indexes-as-keys) |
| 65 | [Is it good to use setState() in componentWillMount() method?](#is-it-good-to-use-setstate-in-componentwillmount-method) |
| 66 | [What will happen if you use props in initial state?](#what-will-happen-if-you-use-props-in-initial-state) |
| 67 | [How do you conditionally render components?](#how-do-you-conditionally-render-components) |
| 68 | [Why we need to be careful when spreading props on DOM elements??](#why-we-need-to-be-careful-when-spreading-props-on-dom-elements) |
| 69 | [How you use decorators in React?](#how-you-use-decorators-in-react) |
| 70 | [How do you memoize a component?](#how-do-you-memoize-a-component) |
| 71 | [How you implement Server-Side Rendering or SSR?](#how-you-implement-server-side-rendering-or-ssr) |
| 72 | [How to enable production mode in React?](#how-to-enable-production-mode-in-react) |
| 73 | [What is CRA and its benefits?](#what-is-cra-and-its-benefits) |
| 74 | [What is the lifecycle methods order in mounting?](#what-is-the-lifecycle-methods-order-in-mounting) |
| 75 | [What are the lifecycle methods going to be deprecated in React v16?](#what-are-the-lifecycle-methods-going-to-be-deprecated-in-react-v16) |
| 76 | [What is the purpose of getDerivedStateFromProps() lifecycle method?](#what-is-the-purpose-of-getderivedstatefromprops-lifecycle-method) |
| 77 | [What is the purpose of getSnapshotBeforeUpdate() lifecycle method?](#what-is-the-purpose-of-getsnapshotbeforeupdate-lifecycle-method) |
| 78 | [Do Hooks replace render props and higher order components?](#do-hooks-replace-render-props-and-higher-order-components) |
| 79 | [What is the recommended way for naming components?](#what-is-the-recommended-way-for-naming-components) |
| 80 | [What is the recommended ordering of methods in component class?](#what-is-the-recommended-ordering-of-methods-in-component-class) |
| 81 | [What is a switching component?](#what-is-a-switching-component) |
| 82 | [Why we need to pass a function to setState()?](#why-we-need-to-pass-a-function-to-setstate) |
| 83 | [What is strict mode in React?](#what-is-strict-mode-in-react) |
| 84 | [What are React Mixins?](#what-are-react-mixins) |
| 85 | [Why is isMounted() an anti-pattern and what is the proper solution?](#why-is-ismounted-an-anti-pattern-and-what-is-the-proper-solution) |
| 86 | [What are the Pointer Events supported in React?](#what-are-the-pointer-events-supported-in-react) |
| 87 | [Why should component names start with capital letter?](#why-should-component-names-start-with-capital-letter) |
| 88 | [Are custom DOM attributes supported in React v16?](#are-custom-dom-attributes-supported-in-react-v16) |
| 89 | [What is the difference between constructor and getInitialState?](#what-is-the-difference-between-constructor-and-getinitialstate) |
| 90 | [Can you force a component to re-render without calling setState?](#can-you-force-a-component-to-re-render-without-calling-setstate) |
| 91 | [What is the difference between super() and super(props) in React using ES6 classes?](#what-is-the-difference-between-super-and-superprops-in-react-using-es6-classes) |
| 92 | [How to loop inside JSX?](#how-to-loop-inside-jsx) |
| 93 | [How do you access props in attribute quotes?](#how-do-you-access-props-in-attribute-quotes) |
| 94 | [What is React PropType array with shape?](#what-is-react-proptype-array-with-shape) |
| 95 | [How to conditionally apply class attributes?](#how-to-conditionally-apply-class-attributes) |
| 96 | [What is the difference between React and ReactDOM?](#what-is-the-difference-between-react-and-reactdom) |
| 97 | [Why ReactDOM is separated from React?](#why-reactdom-is-separated-from-react) |
| 98 | [How to use React label element?](#how-to-use-react-label-element) |
| 99 | [How to combine multiple inline style objects?](#how-to-combine-multiple-inline-style-objects) |
| 100 | [How to re-render the view when the browser is resized?](#how-to-re-render-the-view-when-the-browser-is-resized) |
| 101 | [What is the difference between setState and replaceState methods?](#what-is-the-difference-between-setstate-and-replacestate-methods) |
| 102 | [How to listen to state changes?](#how-to-listen-to-state-changes) |
| 103 | [What is the recommended approach of removing an array element in react state?](#what-is-the-recommended-approach-of-removing-an-array-element-in-react-state) |
| 104 | [Is it possible to use React without rendering HTML?](#is-it-possible-to-use-react-without-rendering-html) |
| 105 | [How to pretty print JSON with React?](#how-to-pretty-print-json-with-react) |
| 106 | [Why you can't update props in React?](#why-you-cant-update-props-in-react) |
| 107 | [How to focus an input element on page load?](#how-to-focus-an-input-element-on-page-load) |
| 108 | [What are the possible ways of updating objects in state?](#what-are-the-possible-ways-of-updating-objects-in-state) |
| 110 | [How can we find the version of React at runtime in the browser?](#how-can-we-find-the-version-of-react-at-runtime-in-the-browser) |
| 111 | [What are the approaches to include polyfills in your create-react-app?](#what-are-the-approaches-to-include-polyfills-in-your-create-react-app) |
| 112 | [How to use https instead of http in create-react-app?](#how-to-use-https-instead-of-http-in-create-react-app) |
| 113 | [How to avoid using relative path imports in create-react-app?](#how-to-avoid-using-relative-path-imports-in-create-react-app) |
| 114 | [How to add Google Analytics for react-router?](#how-to-add-google-analytics-for-react-router) |
| 115 | [How to update a component every second?](#how-to-update-a-component-every-second) |
| 116 | [How do you apply vendor prefixes to inline styles in React?](#how-do-you-apply-vendor-prefixes-to-inline-styles-in-react) |
| 117 | [How to import and export components using react and ES6?](#how-to-import-and-export-components-using-react-and-es6) |
| 118 | [What are the exceptions on React component naming?](#what-are-the-exceptions-on-react-component-naming) |
| 119 | [Why is a component constructor called only once?](#why-is-a-component-constructor-called-only-once) |
| 120 | [How to define constants in React?](#how-to-define-constants-in-react) |
| 121 | [How to programmatically trigger click event in React?](#how-to-programmatically-trigger-click-event-in-react) |
| 122 | [Is it possible to use async/await in plain React?](#is-it-possible-to-use-asyncawait-in-plain-react) |
| 123 | [What are the common folder structures for React?](#what-are-the-common-folder-structures-for-react) |
| 124 | [What are the popular packages for animation?](#what-are-the-popular-packages-for-animation) |
| 125 | [What is the benefit of styles modules?](#what-is-the-benefit-of-styles-modules) |
| 126 | [What are the popular React-specific linters?](#what-are-the-popular-react-specific-linters) |
| 127 | [How to make AJAX call and In which component lifecycle methods should I make an AJAX call?](#how-to-make-ajax-call-and-in-which-component-lifecycle-methods-should-i-make-an-ajax-call) |
| 128 | [What are render props?](#what-are-render-props) |
| | **React Router** |
| 129 | [What is React Router?](#what-is-react-router) |
| 130 | [How React Router is different from history library?](#how-react-router-is-different-from-history-library) |
| 131 | [What are the \ components of React Router v4?](#what-are-the-router-components-of-react-router-v4) |
| 132 | [What is the purpose of push and replace methods of history?](#what-is-the-purpose-of-push-and-replace-methods-of-history) |
| 133 | [How do you programmatically navigate using React router v4?](#how-do-you-programmatically-navigate-using-react-router-v4) |
| 134 | [How to get query parameters in React Router v4](#how-to-get-query-parameters-in-react-router-v4) |
| 135 | [Why you get "Router may have only one child element" warning?](#why-you-get-router-may-have-only-one-child-element-warning) |
| 136 | [How to pass params to history.push method in React Router v4?](#how-to-pass-params-to-historypush-method-in-react-router-v4) |
| 137 | [How to implement default or NotFound page?](#how-to-implement-default-or-notfound-page) |
| 138 | [How to get history on React Router v4?](#how-to-get-history-on-react-router-v4) |
| 139 | [How to perform automatic redirect after login?](#how-to-perform-automatic-redirect-after-login) |
| | **React Internationalization** |
| 140 | [What is React-Intl?](#what-is-react-intl) |
| 141 | [What are the main features of React Intl?](#what-are-the-main-features-of-react-intl) |
| 142 | [What are the two ways of formatting in React Intl?](#what-are-the-two-ways-of-formatting-in-react-intl) |
| 143 | [How to use FormattedMessage as placeholder using React Intl?](#how-to-use-formattedmessage-as-placeholder-using-react-intl) |
| 144 | [How to access current locale with React Intl](#how-to-access-current-locale-with-react-intl) |
| 145 | [How to format date using React Intl?](#how-to-format-date-using-react-intl) |
| | **React Testing** |
| 146 | [What is Shallow Renderer in React testing?](#what-is-shallow-renderer-in-react-testing) |
| 147 | [What is TestRenderer package in React?](#what-is-testrenderer-package-in-react) |
| 148 | [What is the purpose of ReactTestUtils package?](#what-is-the-purpose-of-reacttestutils-package) |
| 149 | [What is Jest?](#what-is-jest) |
| 150 | [What are the advantages of Jest over Jasmine?](#what-are-the-advantages-of-jest-over-jasmine) |
| 151 | [Give a simple example of Jest test case](#give-a-simple-example-of-jest-test-case) |
| | **React Redux** |
| 152 | [What is Flux?](#what-is-flux) |
| 153 | [What is Redux?](#what-is-redux) |
| 154 | [What are the core principles of Redux?](#what-are-the-core-principles-of-redux) |
| 155 | [What are the downsides of Redux compared to Flux?](#what-are-the-downsides-of-redux-compared-to-flux) |
| 156 | [What is the difference between mapStateToProps() and mapDispatchToProps()?](#what-is-the-difference-between-mapstatetoprops-and-mapdispatchtoprops) |
| 157 | [Can I dispatch an action in reducer?](#can-i-dispatch-an-action-in-reducer) |
| 158 | [How to access Redux store outside a component?](#how-to-access-redux-store-outside-a-component) |
| 159 | [What are the drawbacks of MVW pattern](#what-are-the-drawbacks-of-mvw-pattern) |
| 160 | [Are there any similarities between Redux and RxJS?](#are-there-any-similarities-between-redux-and-rxjs) |
| 161 | [How to dispatch an action on load?](#how-to-dispatch-an-action-on-load) |
| 162 | [How to use connect from React Redux?](#how-to-use-connect-from-react-redux) |
| 163 | [How to reset state in Redux?](#how-to-reset-state-in-redux) |
| 164 | [Whats the purpose of at symbol in the redux connect decorator?](#whats-the-purpose-of-at-symbol-in-the-redux-connect-decorator) |
| 165 | [What is the difference between React context and React Redux?](#what-is-the-difference-between-react-context-and-react-redux) |
| 166 | [Why are Redux state functions called reducers?](#why-are-redux-state-functions-called-reducers) |
| 167 | [How to make AJAX request in Redux?](#how-to-make-ajax-request-in-redux) |
| 168 | [Should I keep all component's state in Redux store?](#should-i-keep-all-components-state-in-redux-store) |
| 169 | [What is the proper way to access Redux store?](#what-is-the-proper-way-to-access-redux-store) |
| 170 | [What is the difference between component and container in React Redux?](#what-is-the-difference-between-component-and-container-in-react-redux) |
| 171 | [What is the purpose of the constants in Redux? ](#what-is-the-purpose-of-the-constants-in-redux) |
| 172 | [What are the different ways to write mapDispatchToProps()?](#what-are-the-different-ways-to-write-mapdispatchtoprops) |
| 173 | [What is the use of the ownProps parameter in mapStateToProps() and mapDispatchToProps()?](#what-is-the-use-of-the-ownprops-parameter-in-mapstatetoprops-and-mapdispatchtoprops) |
| 174 | [How to structure Redux top level directories?](#how-to-structure-redux-top-level-directories) |
| 175 | [What is redux-saga?](#what-is-redux-saga) |
| 176 | [What is the mental model of redux-saga?](#what-is-the-mental-model-of-redux-saga) |
| 177 | [What are the differences between call and put in redux-saga](#what-are-the-differences-between-call-and-put-in-redux-saga) |
| 178 | [What is Redux Thunk?](#what-is-redux-thunk) |
| 179 | [What are the differences between redux-saga and redux-thunk](#what-are-the-differences-between-redux-saga-and-redux-thunk) |
| 180 | [What is Redux DevTools?](#what-is-redux-devtools) |
| 181 | [What are the features of Redux DevTools?](#what-are-the-features-of-redux-devtools) |
| 182 | [What are Redux selectors and Why to use them?](#what-are-redux-selectors-and-why-to-use-them) |
| 183 | [What is Redux Form?](#what-is-redux-form) |
| 184 | [What are the main features of Redux Form?](#what-are-the-main-features-of-redux-form) |
| 185 | [How to add multiple middlewares to Redux?](#how-to-add-multiple-middlewares-to-redux) |
| 186 | [How to set initial state in Redux?](#how-to-set-initial-state-in-redux) |
| 187 | [How Relay is different from Redux?](#how-relay-is-different-from-redux) |
| 188 | [What is an action in Redux?](#what-is-an-action-in-redux) |
| | **React Native** |
| 188 | [What is the difference between React Native and React?](#what-is-the-difference-between-react-native-and-react) |
| 189 | [How to test React Native apps?](#how-to-test-react-native-apps) |
| 190 | [How to do logging in React Native?](#how-to-do-logging-in-react-native) |
| 191 | [How to debug your React Native?](#how-to-debug-your-react-native) |
| | **React supported libraries and Integration** |
| 192 | [What is reselect and how it works?](#what-is-reselect-and-how-it-works) |
| 193 | [What is Flow?](#what-is-flow) |
| 194 | [What is the difference between Flow and PropTypes?](#what-is-the-difference-between-flow-and-proptypes) |
| 195 | [How to use font-awesome icons in React?](#how-to-use-font-awesome-icons-in-react) |
| 196 | [What is React Dev Tools?](#what-is-react-dev-tools) |
| 197 | [Why is DevTools not loading in Chrome for local files?](#why-is-devtools-not-loading-in-chrome-for-local-files) |
| 198 | [How to use Polymer in React?](#how-to-use-polymer-in-react) |
| 199 | [What are the advantages of React over Vue.js?](#what-are-the-advantages-of-react-over-vuejs) |
| 200 | [What is the difference between React and Angular?](#what-is-the-difference-between-react-and-angular) |
| 201 | [Why React tab is not showing up in DevTools?](#why-react-tab-is-not-showing-up-in-devtools) |
| 202 | [What are styled components?](#what-are-styled-components) |
| 203 | [Give an example of Styled Components?](#give-an-example-of-styled-components) |
| 204 | [What is Relay?](#what-is-relay) |
| 205 | [How to use TypeScript in create-react-app application?](#how-to-use-typescript-in-create-react-app-application) |
| | **Miscellaneous** |
| 206 | [What are the main features of reselect library?](#what-are-the-main-features-of-reselect-library) |
| 207 | [Give an example of reselect usage?](#give-an-example-of-reselect-usage) |
| 209 | [Does the statics object work with ES6 classes in React?](#does-the-statics-object-work-with-es6-classes-in-react) |
| 210 | [Can Redux only be used with React?](#can-redux-only-be-used-with-react) |
| 211 | [Do you need to have a particular build tool to use Redux?](#do-you-need-to-have-a-particular-build-tool-to-use-redux) |
| 212 | [How Redux Form initialValues get updated from state?](#how-redux-form-initialvalues-get-updated-from-state) |
| 213 | [How React PropTypes allow different type for one prop?](#how-react-proptypes-allow-different-types-for-one-prop) |
| 214 | [Can I import an SVG file as react component?](#can-i-import-an-svg-file-as-react-component) |
| 215 | [Why are inline ref callbacks or functions not recommended?](#why-are-inline-ref-callbacks-or-functions-not-recommended) |
| 216 | [What is render hijacking in React?](#what-is-render-hijacking-in-react) |
| 217 | [What are HOC factory implementations?](#what-are-hoc-factory-implementations) |
| 218 | [How to pass numbers to React component?](#how-to-pass-numbers-to-react-component) |
| 219 | [Do I need to keep all my state into Redux? Should I ever use react internal state?](#do-i-need-to-keep-all-my-state-into-redux-should-i-ever-use-react-internal-state) |
| 220 | [What is the purpose of registerServiceWorker in React?](#what-is-the-purpose-of-registerserviceworker-in-react) |
| 221 | [What is React memo function?](#what-is-react-memo-function) |
| 222 | [What is React lazy function?](#what-is-react-lazy-function) |
| 223 | [How to prevent unnecessary updates using setState?](#how-to-prevent-unnecessary-updates-using-setstate) |
| 224 | [How do you render Array, Strings and Numbers in React 16 Version?](#how-do-you-render-array-strings-and-numbers-in-react-16-version) |
| 225 | [How to use class field declarations syntax in React classes?](#how-to-use-class-field-declarations-syntax-in-react-classes) |
| 226 | [What are hooks?](#what-are-hooks) |
| 227 | [What rules need to be followed for hooks?](#what-rules-need-to-be-followed-for-hooks) |
| 228 | [How to ensure hooks followed the rules in your project?](#how-to-ensure-hooks-followed-the-rules-in-your-project) |
| 229 | [What are the differences between Flux and Redux?](#what-are-the-differences-between-flux-and-redux) |
| 230 | [What are the benefits of React Router V4?](#what-are-the-benefits-of-react-router-v4) |
| 231 | [Can you describe about componentDidCatch lifecycle method signature?](#can-you-describe-about-componentdidcatch-lifecycle-method-signature) |
| 232 | [In which scenarios error boundaries do not catch errors?](#in-which-scenarios-error-boundaries-do-not-catch-errors) |
| 233 | [Why do you not need error boundaries for event handlers?](#why-do-you-not-need-error-boundaries-for-event-handlers) |
| 234 | [What is the difference between try catch block and error boundaries?](#what-is-the-difference-between-try-catch-block-and-error-boundaries) |
| 235 | [What is the behavior of uncaught errors in react 16?](#what-is-the-behavior-of-uncaught-errors-in-react-16) |
| 236 | [What is the proper placement for error boundaries?](#what-is-the-proper-placement-for-error-boundaries) |
| 237 | [What is the benefit of component stack trace from error boundary?](#what-is-the-benefit-of-component-stack-trace-from-error-boundary) |
| 238 | [What is the required method to be defined for a class component?](#what-is-the-required-method-to-be-defined-for-a-class-component) |
| 239 | [What are the possible return types of render method?](#what-are-the-possible-return-types-of-render-method) |
| 240 | [What is the main purpose of constructor?](#what-is-the-main-purpose-of-constructor) |
| 241 | [Is it mandatory to define constructor for React component?](#is-it-mandatory-to-define-constructor-for-react-component) |
| 242 | [What are default props?](#what-are-default-props) |
| 243 | [Why should not call setState in componentWillUnmount?](#why-should-not-call-setstate-in-componentwillunmount) |
| 244 | [What is the purpose of getDerivedStateFromError?](#what-is-the-purpose-of-getderivedstatefromerror) |
| 245 | [What is the methods order when component re-rendered?](#what-is-the-methods-order-when-component-re-rendered) |
| 246 | [What are the methods invoked during error handling?](#what-are-the-methods-invoked-during-error-handling) |
| 247 | [What is the purpose of displayName class property?](#what-is-the-purpose-of-displayname-class-property) |
| 248 | [What is the browser support for react applications?](#what-is-the-browser-support-for-react-applications) |
| 249 | [What is the purpose of unmountComponentAtNode method?](#what-is-the-purpose-of-unmountcomponentatnode-method) |
| 250 | [What is code-splitting?](#what-is-code-splitting) |
| 251 | [What is the benefit of strict mode?](#what-is-the-benefit-of-strict-mode) |
| 252 | [What are Keyed Fragments?](#what-are-keyed-fragments) |
| 253 | [Does React support all HTML attributes?](#does-react-support-all-html-attributes) |
| 254 | [What are the limitations with HOCs?](#what-are-the-limitations-with-hocs) |
| 255 | [How to debug forwardRefs in DevTools?](#how-to-debug-forwardrefs-in-devtools) |
| 256 | [When component props defaults to true?](#when-component-props-defaults-to-true) |
| 257 | [What is NextJS and major features of it?](#what-is-nextjs-and-major-features-of-it) |
| 258 | [How do you pass an event handler to a component?](#how-do-you-pass-an-event-handler-to-a-component) |
| 259 | [Is it good to use arrow functions in render methods?](#is-it-good-to-use-arrow-functions-in-render-methods) |
| 260 | [How to prevent a function from being called multiple times?](#how-to-prevent-a-function-from-being-called-multiple-times) |
| 261 | [How JSX prevents Injection Attacks?](#how-jsx-prevents-injection-attacks) |
| 262 | [How do you update rendered elements?](#how-do-you-update-rendered-elements) |
| 263 | [How do you say that props are read only?](#how-do-you-say-that-props-are-read-only) |
| 264 | [How do you say that state updates are merged?](#how-do-you-say-that-state-updates-are-merged) |
| 265 | [How do you pass arguments to an event handler?](#how-do-you-pass-arguments-to-an-event-handler) |
| 266 | [How to prevent component from rendering?](#how-to-prevent-component-from-rendering) |
| 267 | [What are the conditions to safely use the index as a key?](#what-are-the-conditions-to-safely-use-the-index-as-a-key) |
| 268 | [Is it keys should be globally unique?](#is-it-keys-should-be-globally-unique) |
| 269 | [What is the popular choice for form handling?](#what-is-the-popular-choice-for-form-handling) |
| 270 | [What are the advantages of formik over redux form library?](#what-are-the-advantages-of-formik-over-redux-form-library) |
| 271 | [Why do you not required to use inheritance?](#why-do-you-not-required-to-use-inheritance) |
| 272 | [Can I use web components in react application?](#can-i-use-web-components-in-react-application) |
| 273 | [What is dynamic import?](#what-is-dynamic-import) |
| 274 | [What are loadable components?](#what-are-loadable-components) |
| 275 | [What is suspense component?](#what-is-suspense-component) |
| 276 | [What is route based code splitting?](#what-is-route-based-code-splitting) |
| 277 | [Give an example on How to use context?](#give-an-example-on-how-to-use-context) |
| 278 | [What is the purpose of default value in context?](#what-is-the-purpose-of-default-value-in-context) |
| 279 | [How do you use contextType?](#how-do-you-use-contexttype) |
| 280 | [What is a consumer?](#what-is-a-consumer) |
| 281 | [How do you solve performance corner cases while using context?](#how-do-you-solve-performance-corner-cases-while-using-context) |
| 282 | [What is the purpose of forward ref in HOCs?](#what-is-the-purpose-of-forward-ref-in-hocs) |
| 283 | [Is it ref argument available for all functions or class components?](#is-it-ref-argument-available-for-all-functions-or-class-components) |
| 284 | [Why do you need additional care for component libraries while using forward refs?](#why-do-you-need-additional-care-for-component-libraries-while-using-forward-refs) |
| 285 | [How to create react class components without ES6?](#how-to-create-react-class-components-without-es6) |
| 286 | [Is it possible to use react without JSX?](#is-it-possible-to-use-react-without-jsx) |
| 287 | [What is diffing algorithm?](#what-is-diffing-algorithm) |
| 288 | [What are the rules covered by diffing algorithm?](#what-are-the-rules-covered-by-diffing-algorithm) |
| 289 | [When do you need to use refs?](#when-do-you-need-to-use-refs) |
| 290 | [Is it prop must be named as render for render props?](#is-it-prop-must-be-named-as-render-for-render-props) |
| 291 | [What are the problems of using render props with pure components?](#what-are-the-problems-of-using-render-props-with-pure-components) |
| 292 | [How do you create HOC using render props?](#how-do-you-create-hoc-using-render-props) |
| 293 | [What is windowing technique?](#what-is-windowing-technique) |
| 294 | [How do you print falsy values in JSX?](#how-do-you-print-falsy-values-in-jsx) |
| 295 | [What is the typical use case of portals?](#what-is-the-typical-use-case-of-portals) |
| 296 | [How do you set default value for uncontrolled component?](#how-do-you-set-default-value-for-uncontrolled-component) |
| 297 | [What is your favorite React stack?](#what-is-your-favorite-react-stack) |
| 298 | [What is the difference between Real DOM and Virtual DOM?](#what-is-the-difference-between-real-dom-and-virtual-dom) |
| 299 | [How to add Bootstrap to a react application?](#how-to-add-bootstrap-to-a-react-application) |
| 300 | [Can you list down top websites or applications using react as front end framework?](#can-you-list-down-top-websites-or-applications-using-react-as-front-end-framework) |
| 301 | [Is it recommended to use CSS In JS technique in React?](#is-it-recommended-to-use-css-in-js-technique-in-react) |
| 302 | [Do I need to rewrite all my class components with hooks?](#do-i-need-to-rewrite-all-my-class-components-with-hooks) |
| 303 | [How to fetch data with React Hooks?](#how-to-fetch-data-with-react-hooks) |
| 304 | [Is Hooks cover all use cases for classes?](#is-hooks-cover-all-use-cases-for-classes) |
| 305 | [What is the stable release for hooks support?](#what-is-the-stable-release-for-hooks-support) |
| 306 | [Why do we use array destructuring (square brackets notation) in useState?](#why-do-we-use-array-destructuring-square-brackets-notation-in-usestate) |
| 307 | [What are the sources used for introducing hooks?](#what-are-the-sources-used-for-introducing-hooks) |
| 308 | [How do you access imperative API of web components?](#how-do-you-access-imperative-api-of-web-components) |
| 309 | [What is formik?](#what-is-formik) |
| 310 | [What are typical middleware choices for handling asynchronous calls in Redux?](#what-are-typical-middleware-choices-for-handling-asynchronous-calls-in-redux) |
| 311 | [Do browsers understand JSX code?](#do-browsers-understand-jsx-code) |
| 312 | [Describe about data flow in react?](#describe-about-data-flow-in-react) |
| 313 | [What is react scripts?](#what-is-react-scripts) |
| 314 | [What are the features of create react app?](#what-are-the-features-of-create-react-app) |
| 315 | [What is the purpose of renderToNodeStream method?](#what-is-the-purpose-of-rendertonodestream-method) |
| 316 | [What is MobX?](#what-is-mobx) |
| 317 | [What are the differences between Redux and MobX?](#what-are-the-differences-between-redux-and-mobx) |
| 318 | [Should I learn ES6 before learning ReactJS?](#should-i-learn-es6-before-learning-reactjs) |
| 319 | [What is Concurrent Rendering?](#what-is-concurrent-rendering) |
| 320 | [What is the difference between async mode and concurrent mode?](#what-is-the-difference-between-async-mode-and-concurrent-mode) |
| 321 | [Can I use javascript urls in react16.9?](#can-i-use-javascript-urls-in-react169) |
| 322 | [What is the purpose of eslint plugin for hooks?](#what-is-the-purpose-of-eslint-plugin-for-hooks) |
| 323 | [What is the difference between Imperative and Declarative in React?](#what-is-the-difference-between-imperative-and-declarative-in-react) |
| 324 | [What are the benefits of using typescript with reactjs?](#what-are-the-benefits-of-using-typescript-with-reactjs) |
| 325 | [How do you make sure that user remains authenticated on page refresh while using Context API State Management?](#how-do-you-make-sure-that-user-remains-authenticated-on-page-refresh-while-using-context-api-state-management) |
| 326 | [What are the benefits of new JSX transform?](#what-are-the-benefits-of-new-jsx-transform) |
| 327 | [How is the new JSX transform different from old transform?](#how-is-the-new-jsx-transform-different-from-old-transform) |
| 328 | [How do you get redux scaffolding using create-react-app?](#how-do-you-get-redux-scaffolding-using-create-react-app) |
| 329 | [What are React Server components?](#what-are-react-server-components) |
| 330 | [What is prop drilling?](#what-is-prop-drilling) |
| 331 | [What is state mutation and how to prevent it?](#what-is-state-mutation-and-how-to-prevent-it) |
| 332 | [What is the difference between useState and useRef hook?](#what-is-the-difference-between-usestate-and-useref-hook) |
| 333 | [What is a wrapper component ](#what-is-a-wrapper-component) |
| 334 | [What are the differences between useEffect and useLayoutEffect hooks](#what-are-the-differences-between-useEffect-and-useLayoutEffect-hooks) |
| 335 | [What are the differences between Functional and Class Components ](#what-are-the-differences-between-functional-and-class-components) |
| 336 | [Why does strict mode render twice in React?](#why-does-strict-mode-render-twice-in-react) |
## Core React
1. ### What is React?
React(aka React.js or ReactJS) is an **open-source front-end JavaScript library** that is used for building composable user interfaces, especially for single-page applications. It is used for handling view layer for web and mobile apps based on components in a declarative approach.
React was created by [Jordan Walke](https://github.com/jordwalke), a software engineer working for Facebook. React was first deployed on Facebook's News Feed in 2011 and on Instagram in 2012.
**[⬆ Back to Top](#table-of-contents)**
2. ### What is the history behind React evolution?
The history of ReactJS started in 2010 with the creation of **XHP**. XHP is a PHP extension which improved the syntax of the language such that XML document fragments become valid PHP expressions and the primary purpose was used to create custom and reusable HTML elements.
The main principle of this extension was to make front-end code easier to understand and to help avoid cross-site scripting attacks. The project was successful to prevent the malicious content submitted by the scrubbing user.
But there was a different problem with XHP in which dynamic web applications require many roundtrips to the server, and XHP did not solve this problem. Also, the whole UI was re-rendered for small change in the application. Later, the initial prototype of React is created with the name **FaxJ** by Jordan inspired from XHP. Finally after sometime React has been introduced as a new library into JavaScript world.
**Note:** JSX comes from the idea of XHP
**[⬆ Back to Top](#table-of-contents)**
2. ### What are the major features of React?
The major features of React are:
- Uses **JSX** syntax, a syntax extension of JS that allows developers to write HTML in their JS code.
- It uses **Virtual DOM** instead of Real DOM considering that Real DOM manipulations are expensive.
- Supports **server-side rendering** which is useful for Search Engine Optimizations(SEO).
- Follows **Unidirectional or one-way** data flow or data binding.
- Uses **reusable/composable** UI components to develop the view.
**[⬆ Back to Top](#table-of-contents)**
3. ### What is JSX?
_JSX_ stands for _JavaScript XML_ and it is an XML-like syntax extension to ECMAScript. Basically it just provides the syntactic sugar for the `React.createElement(type, props, ...children)` function, giving us expressiveness of JavaScript along with HTML like template syntax.
In the example below, the text inside `
` tag is returned as JavaScript function to the render function.
```jsx harmony
export default function App() {
return (
{"Hello, this is a JSX Code!"}
);
}
```
If you don't use JSX syntax then the respective JavaScript code should be written as below,
```javascript
import { createElement } from 'react';
export default function App() {
return createElement(
'h1',
{ className: 'greeting' },
'Hello, this is a JSX Code!'
);
}
```
See Class
```jsx harmony
class App extends React.Component {
render() {
return (
{"Hello, this is a JSX Code!"}
);
}
}
```
**Note:** JSX is stricter than HTML
**[⬆ Back to Top](#table-of-contents)**
4. ### What is the difference between Element and Component?
An _Element_ is a plain object describing what you want to appear on the screen in terms of the DOM nodes or other components. _Elements_ can contain other _Elements_ in their props. Creating a React element is cheap. Once an element is created, it cannot be mutated.
The JavaScript representation(Without JSX) of React Element would be as follows:
```javascript
const element = React.createElement("div", { id: "login-btn" }, "Login");
```
and this element can be simiplified using JSX
```html
Login
```
The above `React.createElement()` function returns an object as below:
```
{
type: 'div',
props: {
children: 'Login',
id: 'login-btn'
}
}
```
Finally, this element renders to the DOM using `ReactDOM.render()`.
Whereas a **component** can be declared in several different ways. It can be a class with a `render()` method or it can be defined as a function. In either case, it takes props as an input, and returns a JSX tree as the output:
```javascript
const Button = ({ handleLogin }) => (
Login
);
```
Then JSX gets transpiled to a `React.createElement()` function tree:
```javascript
const Button = ({ handleLogin }) =>
React.createElement(
"div",
{ id: "login-btn", onClick: handleLogin },
"Login"
);
```
**[⬆ Back to Top](#table-of-contents)**
5. ### How to create components in React?
Components are the building blocks of creating User Interfaces(UI) in React. There are two possible ways to create a component.
1. **Function Components:** This is the simplest way to create a component. Those are pure JavaScript functions that accept props object as the first parameter and return React elements to render the output:
```jsx harmony
function Greeting({ message }) {
return
{`Hello, ${message}`}
;
}
```
2. **Class Components:** You can also use ES6 class to define a component. The above function component can be written as a class component:
```jsx harmony
class Greeting extends React.Component {
render() {
return
{`Hello, ${this.props.message}`}
;
}
}
```
**[⬆ Back to Top](#table-of-contents)**
6. ### When to use a Class Component over a Function Component?
After the addition of Hooks(i.e. React 16.8 onwards) it is always recommended to use Function components over Class components in React. Because you could use state, lifecycle methods and other features that were only available in class component present in function component too.
But even there are two reasons to use Class components over Function components.
1. If you need a React functionality whose Function component equivalent is not present yet, like Error Boundaries.
2. In older versions, If the component needs _state or lifecycle methods_ then you need to use class component.
**Note:** You can also use reusable [react error boundary](https://github.com/bvaughn/react-error-boundary) third-party component without writing any class. i.e, No need to use class components for Error boundaries.
The usage of Error boundaries from the above library is quite straight forward.
```jsx
"use client";
import { ErrorBoundary } from "react-error-boundary";
Something went wrong}>
```
**[⬆ Back to Top](#table-of-contents)**
7. ### What are Pure Components?
Pure components are the components which render the same output for the same state and props. In function components, you can achieve these pure components through memoized `React.memo()` API wrapping around the component. This API prevents unnecessary re-renders by comparing the previous props and new props using shallow comparison. So it will be helpful for performance optimizations.
But at the same time, it won't compare the previous state with the current state because function component itself prevents the unnecessary rendering by default when you set the same state again.
The syntactic representation of memoized components looks like below,
```jsx
const MemoizedComponent = memo(SomeComponent, arePropsEqual?);
```
Below is the example of how child component(i.e., EmployeeProfile) prevents re-renders for the same props passed by parent component(i.e.,EmployeeRegForm).
```jsx
import { memo, useState } from 'react';
const EmployeeProfile = memo(function EmployeeProfile({ name, email }) {
return (<>
Name:{name}
Email: {email}
>);
});
export default function EmployeeRegForm() {
const [name, setName] = useState('');
const [email, setEmail] = useState('');
return (
<>
Name: setName(e.target.value)} />
Email: setEmail(e.target.value)} />
>
);
}
```
In the above code, the email prop has not been passed to child component. So there won't be any re-renders for email prop change.
In class components, the components extending _`React.PureComponent`_ instead of _`React.Component`_ become the pure components. When props or state changes, _PureComponent_ will do a shallow comparison on both props and state by invoking `shouldComponentUpdate()` lifecycle method.
**Note:** `React.memo()` is a higher-order component.
**[⬆ Back to Top](#table-of-contents)**
8. ### What is state in React?
_State_ of a component is an object that holds some information that may change over the lifetime of the component. The important point is whenever the state object changes, the component re-renders. It is always recommended to make our state as simple as possible and minimize the number of stateful components.
Let's take an example of **User** component with message state. Here, **useState** hook has been used to add state to the User component and it returns an array with current state and function to update it.
```jsx harmony
import React, { useState } from "react";
function User() {
const [message, setMessage] = useState("Welcome to React world");
return (
{message}
);
}
```
See Class
```jsx harmony
import React from 'react';
class User extends React.Component {
constructor(props) {
super(props);
this.state = {
message: "Welcome to React world",
};
}
render() {
return (
{this.state.message}
);
}
}
```
State is similar to props, but it is private and fully controlled by the component ,i.e., it is not accessible to any other component till the owner component decides to pass it.
**[⬆ Back to Top](#table-of-contents)**
9. ### What are props in React?
_Props_ are inputs to components. They are single values or objects containing a set of values that are passed to components on creation similar to HTML-tag attributes. Here, the data is passed down from a parent component to a child component.
The primary purpose of props in React is to provide following component functionality:
1. Pass custom data to your component.
2. Trigger state changes.
3. Use via `this.props.reactProp` inside component's `render()` method.
For example, let us create an element with `reactProp` property:
```jsx harmony
```
This `reactProp` (or whatever you came up with) attribute name then becomes a property attached to React's native props object which originally already exists on all components created using React library.
```jsx harmony
props.reactProp
```
For example, the usage of props in function component looks like below:
```jsx
import React from "react";
import ReactDOM from "react-dom";
const ChildComponent = (props) => {
return (
{props.name}
{props.age}
);
};
const ParentComponent = () => {
return (
);
};
```
The properties from props object can be accessed directly using destructing feature from ES6 (ECMAScript 2015). The above child component can be simplified like below.
```jsx harmony
const ChildComponent = ({name, age}) => {
return (
{name}
{age}
);
};
```
See Class
The Props accessed in Class Based Component as below
```jsx
import React from "react";
import ReactDOM from "react-dom";
class ChildComponent extends React.Component {
render() {
return (
{this.props.name}
{this.props.age}
);
}
}
class ParentComponent extends React.Component {
render() {
return (
);
}
}
```
**[⬆ Back to Top](#table-of-contents)**
10. ### What is the difference between state and props?
In React, both `state` and `props` are plain JavaScript objects and used to manage the data of a component, but they are used in different ways and have different characteristics.
`state` is managed by the component itself and can be updated using the `setState()` function. Unlike props, state can be modified by the component and is used to manage the internal state of the component. Changes in the state trigger a re-render of the component and its children.
`props` (short for "properties") are passed to a component by its parent component and are `read-only`, meaning that they cannot be modified by the component itself. props can be used to configure the behavior of a component and to pass data between components.
**[⬆ Back to Top](#table-of-contents)**
11. ### Why should we not update the state directly?
If you try to update the state directly then it won't re-render the component.
```javascript
//Wrong
this.state.message = "Hello world";
```
Instead use `setState()` method. It schedules an update to a component's state object. When state changes, the component responds by re-rendering.
```javascript
//Correct
this.setState({ message: "Hello World" });
```
**Note:** You can directly assign to the state object either in _constructor_ or using latest javascript's class field declaration syntax.
**[⬆ Back to Top](#table-of-contents)**
12. ### What is the purpose of callback function as an argument of `setState()`?
The callback function is invoked when setState finished and the component gets rendered. Since `setState()` is **asynchronous** the callback function is used for any post action.
**Note:** It is recommended to use lifecycle method rather than this callback function.
```javascript
setState({ name: "John" }, () =>
console.log("The name has updated and component re-rendered")
);
```
**[⬆ Back to Top](#table-of-contents)**
13. ### What is the difference between HTML and React event handling?
Below are some of the main differences between HTML and React event handling,
1. In HTML, the event name usually represents in _lowercase_ as a convention:
```html
```
Whereas in React it follows _camelCase_ convention:
```jsx harmony
```
2. In HTML, you can return `false` to prevent default behavior:
Whereas in React you must call `preventDefault()` explicitly:
```javascript
function handleClick(event) {
event.preventDefault();
console.log("The link was clicked.");
}
```
3. In HTML, you need to invoke the function by appending `()`
Whereas in react you should not append `()` with the function name. (refer "activateLasers" function in the first point for example)
**[⬆ Back to Top](#table-of-contents)**
14. ### How to bind methods or event handlers in JSX callbacks?
There are 3 possible ways to achieve this in class components:
1. **Binding in Constructor:** In JavaScript classes, the methods are not bound by default. The same rule applies for React event handlers defined as class methods. Normally we bind them in constructor.
```javascript
class User extends Component {
constructor(props) {
super(props);
this.handleClick = this.handleClick.bind(this);
}
handleClick() {
console.log("SingOut triggered");
}
render() {
return SingOut;
}
}
```
2. **Public class fields syntax:** If you don't like to use bind approach then _public class fields syntax_ can be used to correctly bind callbacks. The Create React App eanables this syntax by default.
```jsx harmony
handleClick = () => {
console.log("SingOut triggered", this);
};
```
```jsx harmony
SingOut
```
3. **Arrow functions in callbacks:** It is possible to use _arrow functions_ directly in the callbacks.
```jsx harmony
handleClick() {
console.log('SingOut triggered');
}
render() {
return this.handleClick()}>SignOut;
}
```
**Note:** If the callback is passed as prop to child components, those components might do an extra re-rendering. In those cases, it is preferred to go with `.bind()` or _public class fields syntax_ approach considering performance.
**[⬆ Back to Top](#table-of-contents)**
15. ### How to pass a parameter to an event handler or callback?
You can use an _arrow function_ to wrap around an _event handler_ and pass parameters:
```jsx harmony
this.handleClick(id)} />
```
This is an equivalent to calling `.bind`:
```jsx harmony
```
Apart from these two approaches, you can also pass arguments to a function which is defined as arrow function
```jsx harmony
;
handleClick = (id) => () => {
console.log("Hello, your ticket number is", id);
};
```
**[⬆ Back to Top](#table-of-contents)**
16. ### What are synthetic events in React?
`SyntheticEvent` is a cross-browser wrapper around the browser's native event. Its API is same as the browser's native event, including `stopPropagation()` and `preventDefault()`, except the events work identically across all browsers. The native events can be accessed directly from synthetic events using `nativeEvent` attribute.
Let's take an example of BookStore title search component with the ability to get all native event properties
```js
function BookStore() {
handleTitleChange(e) {
console.log('The new title is:', e.target.value);
// 'e' represents synthetic event
const nativeEvent = e.nativeEvent;
console.log(nativeEvent);
e.stopPropogation();
e.preventDefault();
}
return
}
```
**[⬆ Back to Top](#table-of-contents)**
17. ### What are inline conditional expressions?
You can use either _if statements_ or _ternary expressions_ which are available from JS to conditionally render expressions. Apart from these approaches, you can also embed any expressions in JSX by wrapping them in curly braces and then followed by JS logical operator `&&`.
```jsx harmony
Hello!
;
{
messages.length > 0 && !isLogin ? (
You have {messages.length} unread messages.
) : (
You don't have unread messages.
);
}
```
**[⬆ Back to Top](#table-of-contents)**
18. ### What is "key" prop and what is the benefit of using it in arrays of elements?
A `key` is a special attribute you **should** include when mapping over arrays to render data. _Key_ prop helps React identify which items have changed, are added, or are removed.
Keys should be unique among its siblings. Most often we use ID from our data as _key_:
```jsx harmony
const todoItems = todos.map((todo) =>
```
When you don't have stable IDs for rendered items, you may use the item _index_ as a _key_ as a last resort:
```jsx harmony
const todoItems = todos.map((todo, index) => (
));
```
**Note:**
1. Using _indexes_ for _keys_ is **not recommended** if the order of items may change. This can negatively impact performance and may cause issues with component state.
2. If you extract list item as separate component then apply _keys_ on list component instead of `li` tag.
3. There will be a warning message in the console if the `key` prop is not present on list items.
4. The key attribute accepts either string or number and internally convert it as string type.
**[⬆ Back to Top](#table-of-contents)**
19. ### What is the use of refs?
The _ref_ is used to return a reference to the element. They _should be avoided_ in most cases, however, they can be useful when you need a direct access to the DOM element or an instance of a component.
**[⬆ Back to Top](#table-of-contents)**
20. ### How to create refs?
There are two approaches
1. This is a recently added approach. _Refs_ are created using `React.createRef()` method and attached to React elements via the `ref` attribute. In order to use _refs_ throughout the component, just assign the _ref_ to the instance property within constructor.
```jsx harmony
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.myRef = React.createRef();
}
render() {
return
}
}
```
2. You can also use ref callbacks approach regardless of React version. For example, the search bar component's input element is accessed as follows,
```jsx harmony
class SearchBar extends Component {
constructor(props) {
super(props);
this.txtSearch = null;
this.state = { term: "" };
this.setInputSearchRef = (e) => {
this.txtSearch = e;
};
}
onInputChange(event) {
this.setState({ term: this.txtSearch.value });
}
render() {
return (
);
}
}
```
You can also use _refs_ in function components using **closures**.
**Note**: You can also use inline ref callbacks even though it is not a recommended approach.
**[⬆ Back to Top](#table-of-contents)**
21. ### What are forward refs?
_Ref forwarding_ is a feature that lets some components take a _ref_ they receive, and pass it further down to a child.
```jsx harmony
const ButtonElement = React.forwardRef((props, ref) => (
{props.children}
));
// Create ref to the DOM button:
const ref = React.createRef();
{"Forward Ref"};
```
**[⬆ Back to Top](#table-of-contents)**
22. ### Which is preferred option with in callback refs and findDOMNode()?
It is preferred to use _callback refs_ over `findDOMNode()` API. Because `findDOMNode()` prevents certain improvements in React in the future.
The **legacy** approach of using `findDOMNode`:
```javascript
class MyComponent extends Component {
componentDidMount() {
findDOMNode(this).scrollIntoView();
}
render() {
return
}
}
```
The recommended approach is:
```javascript
class MyComponent extends Component {
constructor(props) {
super(props);
this.node = createRef();
}
componentDidMount() {
this.node.current.scrollIntoView();
}
render() {
return
}
}
```
**[⬆ Back to Top](#table-of-contents)**
23. ### Why are String Refs legacy?
If you worked with React before, you might be familiar with an older API where the `ref` attribute is a string, like `ref={'textInput'}`, and the DOM node is accessed as `this.refs.textInput`. We advise against it because _string refs have below issues_, and are considered legacy. String refs were **removed in React v16**.
1. They _force React to keep track of currently executing component_. This is problematic because it makes react module stateful, and thus causes weird errors when react module is duplicated in the bundle.
2. They are _not composable_ — if a library puts a ref on the passed child, the user can't put another ref on it. Callback refs are perfectly composable.
3. They _don't work with static analysis_ like Flow. Flow can't guess the magic that framework does to make the string ref appear on `this.refs`, as well as its type (which could be different). Callback refs are friendlier to static analysis.
4. It doesn't work as most people would expect with the "render callback" pattern (e.g. )
```jsx harmony
class MyComponent extends Component {
renderRow = (index) => {
// This won't work. Ref will get attached to DataTable rather than MyComponent:
return ;
// This would work though! Callback refs are awesome.
return (this["input-" + index] = input)} />;
};
render() {
return (
);
}
}
```
**[⬆ Back to Top](#table-of-contents)**
24. ### What is Virtual DOM?
The _Virtual DOM_ (VDOM) is an in-memory representation of _Real DOM_. The representation of a UI is kept in memory and synced with the "real" DOM. It's a step that happens between the render function being called and the displaying of elements on the screen. This entire process is called _reconciliation_.
**[⬆ Back to Top](#table-of-contents)**
25. ### How Virtual DOM works?
The _Virtual DOM_ works in three simple steps.
1. Whenever any underlying data changes, the entire UI is re-rendered in Virtual DOM representation.
2. Then the difference between the previous DOM representation and the new one is calculated.
3. Once the calculations are done, the real DOM will be updated with only the things that have actually changed.
**[⬆ Back to Top](#table-of-contents)**
26. ### What is the difference between Shadow DOM and Virtual DOM?
The _Shadow DOM_ is a browser technology designed primarily for scoping variables and CSS in _web components_. The _Virtual DOM_ is a concept implemented by libraries in JavaScript on top of browser APIs.
**[⬆ Back to Top](#table-of-contents)**
27. ### What is React Fiber?
Fiber is the new _reconciliation_ engine or reimplementation of core algorithm in React v16. The goal of React Fiber is to increase its suitability for areas like animation, layout, gestures, ability to pause, abort, or reuse work and assign priority to different types of updates; and new concurrency primitives.
**[⬆ Back to Top](#table-of-contents)**
28. ### What is the main goal of React Fiber?
The goal of _React Fiber_ is to increase its suitability for areas like animation, layout, and gestures. Its headline feature is **incremental rendering**: the ability to split rendering work into chunks and spread it out over multiple frames.
_from documentation_
Its main goals are:
1. Ability to split interruptible work in chunks.
2. Ability to prioritize, rebase and reuse work in progress.
3. Ability to yield back and forth between parents and children to support layout in React.
4. Ability to return multiple elements from render().
5. Better support for error boundaries.
**[⬆ Back to Top](#table-of-contents)**
29. ### What are controlled components?
A component that controls the input elements within the forms on subsequent user input is called **Controlled Component**, i.e, every state mutation will have an associated handler function.
For example, to write all the names in uppercase letters, we use handleChange as below,
```javascript
handleChange(event) {
this.setState({value: event.target.value.toUpperCase()})
}
```
**[⬆ Back to Top](#table-of-contents)**
30. ### What are uncontrolled components?
The **Uncontrolled Components** are the ones that store their own state internally, and you query the DOM using a ref to find its current value when you need it. This is a bit more like traditional HTML.
In the below UserProfile component, the `name` input is accessed using ref.
```jsx harmony
class UserProfile extends React.Component {
constructor(props) {
super(props);
this.handleSubmit = this.handleSubmit.bind(this);
this.input = React.createRef();
}
handleSubmit(event) {
alert("A name was submitted: " + this.input.current.value);
event.preventDefault();
}
render() {
return (
{"Name:"}
);
}
}
```
In most cases, it's recommend to use controlled components to implement forms. In a controlled component, form data is handled by a React component. The alternative is uncontrolled components, where form data is handled by the DOM itself.
**[⬆ Back to Top](#table-of-contents)**
31. ### What is the difference between createElement and cloneElement?
JSX elements will be transpiled to `React.createElement()` functions to create React elements which are going to be used for the object representation of UI. Whereas `cloneElement` is used to clone an element and pass it new props.
**[⬆ Back to Top](#table-of-contents)**
32. ### What is Lifting State Up in React?
When several components need to share the same changing data then it is recommended to _lift the shared state up_ to their closest common ancestor. That means if two child components share the same data from its parent, then move the state to parent instead of maintaining local state in both of the child components.
**[⬆ Back to Top](#table-of-contents)**
33. ### What are the different phases of component lifecycle?
The component lifecycle has three distinct lifecycle phases:
1. **Mounting:** The component is ready to mount in the browser DOM. This phase covers initialization from `constructor()`, `getDerivedStateFromProps()`, `render()`, and `componentDidMount()` lifecycle methods.
2. **Updating:** In this phase, the component gets updated in two ways, sending the new props and updating the state either from `setState()` or `forceUpdate()`. This phase covers `getDerivedStateFromProps()`, `shouldComponentUpdate()`, `render()`, `getSnapshotBeforeUpdate()` and `componentDidUpdate()` lifecycle methods.
3. **Unmounting:** In this last phase, the component is not needed and gets unmounted from the browser DOM. This phase includes `componentWillUnmount()` lifecycle method.
It's worth mentioning that React internally has a concept of phases when applying changes to the DOM. They are separated as follows
1. **Render** The component will render without any side effects. This applies to Pure components and in this phase, React can pause, abort, or restart the render.
2. **Pre-commit** Before the component actually applies the changes to the DOM, there is a moment that allows React to read from the DOM through the `getSnapshotBeforeUpdate()`.
3. **Commit** React works with the DOM and executes the final lifecycles respectively `componentDidMount()` for mounting, `componentDidUpdate()` for updating, and `componentWillUnmount()` for unmounting.
React 16.3+ Phases (or an [interactive version](http://projects.wojtekmaj.pl/react-lifecycle-methods-diagram/))
Before React 16.3
**[⬆ Back to Top](#table-of-contents)**
34. ### What are the lifecycle methods of React?
Before React 16.3
- **componentWillMount:** Executed before rendering and is used for App level configuration in your root component.
- **componentDidMount:** Executed after first rendering and here all AJAX requests, DOM or state updates, and set up event listeners should occur.
- **componentWillReceiveProps:** Executed when particular prop updates to trigger state transitions.
- **shouldComponentUpdate:** Determines if the component will be updated or not. By default it returns `true`. If you are sure that the component doesn't need to render after state or props are updated, you can return false value. It is a great place to improve performance as it allows you to prevent a re-render if component receives new prop.
- **componentWillUpdate:** Executed before re-rendering the component when there are props & state changes confirmed by `shouldComponentUpdate()` which returns true.
- **componentDidUpdate:** Mostly it is used to update the DOM in response to prop or state changes.
- **componentWillUnmount:** It will be used to cancel any outgoing network requests, or remove all event listeners associated with the component.
React 16.3+
- **getDerivedStateFromProps:** Invoked right before calling `render()` and is invoked on _every_ render. This exists for rare use cases where you need a derived state. Worth reading [if you need derived state](https://reactjs.org/blog/2018/06/07/you-probably-dont-need-derived-state.html).
- **componentDidMount:** Executed after first rendering and where all AJAX requests, DOM or state updates, and set up event listeners should occur.
- **shouldComponentUpdate:** Determines if the component will be updated or not. By default, it returns `true`. If you are sure that the component doesn't need to render after the state or props are updated, you can return a false value. It is a great place to improve performance as it allows you to prevent a re-render if component receives a new prop.
- **getSnapshotBeforeUpdate:** Executed right before rendered output is committed to the DOM. Any value returned by this will be passed into `componentDidUpdate()`. This is useful to capture information from the DOM i.e. scroll position.
- **componentDidUpdate:** Mostly it is used to update the DOM in response to prop or state changes. This will not fire if `shouldComponentUpdate()` returns `false`.
- **componentWillUnmount** It will be used to cancel any outgoing network requests, or remove all event listeners associated with the component.
**[⬆ Back to Top](#table-of-contents)**
35. ### What are Higher-Order Components?
A _higher-order component_ (_HOC_) is a function that takes a component and returns a new component. Basically, it's a pattern that is derived from React's compositional nature.
We call them **pure components** because they can accept any dynamically provided child component but they won't modify or copy any behavior from their input components.
```javascript
const EnhancedComponent = higherOrderComponent(WrappedComponent);
```
HOC can be used for many use cases:
1. Code reuse, logic and bootstrap abstraction.
2. Render hijacking.
3. State abstraction and manipulation.
4. Props manipulation.
**[⬆ Back to Top](#table-of-contents)**
36. ### How to create props proxy for HOC component?
You can add/edit props passed to the component using _props proxy_ pattern like this:
```jsx harmony
function HOC(WrappedComponent) {
return class Test extends Component {
render() {
const newProps = {
title: "New Header",
footer: false,
showFeatureX: false,
showFeatureY: true,
};
return ;
}
};
}
```
**[⬆ Back to Top](#table-of-contents)**
37. ### What is context?
_Context_ provides a way to pass data through the component tree without having to pass props down manually at every level.
For example, authenticated users, locale preferences, UI themes need to be accessed in the application by many components.
```javascript
const { Provider, Consumer } = React.createContext(defaultValue);
```
**[⬆ Back to Top](#table-of-contents)**
38. ### What is children prop?
_Children_ is a prop (`this.props.children`) that allows you to pass components as data to other components, just like any other prop you use. Component tree put between component's opening and closing tag will be passed to that component as `children` prop.
There are several methods available in the React API to work with this prop. These include `React.Children.map`, `React.Children.forEach`, `React.Children.count`, `React.Children.only`, `React.Children.toArray`.
A simple usage of children prop looks as below,
```jsx harmony
const MyDiv = React.createClass({
render: function () {
return
},
});
ReactDOM.render(
{"Hello"}
{"World"}
,
node
);
```
**[⬆ Back to Top](#table-of-contents)**
39. ### How to write comments in React?
The comments in React/JSX are similar to JavaScript Multiline comments but are wrapped in curly braces.
**Single-line comments:**
```jsx harmony
{/* Single-line comments(In vanilla JavaScript, the single-line comments are represented by double slash(//)) */}
{`Welcome ${user}, let's play React`}
```
**Multi-line comments:**
```jsx harmony
{/* Multi-line comments for more than
one line */}
{`Welcome ${user}, let's play React`}
```
**[⬆ Back to Top](#table-of-contents)**
40. ### What is the purpose of using super constructor with props argument?
A child class constructor cannot make use of `this` reference until the `super()` method has been called. The same applies to ES6 sub-classes as well. The main reason for passing props parameter to `super()` call is to access `this.props` in your child constructors.
**Passing props:**
```javascript
class MyComponent extends React.Component {
constructor(props) {
super(props);
console.log(this.props); // prints { name: 'John', age: 42 }
}
}
```
**Not passing props:**
```javascript
class MyComponent extends React.Component {
constructor(props) {
super();
console.log(this.props); // prints undefined
// but props parameter is still available
console.log(props); // prints { name: 'John', age: 42 }
}
render() {
// no difference outside constructor
console.log(this.props); // prints { name: 'John', age: 42 }
}
}
```
The above code snippets reveals that `this.props` is different only within the constructor. It would be the same outside the constructor.
**[⬆ Back to Top](#table-of-contents)**
41. ### What is reconciliation?
`Reconciliation` is the process through which React updates the Browser DOM and makes React work faster. React use a `diffing algorithm` so that component updates are predictable and faster. React would first calculate the difference between the `real DOM` and the copy of DOM `(Virtual DOM)` when there's an update of components.
React stores a copy of Browser DOM which is called `Virtual DOM`. When we make changes or add data, React creates a new Virtual DOM and compares it with the previous one. This comparison is done by `Diffing Algorithm`.
Now React compares the Virtual DOM with Real DOM. It finds out the changed nodes and updates only the changed nodes in Real DOM leaving the rest nodes as it is. This process is called _Reconciliation_.
**[⬆ Back to Top](#table-of-contents)**
42. ### How to set state with a dynamic key name?
If you are using ES6 or the Babel transpiler to transform your JSX code then you can accomplish this with _computed property names_.
```javascript
handleInputChange(event) {
this.setState({ [event.target.id]: event.target.value })
}
```
**[⬆ Back to Top](#table-of-contents)**
43. ### What would be the common mistake of function being called every time the component renders?
You need to make sure that function is not being called while passing the function as a parameter.
```jsx harmony
render() {
// Wrong: handleClick is called instead of passed as a reference!
return {'Click Me'}
}
```
Instead, pass the function itself without parenthesis:
```jsx harmony
render() {
// Correct: handleClick is passed as a reference!
return {'Click Me'}
}
```
**[⬆ Back to Top](#table-of-contents)**
44. ### Is lazy function supports named exports?
No, currently `React.lazy` function supports default exports only. If you would like to import modules which are named exports, you can create an intermediate module that reexports it as the default. It also ensures that tree shaking keeps working and don’t pull unused components.
Let's take a component file which exports multiple named components,
```javascript
// MoreComponents.js
export const SomeComponent = /* ... */;
export const UnusedComponent = /* ... */;
```
and reexport `MoreComponents.js` components in an intermediate file `IntermediateComponent.js`
```javascript
// IntermediateComponent.js
export { SomeComponent as default } from "./MoreComponents.js";
```
Now you can import the module using lazy function as below,
```javascript
import React, { lazy } from "react";
const SomeComponent = lazy(() => import("./IntermediateComponent.js"));
```
**[⬆ Back to Top](#table-of-contents)**
45. ### Why React uses `className` over `class` attribute?
The attribute `class` is a keyword in JavaScript, and JSX is an extension of JavaScript. That's the principal reason why React uses `className` instead of `class`. Pass a string as the `className` prop.
```jsx harmony
render() {
return {'Menu'}
}
```
**[⬆ Back to Top](#table-of-contents)**
46. ### What are fragments?
It's a common pattern or practice in React for a component to return multiple elements. _Fragments_ let you group a list of children without adding extra nodes to the DOM.
You need to use either **** or a shorter syntax having empty tag (**<>>**).
Below is the example of how to use fragment inside _Story_ component.
```jsx harmony
function Story({title, description, date}) {
return (
{title}
{description}
{date}
);
}
```
It is also possible to render list of fragments inside a loop with the mandatory **key** attribute supplied.
```jsx harmony
function StoryBook() {
return stories.map(story =>
{story.title}
{story.description}
{story.date}
);
}
```
Ususally you don't need to use **** until unless there is a need of _key_ attribute. The usage of shorter syntax looks like below.
```jsx harmony
function Story({title, description, date}) {
return (
<>
{title}
{description}
{date}
>
);
}
```
**[⬆ Back to Top](#table-of-contents)**
47. ### Why fragments are better than container divs?
Below are the list of reasons to prefer fragments over container DOM elements,
1. Fragments are a bit faster and use less memory by not creating an extra DOM node. This only has a real benefit on very large and deep trees.
2. Some CSS mechanisms like _Flexbox_ and _CSS Grid_ have a special parent-child relationships, and adding divs in the middle makes it hard to keep the desired layout.
3. The DOM Inspector is less cluttered.
**[⬆ Back to Top](#table-of-contents)**
48. ### What are portals in React?
_Portal_ is a recommended way to render children into a DOM node that exists outside the DOM hierarchy of the parent component.
```javascript
ReactDOM.createPortal(child, container);
```
The first argument is any render-able React child, such as an element, string, or fragment. The second argument is a DOM element.
**[⬆ Back to Top](#table-of-contents)**
49. ### What are stateless components?
If the behaviour of a component is independent of its state then it can be a stateless component. You can use either a function or a class for creating stateless components. But unless you need to use a lifecycle hook in your components, you should go for function components. There are a lot of benefits if you decide to use function components here; they are easy to write, understand, and test, a little faster, and you can avoid the `this` keyword altogether.
**[⬆ Back to Top](#table-of-contents)**
50. ### What are stateful components?
If the behaviour of a component is dependent on the _state_ of the component then it can be termed as stateful component. These _stateful components_ are either function components with hooks or _class components_.
Let's take an example of function stateful component which update the state based on click event,
```javascript
import React, {useState} from 'react';
const App = (props) => {
const [count, setCount] = useState(0);
handleIncrement() {
setCount(count+1);
}
return (
<>
Increment
Counter: {count}
>
)
}
```
See Class
The equivalent class stateful component with a state that gets initialized in the `constructor`.
```jsx harmony
class App extends Component {
constructor(props) {
super(props);
this.state = { count: 0 };
}
handleIncrement() {
setState({count: this.state.count + 1})
}
render() {
<>
this.handleIncrement}>Increment
Count: {count}
>
}
}
```
**[⬆ Back to Top](#table-of-contents)**
51. ### How to apply validation on props in React?
When the application is running in _development mode_, React will automatically check all props that we set on components to make sure they have _correct type_. If the type is incorrect, React will generate warning messages in the console. It's disabled in _production mode_ due to performance impact. The mandatory props are defined with `isRequired`.
The set of predefined prop types:
1. `PropTypes.number`
2. `PropTypes.string`
3. `PropTypes.array`
4. `PropTypes.object`
5. `PropTypes.func`
6. `PropTypes.node`
7. `PropTypes.element`
8. `PropTypes.bool`
9. `PropTypes.symbol`
10. `PropTypes.any`
We can define `propTypes` for `User` component as below:
```jsx harmony
import React from "react";
import PropTypes from "prop-types";
class User extends React.Component {
static propTypes = {
name: PropTypes.string.isRequired,
age: PropTypes.number.isRequired,
};
render() {
return (
<>
{`Welcome, ${this.props.name}`}
{`Age, ${this.props.age}`}
>
);
}
}
```
**Note:** In React v15.5 _PropTypes_ were moved from `React.PropTypes` to `prop-types` library.
_The Equivalent Functional Component_
```jsx harmony
import React from "react";
import PropTypes from "prop-types";
function User({ name, age }) {
return (
<>
{`Welcome, ${name}`}
{`Age, ${age}`}
>
);
}
User.propTypes = {
name: PropTypes.string.isRequired,
age: PropTypes.number.isRequired,
};
```
**[⬆ Back to Top](#table-of-contents)**
52. ### What are the advantages of React?
Below are the list of main advantages of React,
1. Increases the application's performance with _Virtual DOM_.
2. JSX makes code easy to read and write.
3. It renders both on client and server side (_SSR_).
4. Easy to integrate with frameworks (Angular, Backbone) since it is only a view library.
5. Easy to write unit and integration tests with tools such as Jest.
**[⬆ Back to Top](#table-of-contents)**
53. ### What are the limitations of React?
Apart from the advantages, there are few limitations of React too,
1. React is just a view library, not a full framework.
2. There is a learning curve for beginners who are new to web development.
3. Integrating React into a traditional MVC framework requires some additional configuration.
4. The code complexity increases with inline templating and JSX.
5. Too many smaller components leading to over engineering or boilerplate.
**[⬆ Back to Top](#table-of-contents)**
54. ### What are error boundaries in React v16?
_Error boundaries_ are components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of the component tree that crashed.
A class component becomes an error boundary if it defines a new lifecycle method called `componentDidCatch(error, info)` or `static getDerivedStateFromError() `:
```jsx harmony
class ErrorBoundary extends React.Component {
constructor(props) {
super(props);
this.state = { hasError: false };
}
componentDidCatch(error, info) {
// You can also log the error to an error reporting service
logErrorToMyService(error, info);
}
static getDerivedStateFromError(error) {
// Update state so the next render will show the fallback UI.
return { hasError: true };
}
render() {
if (this.state.hasError) {
// You can render any custom fallback UI
return
{"Something went wrong."}
;}
return this.props.children;
}
}
```
After that use it as a regular component:
```jsx harmony
```
**[⬆ Back to Top](#table-of-contents)**
55. ### How are error boundaries handled in React v15?
React v15 provided very basic support for _error boundaries_ using `unstable_handleError` method. It has been renamed to `componentDidCatch` in React v16.
**[⬆ Back to Top](#table-of-contents)**
56. ### What are the recommended ways for static type checking?
Normally we use _PropTypes library_ (`React.PropTypes` moved to a `prop-types` package since React v15.5) for _type checking_ in the React applications. For large code bases, it is recommended to use _static type checkers_ such as Flow or TypeScript, that perform type checking at compile time and provide auto-completion features.
**[⬆ Back to Top](#table-of-contents)**
57. ### What is the use of `react-dom` package?
The `react-dom` package provides _DOM-specific methods_ that can be used at the top level of your app. Most of the components are not required to use this module. Some of the methods of this package are:
1. `render()`
2. `hydrate()`
3. `unmountComponentAtNode()`
4. `findDOMNode()`
5. `createPortal()`
**[⬆ Back to Top](#table-of-contents)**
58. ### What is the purpose of render method of `react-dom`?
This method is used to render a React element into the DOM in the supplied container and return a reference to the component. If the React element was previously rendered into container, it will perform an update on it and only mutate the DOM as necessary to reflect the latest changes.
```
ReactDOM.render(element, container, [callback])
```
If the optional callback is provided, it will be executed after the component is rendered or updated.
**[⬆ Back to Top](#table-of-contents)**
59. ### What is ReactDOMServer?
The `ReactDOMServer` object enables you to render components to static markup (typically used on node server). This object is mainly used for _server-side rendering_ (SSR). The following methods can be used in both the server and browser environments:
1. `renderToString()`
2. `renderToStaticMarkup()`
For example, you generally run a Node-based web server like Express, Hapi, or Koa, and you call `renderToString` to render your root component to a string, which you then send as response.
```javascript
// using Express
import { renderToString } from "react-dom/server";
import MyPage from "./MyPage";
app.get("/", (req, res) => {
res.write(
"My Page"
);
res.write('
res.write(renderToString());
res.write("
res.end();
});
```
**[⬆ Back to Top](#table-of-contents)**
60. ### How to use innerHTML in React?
The `dangerouslySetInnerHTML` attribute is React's replacement for using `innerHTML` in the browser DOM. Just like `innerHTML`, it is risky to use this attribute considering cross-site scripting (XSS) attacks. You just need to pass a `__html` object as key and HTML text as value.
In this example MyComponent uses `dangerouslySetInnerHTML` attribute for setting HTML markup:
```jsx harmony
function createMarkup() {
return { __html: "First · Second" };
}
function MyComponent() {
return
}
```
**[⬆ Back to Top](#table-of-contents)**
61. ### How to use styles in React?
The `style` attribute accepts a JavaScript object with camelCased properties rather than a CSS string. This is consistent with the DOM style JavaScript property, is more efficient, and prevents XSS security holes.
```jsx harmony
const divStyle = {
color: "blue",
backgroundImage: "url(" + imgUrl + ")",
};
function HelloWorldComponent() {
return
}
```
Style keys are camelCased in order to be consistent with accessing the properties on DOM nodes in JavaScript (e.g. `node.style.backgroundImage`).
**[⬆ Back to Top](#table-of-contents)**
62. ### How events are different in React?
Handling events in React elements has some syntactic differences:
1. React event handlers are named using camelCase, rather than lowercase.
2. With JSX you pass a function as the event handler, rather than a string.
**[⬆ Back to Top](#table-of-contents)**
63. ### What will happen if you use `setState()` in constructor?
When you use `setState()`, then apart from assigning to the object state React also re-renders the component and all its children. You would get error like this: _Can only update a mounted or mounting component._ So we need to use `this.state` to initialize variables inside constructor.
**[⬆ Back to Top](#table-of-contents)**
64. ### What is the impact of indexes as keys?
Keys should be stable, predictable, and unique so that React can keep track of elements.
In the below code snippet each element's key will be based on ordering, rather than tied to the data that is being represented. This limits the optimizations that React can do.
```jsx harmony
{
todos.map((todo, index) => );
}
```
If you use element data for unique key, assuming todo.id is unique to this list and stable, React would be able to reorder elements without needing to reevaluate them as much.
```jsx harmony
{
todos.map((todo) => );
}
```
**[⬆ Back to Top](#table-of-contents)**
65. ### Is it good to use `setState()` in `componentWillMount()` method?
Yes, it is safe to use `setState()` inside `componentWillMount()` method. But at the same it is recommended to avoid async initialization in `componentWillMount()` lifecycle method. `componentWillMount()` is invoked immediately before mounting occurs. It is called before `render()`, therefore setting state in this method will not trigger a re-render. Avoid introducing any side-effects or subscriptions in this method. We need to make sure async calls for component initialization happened in `componentDidMount()` instead of `componentWillMount()`.
```jsx harmony
componentDidMount() {
axios.get(`api/todos`)
.then((result) => {
this.setState({
messages: [...result.data]
})
})
}
```
**[⬆ Back to Top](#table-of-contents)**
66. ### What will happen if you use props in initial state?
If the props on the component are changed without the component being refreshed, the new prop value will never be displayed because the constructor function will never update the current state of the component. The initialization of state from props only runs when the component is first created.
The below component won't display the updated input value:
```jsx harmony
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.state = {
records: [],
inputValue: this.props.inputValue,
};
}
render() {
return
}
}
```
Using props inside render method will update the value:
```jsx harmony
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.state = {
record: [],
};
}
render() {
return
}
}
```
**[⬆ Back to Top](#table-of-contents)**
67. ### How do you conditionally render components?
In some cases you want to render different components depending on some state. JSX does not render `false` or `undefined`, so you can use conditional _short-circuiting_ to render a given part of your component only if a certain condition is true.
```jsx harmony
const MyComponent = ({ name, address }) => (
{name}
{address &&
{address}
});
```
If you need an `if-else` condition then use _ternary operator_.
```jsx harmony
const MyComponent = ({ name, address }) => (
{name}
{address ?
{address}
:{"Address is not available"}
});
```
**[⬆ Back to Top](#table-of-contents)**
68. ### Why we need to be careful when spreading props on DOM elements?
When we _spread props_ we run into the risk of adding unknown HTML attributes, which is a bad practice. Instead we can use prop destructuring with `...rest` operator, so it will add only required props.
For example,
```jsx harmony
const ComponentA = () => (
);
const ComponentB = ({ isDisplay, ...domProps }) => (
);
```
**[⬆ Back to Top](#table-of-contents)**
69. ### How you use decorators in React?
You can _decorate_ your _class_ components, which is the same as passing the component into a function. **Decorators** are flexible and readable way of modifying component functionality.
```jsx harmony
@setTitle("Profile")
class Profile extends React.Component {
//....
}
/*
title is a string that will be set as a document title
WrappedComponent is what our decorator will receive when
put directly above a component class as seen in the example above
*/
const setTitle = (title) => (WrappedComponent) => {
return class extends React.Component {
componentDidMount() {
document.title = title;
}
render() {
return ;
}
};
};
```
**Note:** Decorators are a feature that didn't make it into ES7, but are currently a _stage 2 proposal_.
**[⬆ Back to Top](#table-of-contents)**
70. ### How do you memoize a component?
There are memoize libraries available which can be used on function components.
For example `moize` library can memoize the component in another component.
```jsx harmony
import moize from "moize";
import Component from "./components/Component"; // this module exports a non-memoized component
const MemoizedFoo = moize.react(Component);
const Consumer = () => {
{"I will memoize the following entry:"}
};
```
**Update:** Since React v16.6.0, we have a `React.memo`. It provides a higher order component which memoizes component unless the props change. To use it, simply wrap the component using React.memo before you use it.
```js
const MemoComponent = React.memo(function MemoComponent(props) {
/* render using props */
});
OR;
export default React.memo(MyFunctionComponent);
```
**[⬆ Back to Top](#table-of-contents)**
71. ### How you implement Server Side Rendering or SSR?
React is already equipped to handle rendering on Node servers. A special version of the DOM renderer is available, which follows the same pattern as on the client side.
```jsx harmony
import ReactDOMServer from "react-dom/server";
import App from "./App";
ReactDOMServer.renderToString();
```
This method will output the regular HTML as a string, which can be then placed inside a page body as part of the server response. On the client side, React detects the pre-rendered content and seamlessly picks up where it left off.
**[⬆ Back to Top](#table-of-contents)**
72. ### How to enable production mode in React?
You should use Webpack's `DefinePlugin` method to set `NODE_ENV` to `production`, by which it strip out things like propType validation and extra warnings. Apart from this, if you minify the code, for example, Uglify's dead-code elimination to strip out development only code and comments, it will drastically reduce the size of your bundle.
**[⬆ Back to Top](#table-of-contents)**
73. ### What is CRA and its benefits?
The `create-react-app` CLI tool allows you to quickly create & run React applications with no configuration step.
Let's create Todo App using _CRA_:
```console
# Installation
$ npm install -g create-react-app
# Create new project
$ create-react-app todo-app
$ cd todo-app
# Build, test and run
$ npm run build
$ npm run test
$ npm start
```
It includes everything we need to build a React app:
1. React, JSX, ES6, and Flow syntax support.
2. Language extras beyond ES6 like the object spread operator.
3. Autoprefixed CSS, so you don’t need -webkit- or other prefixes.
4. A fast interactive unit test runner with built-in support for coverage reporting.
5. A live development server that warns about common mistakes.
6. A build script to bundle JS, CSS, and images for production, with hashes and sourcemaps.
**[⬆ Back to Top](#table-of-contents)**
74. ### What is the lifecycle methods order in mounting?
The lifecycle methods are called in the following order when an instance of a component is being created and inserted into the DOM.
1. `constructor()`
2. `static getDerivedStateFromProps()`
3. `render()`
4. `componentDidMount()`
**[⬆ Back to Top](#table-of-contents)**
75. ### What are the lifecycle methods going to be deprecated in React v16?
The following lifecycle methods going to be unsafe coding practices and will be more problematic with async rendering.
1. `componentWillMount()`
2. `componentWillReceiveProps()`
3. `componentWillUpdate()`
Starting with React v16.3 these methods are aliased with `UNSAFE_` prefix, and the unprefixed version will be removed in React v17.
**[⬆ Back to Top](#table-of-contents)**
76. ### What is the purpose of `getDerivedStateFromProps()` lifecycle method?
The new static `getDerivedStateFromProps()` lifecycle method is invoked after a component is instantiated as well as before it is re-rendered. It can return an object to update state, or `null` to indicate that the new props do not require any state updates.
```javascript
class MyComponent extends React.Component {
static getDerivedStateFromProps(props, state) {
// ...
}
}
```
This lifecycle method along with `componentDidUpdate()` covers all the use cases of `componentWillReceiveProps()`.
**[⬆ Back to Top](#table-of-contents)**
77. ### What is the purpose of `getSnapshotBeforeUpdate()` lifecycle method?
The new `getSnapshotBeforeUpdate()` lifecycle method is called right before DOM updates. The return value from this method will be passed as the third parameter to `componentDidUpdate()`.
```javascript
class MyComponent extends React.Component {
getSnapshotBeforeUpdate(prevProps, prevState) {
// ...
}
}
```
This lifecycle method along with `componentDidUpdate()` covers all the use cases of `componentWillUpdate()`.
**[⬆ Back to Top](#table-of-contents)**
78. ### Do Hooks replace render props and higher order components?
Both render props and higher-order components render only a single child but in most of the cases Hooks are a simpler way to serve this by reducing nesting in your tree.
**[⬆ Back to Top](#table-of-contents)**
79. ### What is the recommended way for naming components?
It is recommended to name the component by reference instead of using `displayName`.
Using `displayName` for naming component:
```javascript
export default React.createClass({
displayName: "TodoApp",
// ...
});
```
The **recommended** approach:
```javascript
export default class TodoApp extends React.Component {
// ...
}
```
also
```javascript
const TodoApp = () => {
//...
};
export default TodoApp;
```
**[⬆ Back to Top](#table-of-contents)**
80. ### What is the recommended ordering of methods in component class?
_Recommended_ ordering of methods from _mounting_ to _render stage_:
1. `static` methods
2. `constructor()`
3. `getChildContext()`
4. `componentWillMount()`
5. `componentDidMount()`
6. `componentWillReceiveProps()`
7. `shouldComponentUpdate()`
8. `componentWillUpdate()`
9. `componentDidUpdate()`
10. `componentWillUnmount()`
11. click handlers or event handlers like `onClickSubmit()` or `onChangeDescription()`
12. getter methods for render like `getSelectReason()` or `getFooterContent()`
13. optional render methods like `renderNavigation()` or `renderProfilePicture()`
14. `render()`
**[⬆ Back to Top](#table-of-contents)**
81. ### What is a switching component?
A _switching component_ is a component that renders one of many components. We need to use object to map prop values to components.
For example, a switching component to display different pages based on `page` prop:
```jsx harmony
import HomePage from "./HomePage";
import AboutPage from "./AboutPage";
import ServicesPage from "./ServicesPage";
import ContactPage from "./ContactPage";
const PAGES = {
home: HomePage,
about: AboutPage,
services: ServicesPage,
contact: ContactPage,
};
const Page = (props) => {
const Handler = PAGES[props.page] || ContactPage;
return ;
};
// The keys of the PAGES object can be used in the prop types to catch dev-time errors.
Page.propTypes = {
page: PropTypes.oneOf(Object.keys(PAGES)).isRequired,
};
```
**[⬆ Back to Top](#table-of-contents)**
82. ### Why we need to pass a function to setState()?
The reason behind for this is that `setState()` is an asynchronous operation. React batches state changes for performance reasons, so the state may not change immediately after `setState()` is called. That means you should not rely on the current state when calling `setState()` since you can't be sure what that state will be. The solution is to pass a function to `setState()`, with the previous state as an argument. By doing this you can avoid issues with the user getting the old state value on access due to the asynchronous nature of `setState()`.
Let's say the initial count value is zero. After three consecutive increment operations, the value is going to be incremented only by one.
```javascript
// assuming this.state.count === 0
this.setState({ count: this.state.count + 1 });
this.setState({ count: this.state.count + 1 });
this.setState({ count: this.state.count + 1 });
// this.state.count === 1, not 3
```
If we pass a function to `setState()`, the count gets incremented correctly.
```javascript
this.setState((prevState, props) => ({
count: prevState.count + props.increment,
}));
// this.state.count === 3 as expected
```
**(OR)**
### Why function is preferred over object for `setState()`?
React may batch multiple `setState()` calls into a single update for performance. Because `this.props` and `this.state` may be updated asynchronously, you should not rely on their values for calculating the next state.
This counter example will fail to update as expected:
```javascript
// Wrong
this.setState({
counter: this.state.counter + this.props.increment,
});
```
The preferred approach is to call `setState()` with function rather than object. That function will receive the previous state as the first argument, and the props at the time the update is applied as the second argument.
```javascript
// Correct
this.setState((prevState, props) => ({
counter: prevState.counter + props.increment,
}));
```
**[⬆ Back to Top](#table-of-contents)**
83. ### What is strict mode in React?
`React.StrictMode` is a useful component for highlighting potential problems in an application. Just like ``, `` does not render any extra DOM elements. It activates additional checks and warnings for its descendants. These checks apply for _development mode_ only.
```jsx harmony
import React from "react";
function ExampleApplication() {
return (
);
}
```
In the example above, the _strict mode_ checks apply to `` and `` components only.
React.StrictMode currently helps you with:
1. Identifying components with unsafe lifecycles
2. Warning about legacy string ref API usage
3. Warning about deprecated `findDOMNode` usage
4. Detecting unexpected side effects
5. Detecting legacy context API
**[⬆ Back to Top](#table-of-contents)**
84. ### What are React Mixins?
_Mixins_ are a way to totally separate components to have a common functionality. Mixins **should not be used** and can be replaced with _higher-order components_ or _decorators_.
One of the most commonly used mixins is `PureRenderMixin`. You might be using it in some components to prevent unnecessary re-renders when the props and state are shallowly equal to the previous props and state:
```javascript
const PureRenderMixin = require("react-addons-pure-render-mixin");
const Button = React.createClass({
mixins: [PureRenderMixin],
// ...
});
```
**[⬆ Back to Top](#table-of-contents)**
85. ### Why is `isMounted()` an anti-pattern and what is the proper solution?
The primary use case for `isMounted()` is to avoid calling `setState()` after a component has been unmounted, because it will emit a warning.
```javascript
if (this.isMounted()) {
this.setState({...})
}
```
Checking `isMounted()` before calling `setState()` does eliminate the warning, but it also defeats the purpose of the warning. Using `isMounted()` is a code smell because the only reason you would check is because you think you might be holding a reference after the component has unmounted.
An optimal solution would be to find places where `setState()` might be called after a component has unmounted, and fix them. Such situations most commonly occur due to callbacks, when a component is waiting for some data and gets unmounted before the data arrives. Ideally, any callbacks should be canceled in `componentWillUnmount()`, prior to unmounting.
**[⬆ Back to Top](#table-of-contents)**
86. ### What are the Pointer Events supported in React?
_Pointer Events_ provide a unified way of handling all input events. In the old days we had a mouse and respective event listeners to handle them but nowadays we have many devices which don't correlate to having a mouse, like phones with touch surface or pens. We need to remember that these events will only work in browsers that support the _Pointer Events_ specification.
The following event types are now available in _React DOM_:
1. `onPointerDown`
2. `onPointerMove`
3. `onPointerUp`
4. `onPointerCancel`
5. `onGotPointerCapture`
6. `onLostPointerCapture`
7. `onPointerEnter`
8. `onPointerLeave`
9. `onPointerOver`
10. `onPointerOut`
**[⬆ Back to Top](#table-of-contents)**
87. ### Why should component names start with capital letter?
If you are rendering your component using JSX, the name of that component has to begin with a capital letter otherwise React will throw an error as an unrecognized tag. This convention is because only HTML elements and SVG tags can begin with a lowercase letter.
```jsx harmony
class SomeComponent extends Component {
// Code goes here
}
```
You can define component class which name starts with lowercase letter, but when it's imported it should have capital letter. Here lowercase is fine:
```jsx harmony
class myComponent extends Component {
render() {
return
}
}
export default myComponent;
```
While when imported in another file it should start with capital letter:
```jsx harmony
import MyComponent from "./myComponent";
```
#### What are the exceptions on React component naming?
The component names should start with an uppercase letter but there are few exceptions to this convention. The lowercase tag names with a dot (property accessors) are still considered as valid component names.
For example, the below tag can be compiled to a valid component,
```jsx harmony
render() {
return (
// `React.createElement(obj.component)`
)
}
```
**[⬆ Back to Top](#table-of-contents)**
88. ### Are custom DOM attributes supported in React v16?
Yes. In the past, React used to ignore unknown DOM attributes. If you wrote JSX with an attribute that React doesn't recognize, React would just skip it.
For example, let's take a look at the below attribute:
```jsx harmony
```
Would render an empty div to the DOM with React v15:
```html
```
In React v16 any unknown attributes will end up in the DOM:
```html
```
This is useful for supplying browser-specific non-standard attributes, trying new DOM APIs, and integrating with opinionated third-party libraries.
**[⬆ Back to Top](#table-of-contents)**
89. ### What is the difference between constructor and getInitialState?
You should initialize state in the constructor when using ES6 classes, and `getInitialState()` method when using `React.createClass()`.
**Using ES6 classes:**
```javascript
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.state = {
/* initial state */
};
}
}
```
**Using `React.createClass()`:**
```javascript
const MyComponent = React.createClass({
getInitialState() {
return {
/* initial state */
};
},
});
```
**Note:** `React.createClass()` is deprecated and removed in React v16. Use plain JavaScript classes instead.
**[⬆ Back to Top](#table-of-contents)**
90. ### Can you force a component to re-render without calling setState?
By default, when your component's state or props change, your component will re-render. If your `render()` method depends on some other data, you can tell React that the component needs re-rendering by calling `forceUpdate()`.
```javascript
component.forceUpdate(callback);
```
It is recommended to avoid all uses of `forceUpdate()` and only read from `this.props` and `this.state` in `render()`.
**[⬆ Back to Top](#table-of-contents)**
91. ### What is the difference between `super()` and `super(props)` in React using ES6 classes?
When you want to access `this.props` in `constructor()` then you should pass props to `super()` method.
**Using `super(props)`:**
```javascript
class MyComponent extends React.Component {
constructor(props) {
super(props);
console.log(this.props); // { name: 'John', ... }
}
}
```
**Using `super()`:**
```javascript
class MyComponent extends React.Component {
constructor(props) {
super();
console.log(this.props); // undefined
}
}
```
Outside `constructor()` both will display same value for `this.props`.
**[⬆ Back to Top](#table-of-contents)**
92. ### How to loop inside JSX?
You can simply use `Array.prototype.map` with ES6 _arrow function_ syntax.
For example, the `items` array of objects is mapped into an array of components:
```jsx harmony
{items.map((item) => (
))}
```
But you can't iterate using `for` loop:
```jsx harmony
for (let i = 0; i < items.length; i++) {
}
```
This is because JSX tags are transpiled into _function calls_, and you can't use statements inside expressions. This may change thanks to `do` expressions which are _stage 1 proposal_.
**[⬆ Back to Top](#table-of-contents)**
93. ### How do you access props in attribute quotes?
React (or JSX) doesn't support variable interpolation inside an attribute value. The below representation won't work:
```jsx harmony
```
But you can put any JS expression inside curly braces as the entire attribute value. So the below expression works:
```jsx harmony
```
Using _template strings_ will also work:
```jsx harmony
```
**[⬆ Back to Top](#table-of-contents)**
94. ### What is React proptype array with shape?
If you want to pass an array of objects to a component with a particular shape then use `React.PropTypes.shape()` as an argument to `React.PropTypes.arrayOf()`.
```javascript
ReactComponent.propTypes = {
arrayWithShape: React.PropTypes.arrayOf(
React.PropTypes.shape({
color: React.PropTypes.string.isRequired,
fontSize: React.PropTypes.number.isRequired,
})
).isRequired,
};
```
**[⬆ Back to Top](#table-of-contents)**
95. ### How to conditionally apply class attributes?
You shouldn't use curly braces inside quotes because it is going to be evaluated as a string.
```jsx harmony
```
Instead you need to move curly braces outside (don't forget to include spaces between class names):
```jsx harmony
```
_Template strings_ will also work:
```jsx harmony
```
**[⬆ Back to Top](#table-of-contents)**
96. ### What is the difference between React and ReactDOM?
The `react` package contains `React.createElement()`, `React.Component`, `React.Children`, and other helpers related to elements and component classes. You can think of these as the isomorphic or universal helpers that you need to build components. The `react-dom` package contains `ReactDOM.render()`, and in `react-dom/server` we have _server-side rendering_ support with `ReactDOMServer.renderToString()` and `ReactDOMServer.renderToStaticMarkup()`.
**[⬆ Back to Top](#table-of-contents)**
97. ### Why ReactDOM is separated from React?
The React team worked on extracting all DOM-related features into a separate library called _ReactDOM_. React v0.14 is the first release in which the libraries are split. By looking at some of the packages, `react-native`, `react-art`, `react-canvas`, and `react-three`, it has become clear that the beauty and essence of React has nothing to do with browsers or the DOM.
To build more environments that React can render to, React team planned to split the main React package into two: `react` and `react-dom`. This paves the way to writing components that can be shared between the web version of React and React Native.
**[⬆ Back to Top](#table-of-contents)**
98. ### How to use React label element?
If you try to render a `` element bound to a text input using the standard `for` attribute, then it produces HTML missing that attribute and prints a warning to the console.
```jsx harmony
{'User'}
```
Since `for` is a reserved keyword in JavaScript, use `htmlFor` instead.
```jsx harmony
{'User'}
```
**[⬆ Back to Top](#table-of-contents)**
99. ### How to combine multiple inline style objects?
You can use _spread operator_ in regular React:
```jsx harmony
{"Submit"}
```
If you're using React Native then you can use the array notation:
```jsx harmony
{"Submit"}
```
**[⬆ Back to Top](#table-of-contents)**
100. ### How to re-render the view when the browser is resized?
You can use the `useState` hook to manage the width and height state variables, and the `useEffect` hook to add and remove the `resize` event listener. The `[]` dependency array passed to useEffect ensures that the effect only runs once (on mount) and not on every re-render.
```javascript
import React, { useState, useEffect } from "react";
function WindowDimensions() {
const [dimensions, setDimensions] = useState({
width: window.innerWidth,
height: window.innerHeight,
});
useEffect(() => {
function handleResize() {
setDimensions({
width: window.innerWidth,
height: window.innerHeight,
});
}
window.addEventListener("resize", handleResize);
return () => window.removeEventListener("resize", handleResize);
}, []);
return (
{dimensions.width} x {dimensions.height}
);
}
```
Using Class Component
You can listen to the `resize` event in `componentDidMount()` and then update the dimensions (`width` and `height`). You should remove the listener in `componentWillUnmount()` method.
```javascript
class WindowDimensions extends React.Component {
constructor(props) {
super(props);
this.updateDimensions = this.updateDimensions.bind(this);
}
componentWillMount() {
this.updateDimensions();
}
componentDidMount() {
window.addEventListener("resize", this.updateDimensions);
}
componentWillUnmount() {
window.removeEventListener("resize", this.updateDimensions);
}
updateDimensions() {
this.setState({
width: window.innerWidth,
height: window.innerHeight,
});
}
render() {
return (
{this.state.width} x {this.state.height}
);
}
}
```
**[⬆ Back to Top](#table-of-contents)**
101. ### What is the difference between `setState()` and `replaceState()` methods?
When you use `setState()` the current and previous states are merged. `replaceState()` throws out the current state, and replaces it with only what you provide. Usually `setState()` is used unless you really need to remove all previous keys for some reason. You can also set state to `false`/`null` in `setState()` instead of using `replaceState()`.
**[⬆ Back to Top](#table-of-contents)**
102. ### How to listen to state changes?
The `componentDidUpdate` lifecycle method will be called when state changes. You can compare provided state and props values with current state and props to determine if something meaningful changed.
```
componentDidUpdate(object prevProps, object prevState)
```
**Note:** The previous releases of ReactJS also uses `componentWillUpdate(object nextProps, object nextState)` for state changes. It has been deprecated in latest releases.
**[⬆ Back to Top](#table-of-contents)**
103. ### What is the recommended approach of removing an array element in React state?
The better approach is to use `Array.prototype.filter()` method.
For example, let's create a `removeItem()` method for updating the state.
```javascript
removeItem(index) {
this.setState({
data: this.state.data.filter((item, i) => i !== index)
})
}
```
**[⬆ Back to Top](#table-of-contents)**
104. ### Is it possible to use React without rendering HTML?
It is possible. Below are the possible options:
```jsx harmony
render() {
return false
}
```
```jsx harmony
render() {
return true
}
```
```jsx harmony
render() {
return null
}
```
React version >=16.0.0:
```jsx harmony
render() {
return []
}
```
```jsx harmony
render() {
return ""
}
```
React version >=16.2.0:
```jsx harmony
render() {
return
}
```
```jsx harmony
render() {
return <>>
}
```
React version >=18.0.0:
```jsx harmony
render() {
return undefined
}
```
**[⬆ Back to Top](#table-of-contents)**
105. ### How to pretty print JSON with React?
We can use `
` tag so that the formatting of the `JSON.stringify()` is retained:```jsx harmony
const data = { name: "John", age: 42 };class User extends React.Component {
render() {
return{JSON.stringify(data, null, 2)};
}
}React.render(, document.getElementById("container"));
```**[⬆ Back to Top](#table-of-contents)**
106. ### Why you can't update props in React?
The React philosophy is that props should be _immutable_ and _top-down_. This means that a parent can send any prop values to a child, but the child can't modify received props.
**[⬆ Back to Top](#table-of-contents)**
107. ### How to focus an input element on page load?
You can do it by creating _ref_ for `input` element and using it in `componentDidMount()`:
```jsx harmony
class App extends React.Component {
componentDidMount() {
this.nameInput.focus();
}render() {
return (
(this.nameInput = input)}
defaultValue={"Will focus"}
/>
);
}
}ReactDOM.render(, document.getElementById("app"));
```Also in Functional component (react 16.08 and above)
```jsx harmony
import React, { useEffect, useRef } from "react";const App = () => {
const inputElRef = useRef(null);useEffect(() => {
inputElRef.current.focus();
}, []);return (
);
};ReactDOM.render(, document.getElementById("app"));
```**[⬆ Back to Top](#table-of-contents)**
108. ### What are the possible ways of updating objects in state?
1. **Calling `setState()` with an object to merge with state:**
- Using `Object.assign()` to create a copy of the object:
```javascript
const user = Object.assign({}, this.state.user, { age: 42 });
this.setState({ user });
```- Using _spread operator_:
```javascript
const user = { ...this.state.user, age: 42 };
this.setState({ user });
```2. **Calling `setState()` with a function:**
```javascript
this.setState((prevState) => ({
user: {
...prevState.user,
age: 42,
},
}));
```**[⬆ Back to Top](#table-of-contents)**
110. ### How can we find the version of React at runtime in the browser?
You can use `React.version` to get the version.
```jsx harmony
const REACT_VERSION = React.version;ReactDOM.render(
{`React version: ${REACT_VERSION}`},
document.getElementById("app")
);
```**[⬆ Back to Top](#table-of-contents)**
111. ### What are the approaches to include polyfills in your `create-react-app`?
There are approaches to include polyfills in create-react-app,
1. **Manual import from `core-js`:**
Create a file called (something like) `polyfills.js` and import it into root `index.js` file. Run `npm install core-js` or `yarn add core-js` and import your specific required features.
```javascript
import "core-js/fn/array/find";
import "core-js/fn/array/includes";
import "core-js/fn/number/is-nan";
```2. **Using Polyfill service:**
Use the polyfill.io CDN to retrieve custom, browser-specific polyfills by adding this line to `index.html`:
```html
```In the above script we had to explicitly request the `Array.prototype.includes` feature as it is not included in the default feature set.
**[⬆ Back to Top](#table-of-contents)**
112. ### How to use https instead of http in create-react-app?
You just need to use `HTTPS=true` configuration. You can edit your `package.json` scripts section:
```json
"scripts": {
"start": "set HTTPS=true && react-scripts start"
}
```or just run `set HTTPS=true && npm start`
**[⬆ Back to Top](#table-of-contents)**
113. ### How to avoid using relative path imports in create-react-app?
Create a file called `.env` in the project root and write the import path:
```
NODE_PATH=src/app
```After that restart the development server. Now you should be able to import anything inside `src/app` without relative paths.
**[⬆ Back to Top](#table-of-contents)**
114. ### How to add Google Analytics for React Router?
Add a listener on the `history` object to record each page view:
```javascript
history.listen(function (location) {
window.ga("set", "page", location.pathname + location.search);
window.ga("send", "pageview", location.pathname + location.search);
});
```**[⬆ Back to Top](#table-of-contents)**
115. ### How to update a component every second?
You need to use `setInterval()` to trigger the change, but you also need to clear the timer when the component unmounts to prevent errors and memory leaks.
```javascript
componentDidMount() {
this.interval = setInterval(() => this.setState({ time: Date.now() }), 1000)
}componentWillUnmount() {
clearInterval(this.interval)
}
```**[⬆ Back to Top](#table-of-contents)**
116. ### How do you apply vendor prefixes to inline styles in React?
React _does not_ apply _vendor prefixes_ automatically. You need to add vendor prefixes manually.
```jsx harmony
```**[⬆ Back to Top](#table-of-contents)**
117. ### How to import and export components using React and ES6?
You should use default for exporting the components
```jsx harmony
import React from "react";
import User from "user";export default class MyProfile extends React.Component {
render() {
return //...;
}
}
```With the export specifier, the MyProfile is going to be the member and exported to this module and the same can be imported without mentioning the name in other components.
**[⬆ Back to Top](#table-of-contents)**
119. ### Why is a component constructor called only once?
React's _reconciliation_ algorithm assumes that without any information to the contrary, if a custom component appears in the same place on subsequent renders, it's the same component as before, so reuses the previous instance rather than creating a new one.
**[⬆ Back to Top](#table-of-contents)**
120. ### How to define constants in React?
You can use ES7 `static` field to define constant.
```javascript
class MyComponent extends React.Component {
static DEFAULT_PAGINATION = 10;
}
```**[⬆ Back to Top](#table-of-contents)**
121. ### How to programmatically trigger click event in React?
You could use the ref prop to acquire a reference to the underlying `HTMLInputElement` object through a callback, store the reference as a class property, then use that reference to later trigger a click from your event handlers using the `HTMLElement.click` method.
This can be done in two steps:
1. Create ref in render method:
```jsx harmony
(this.inputElement = input)} />
```2. Apply click event in your event handler:
```javascript
this.inputElement.click();
```**[⬆ Back to Top](#table-of-contents)**
122. ### Is it possible to use async/await in plain React?
If you want to use `async`/`await` in React, you will need _Babel_ and [transform-async-to-generator](https://babeljs.io/docs/en/babel-plugin-transform-async-to-generator) plugin. React Native ships with Babel and a set of transforms.
**[⬆ Back to Top](#table-of-contents)**
123. ### What are the common folder structures for React?
There are two common practices for React project file structure.
1. **Grouping by features or routes:**
One common way to structure projects is locate CSS, JS, and tests together, grouped by feature or route.
```
common/
├─ Avatar.js
├─ Avatar.css
├─ APIUtils.js
└─ APIUtils.test.js
feed/
├─ index.js
├─ Feed.js
├─ Feed.css
├─ FeedStory.js
├─ FeedStory.test.js
└─ FeedAPI.js
profile/
├─ index.js
├─ Profile.js
├─ ProfileHeader.js
├─ ProfileHeader.css
└─ ProfileAPI.js
```2. **Grouping by file type:**
Another popular way to structure projects is to group similar files together.
```
api/
├─ APIUtils.js
├─ APIUtils.test.js
├─ ProfileAPI.js
└─ UserAPI.js
components/
├─ Avatar.js
├─ Avatar.css
├─ Feed.js
├─ Feed.css
├─ FeedStory.js
├─ FeedStory.test.js
├─ Profile.js
├─ ProfileHeader.js
└─ ProfileHeader.css
```**[⬆ Back to Top](#table-of-contents)**
124. ### What are the popular packages for animation?
_React Transition Group_ and _React Motion_ are popular animation packages in React ecosystem.
**[⬆ Back to Top](#table-of-contents)**
125. ### What is the benefit of styles modules?
It is recommended to avoid hard coding style values in components. Any values that are likely to be used across different UI components should be extracted into their own modules.
For example, these styles could be extracted into a separate component:
```javascript
export const colors = {
white,
black,
blue,
};export const space = [0, 8, 16, 32, 64];
```And then imported individually in other components:
```javascript
import { space, colors } from "./styles";
```**[⬆ Back to Top](#table-of-contents)**
126. ### What are the popular React-specific linters?
ESLint is a popular JavaScript linter. There are plugins available that analyse specific code styles. One of the most common for React is an npm package called `eslint-plugin-react`. By default, it will check a number of best practices, with rules checking things from keys in iterators to a complete set of prop types.
Another popular plugin is `eslint-plugin-jsx-a11y`, which will help fix common issues with accessibility. As JSX offers slightly different syntax to regular HTML, issues with `alt` text and `tabindex`, for example, will not be picked up by regular plugins.
**[⬆ Back to Top](#table-of-contents)**
127. ### How to make AJAX call and in which component lifecycle methods should I make an AJAX call?
You can use AJAX libraries such as Axios, jQuery AJAX, and the browser built-in `fetch`. You should fetch data in the `componentDidMount()` lifecycle method. This is so you can use `setState()` to update your component when the data is retrieved.
For example, the employees list fetched from API and set local state:
```jsx harmony
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.state = {
employees: [],
error: null,
};
}componentDidMount() {
fetch("https://api.example.com/items")
.then((res) => res.json())
.then(
(result) => {
this.setState({
employees: result.employees,
});
},
(error) => {
this.setState({ error });
}
);
}render() {
const { error, employees } = this.state;
if (error) {
returnError: {error.message};
} else {
return (
-
{employee.name}-{employee.experience}
{employees.map((employee) => (
))}
);
}
}
}
```
**[⬆ Back to Top](#table-of-contents)**
128. ### What are render props?
**Render Props** is a simple technique for sharing code between components using a prop whose value is a function. The below component uses render prop which returns a React element.
```jsx harmony
{`Hello ${data.target}`}
} />```
Libraries such as React Router and DownShift are using this pattern.
## React Router
**[⬆ Back to Top](#table-of-contents)**
129. ### What is React Router?
React Router is a powerful routing library built on top of React that helps you add new screens and flows to your application incredibly quickly, all while keeping the URL in sync with what's being displayed on the page.
**[⬆ Back to Top](#table-of-contents)**
130. ### How React Router is different from history library?
React Router is a wrapper around the `history` library which handles interaction with the browser's `window.history` with its browser and hash histories. It also provides memory history which is useful for environments that don't have global history, such as mobile app development (React Native) and unit testing with Node.
**[⬆ Back to Top](#table-of-contents)**
131. ### What are the `` components of React Router v4?
React Router v4 provides below 3 `` components:
1. ``
2. ``
3. ``
The above components will create _browser_, _hash_, and _memory_ history instances. React Router v4 makes the properties and methods of the `history` instance associated with your router available through the context in the `router` object.
**[⬆ Back to Top](#table-of-contents)**
132. ### What is the purpose of `push()` and `replace()` methods of `history`?
A history instance has two methods for navigation purpose.
1. `push()`
2. `replace()`
If you think of the history as an array of visited locations, `push()` will add a new location to the array and `replace()` will replace the current location in the array with the new one.
**[⬆ Back to Top](#table-of-contents)**
133. ### How do you programmatically navigate using React Router v4?
There are three different ways to achieve programmatic routing/navigation within components.
1. **Using the `withRouter()` higher-order function:**
The `withRouter()` higher-order function will inject the history object as a prop of the component. This object provides `push()` and `replace()` methods to avoid the usage of context.
```jsx harmony
import { withRouter } from "react-router-dom"; // this also works with 'react-router-native'
const Button = withRouter(({ history }) => (
{
history.push("/new-location");
}}
>
{"Click Me!"}
));
```
2. **Using `` component and render props pattern:**
The `` component passes the same props as `withRouter()`, so you will be able to access the history methods through the history prop.
```jsx harmony
import { Route } from "react-router-dom";
const Button = () => (
(
{
history.push("/new-location");
}}
>
{"Click Me!"}
)}
/>
);
```
3. **Using context:**
This option is not recommended and treated as unstable API.
```jsx harmony
const Button = (props, context) => (
{
context.history.push("/new-location");
}}
>
{"Click Me!"}
);
Button.contextTypes = {
history: React.PropTypes.shape({
push: React.PropTypes.func.isRequired,
}),
};
```
**[⬆ Back to Top](#table-of-contents)**
134. ### How to get query parameters in React Router v4?
The ability to parse query strings was taken out of React Router v4 because there have been user requests over the years to support different implementation. So the decision has been given to users to choose the implementation they like. The recommended approach is to use query strings library.
```javascript
const queryString = require("query-string");
const parsed = queryString.parse(props.location.search);
```
You can also use `URLSearchParams` if you want something native:
```javascript
const params = new URLSearchParams(props.location.search);
const foo = params.get("name");
```
You should use a _polyfill_ for IE11.
**[⬆ Back to Top](#table-of-contents)**
135. ### Why you get "Router may have only one child element" warning?
You have to wrap your Route's in a `` block because `` is unique in that it renders a route exclusively.
At first you need to add `Switch` to your imports:
```javascript
import { Switch, Router, Route } from "react-router";
```
Then define the routes within `` block:
```jsx harmony
```
**[⬆ Back to Top](#table-of-contents)**
136. ### How to pass params to `history.push` method in React Router v4?
While navigating you can pass props to the `history` object:
```javascript
this.props.history.push({
pathname: "/template",
search: "?name=sudheer",
state: { detail: response.data },
});
```
The `search` property is used to pass query params in `push()` method.
**[⬆ Back to Top](#table-of-contents)**
137. ### How to implement _default_ or _NotFound_ page?
A `` renders the first child `` that matches. A `` with no path always matches. So you just need to simply drop path attribute as below
```jsx harmony
```
**[⬆ Back to Top](#table-of-contents)**
138. ### How to get history on React Router v4?
Below are the list of steps to get history object on React Router v4,
1. Create a module that exports a `history` object and import this module across the project.
For example, create `history.js` file:
```javascript
import { createBrowserHistory } from "history";
export default createBrowserHistory({
/* pass a configuration object here if needed */
});
```
2. You should use the `` component instead of built-in routers. Import the above `history.js` inside `index.js` file:
```jsx harmony
import { Router } from "react-router-dom";
import history from "./history";
import App from "./App";
ReactDOM.render(
,
holder
);
```
3. You can also use push method of `history` object similar to built-in history object:
```javascript
// some-other-file.js
import history from "./history";
history.push("/go-here");
```
**[⬆ Back to Top](#table-of-contents)**
139. ### How to perform automatic redirect after login?
The `react-router` package provides `` component in React Router. Rendering a `` will navigate to a new location. Like server-side redirects, the new location will override the current location in the history stack.
```javascript
import React, { Component } from "react";
import { Redirect } from "react-router";
export default class LoginComponent extends Component {
render() {
if (this.state.isLoggedIn === true) {
return ;
} else {
return
}
}
}
```
## React Internationalization
**[⬆ Back to Top](#table-of-contents)**
140. ### What is React Intl?
The _React Intl_ library makes internationalization in React straightforward, with off-the-shelf components and an API that can handle everything from formatting strings, dates, and numbers, to pluralization. React Intl is part of _FormatJS_ which provides bindings to React via its components and API.
**[⬆ Back to Top](#table-of-contents)**
141. ### What are the main features of React Intl?
Below are the main features of React Intl,
1. Display numbers with separators.
2. Display dates and times correctly.
3. Display dates relative to "now".
4. Pluralize labels in strings.
5. Support for 150+ languages.
6. Runs in the browser and Node.
7. Built on standards.
**[⬆ Back to Top](#table-of-contents)**
142. ### What are the two ways of formatting in React Intl?
The library provides two ways to format strings, numbers, and dates:
1. **Using react components:**
```jsx harmony
```
2. **Using an API:**
```javascript
const messages = defineMessages({
accountMessage: {
id: "account",
defaultMessage: "The amount is less than minimum balance.",
},
});
formatMessage(messages.accountMessage);
```
**[⬆ Back to Top](#table-of-contents)**
143. ### How to use `` as placeholder using React Intl?
The `` components from `react-intl` return elements, not plain text, so they can't be used for placeholders, alt text, etc. In that case, you should use lower level API `formatMessage()`. You can inject the `intl` object into your component using `injectIntl()` higher-order component and then format the message using `formatMessage()` available on that object.
```jsx harmony
import React from "react";
import { injectIntl, intlShape } from "react-intl";
const MyComponent = ({ intl }) => {
const placeholder = intl.formatMessage({ id: "messageId" });
return ;
};
MyComponent.propTypes = {
intl: intlShape.isRequired,
};
export default injectIntl(MyComponent);
```
**[⬆ Back to Top](#table-of-contents)**
144. ### How to access current locale with React Intl?
You can get the current locale in any component of your application using `injectIntl()`:
```jsx harmony
import { injectIntl, intlShape } from "react-intl";
const MyComponent = ({ intl }) => (
);
MyComponent.propTypes = {
intl: intlShape.isRequired,
};
export default injectIntl(MyComponent);
```
**[⬆ Back to Top](#table-of-contents)**
145. ### How to format date using React Intl?
The `injectIntl()` higher-order component will give you access to the `formatDate()` method via the props in your component. The method is used internally by instances of `FormattedDate` and it returns the string representation of the formatted date.
```jsx harmony
import { injectIntl, intlShape } from "react-intl";
const stringDate = this.props.intl.formatDate(date, {
year: "numeric",
month: "numeric",
day: "numeric",
});
const MyComponent = ({ intl }) => (
);
MyComponent.propTypes = {
intl: intlShape.isRequired,
};
export default injectIntl(MyComponent);
```
## React Testing
**[⬆ Back to Top](#table-of-contents)**
146. ### What is Shallow Renderer in React testing?
_Shallow rendering_ is useful for writing unit test cases in React. It lets you render a component _one level deep_ and assert facts about what its render method returns, without worrying about the behavior of child components, which are not instantiated or rendered.
For example, if you have the following component:
```javascript
function MyComponent() {
return (
{"Title"}
{"Description"}
);
}
```
Then you can assert as follows:
```jsx harmony
import ShallowRenderer from "react-test-renderer/shallow";
// in your test
const renderer = new ShallowRenderer();
renderer.render();
const result = renderer.getRenderOutput();
expect(result.type).toBe("div");
expect(result.props.children).toEqual([
{"Title"},
{"Description"},
]);
```
**[⬆ Back to Top](#table-of-contents)**
147. ### What is `TestRenderer` package in React?
This package provides a renderer that can be used to render components to pure JavaScript objects, without depending on the DOM or a native mobile environment. This package makes it easy to grab a snapshot of the platform view hierarchy (similar to a DOM tree) rendered by a ReactDOM or React Native without using a browser or `jsdom`.
```jsx harmony
import TestRenderer from "react-test-renderer";
const Link = ({ page, children }) => {children};
const testRenderer = TestRenderer.create(
{"Facebook"}
);
console.log(testRenderer.toJSON());
// {
// type: 'a',
// props: { href: 'https://www.facebook.com/' },
// children: [ 'Facebook' ]
// }
```
**[⬆ Back to Top](#table-of-contents)**
148. ### What is the purpose of ReactTestUtils package?
_ReactTestUtils_ are provided in the `with-addons` package and allow you to perform actions against a simulated DOM for the purpose of unit testing.
**[⬆ Back to Top](#table-of-contents)**
149. ### What is Jest?
_Jest_ is a JavaScript unit testing framework created by Facebook based on Jasmine and provides automated mock creation and a `jsdom` environment. It's often used for testing components.
**[⬆ Back to Top](#table-of-contents)**
150. ### What are the advantages of Jest over Jasmine?
There are couple of advantages compared to Jasmine:
- Automatically finds tests to execute in your source code.
- Automatically mocks dependencies when running your tests.
- Allows you to test asynchronous code synchronously.
- Runs your tests with a fake DOM implementation (via `jsdom`) so that your tests can be run on the command line.
- Runs tests in parallel processes so that they finish sooner.
**[⬆ Back to Top](#table-of-contents)**
151. ### Give a simple example of Jest test case
Let's write a test for a function that adds two numbers in `sum.js` file:
```javascript
const sum = (a, b) => a + b;
export default sum;
```
Create a file named `sum.test.js` which contains actual test:
```javascript
import sum from "./sum";
test("adds 1 + 2 to equal 3", () => {
expect(sum(1, 2)).toBe(3);
});
```
And then add the following section to your `package.json`:
```json
{
"scripts": {
"test": "jest"
}
}
```
Finally, run `yarn test` or `npm test` and Jest will print a result:
```console
$ yarn test
PASS ./sum.test.js
✓ adds 1 + 2 to equal 3 (2ms)
```
## React Redux
**[⬆ Back to Top](#table-of-contents)**
152. ### What is flux?
_Flux_ is an _application design paradigm_ used as a replacement for the more traditional MVC pattern. It is not a framework or a library but a new kind of architecture that complements React and the concept of Unidirectional Data Flow. Facebook uses this pattern internally when working with React.
The workflow between dispatcher, stores and views components with distinct inputs and outputs as follows:
**[⬆ Back to Top](#table-of-contents)**
153. ### What is Redux?
_Redux_ is a predictable state container for JavaScript apps based on the _Flux design pattern_. Redux can be used together with React, or with any other view library. It is tiny (about 2kB) and has no dependencies.
**[⬆ Back to Top](#table-of-contents)**
154. ### What are the core principles of Redux?
Redux follows three fundamental principles:
1. **Single source of truth:** The state of your whole application is stored in an object tree within a single store. The single state tree makes it easier to keep track of changes over time and debug or inspect the application.
2. **State is read-only:** The only way to change the state is to emit an action, an object describing what happened. This ensures that neither the views nor the network callbacks will ever write directly to the state.
3. **Changes are made with pure functions:** To specify how the state tree is transformed by actions, you write reducers. Reducers are just pure functions that take the previous state and an action as parameters, and return the next state.
**[⬆ Back to Top](#table-of-contents)**
155. ### What are the downsides of Redux compared to Flux?
Instead of saying downsides we can say that there are few compromises of using Redux over Flux. Those are as follows:
1. **You will need to learn to avoid mutations:** Flux is un-opinionated about mutating data, but Redux doesn't like mutations and many packages complementary to Redux assume you never mutate the state. You can enforce this with dev-only packages like `redux-immutable-state-invariant`, Immutable.js, or instructing your team to write non-mutating code.
2. **You're going to have to carefully pick your packages:** While Flux explicitly doesn't try to solve problems such as undo/redo, persistence, or forms, Redux has extension points such as middleware and store enhancers, and it has spawned a rich ecosystem.
3. **There is no nice Flow integration yet:** Flux currently lets you do very impressive static type checks which Redux doesn't support yet.
**[⬆ Back to Top](#table-of-contents)**
156. ### What is the difference between `mapStateToProps()` and `mapDispatchToProps()`?
`mapStateToProps()` is a utility which helps your component get updated state (which is updated by some other components):
```javascript
const mapStateToProps = (state) => {
return {
todos: getVisibleTodos(state.todos, state.visibilityFilter),
};
};
```
`mapDispatchToProps()` is a utility which will help your component to fire an action event (dispatching action which may cause change of application state):
```javascript
const mapDispatchToProps = (dispatch) => {
return {
onTodoClick: (id) => {
dispatch(toggleTodo(id));
},
};
};
```
It is recommended to always use the “object shorthand” form for the `mapDispatchToProps`.
Redux wraps it in another function that looks like (…args) => dispatch(onTodoClick(…args)), and pass that wrapper function as a prop to your component.
```javascript
const mapDispatchToProps = {
onTodoClick,
};
```
**[⬆ Back to Top](#table-of-contents)**
157. ### Can I dispatch an action in reducer?
Dispatching an action within a reducer is an **anti-pattern**. Your reducer should be _without side effects_, simply digesting the action payload and returning a new state object. Adding listeners and dispatching actions within the reducer can lead to chained actions and other side effects.
**[⬆ Back to Top](#table-of-contents)**
158. ### How to access Redux store outside a component?
You just need to export the store from the module where it created with `createStore()`. Also, it shouldn't pollute the global window object.
```javascript
store = createStore(myReducer);
export default store;
```
**[⬆ Back to Top](#table-of-contents)**
159. ### What are the drawbacks of MVW pattern?
1. DOM manipulation is very expensive which causes applications to behave slow and inefficient.
2. Due to circular dependencies, a complicated model was created around models and views.
3. Lot of data changes happens for collaborative applications(like Google Docs).
4. No way to do undo (travel back in time) easily without adding so much extra code.
**[⬆ Back to Top](#table-of-contents)**
160. ### Are there any similarities between Redux and RxJS?
These libraries are very different for very different purposes, but there are some vague similarities.
Redux is a tool for managing state throughout the application. It is usually used as an architecture for UIs. Think of it as an alternative to (half of) Angular. RxJS is a reactive programming library. It is usually used as a tool to accomplish asynchronous tasks in JavaScript. Think of it as an alternative to Promises. Redux uses the Reactive paradigm because the Store is reactive. The Store observes actions from a distance, and changes itself. RxJS also uses the Reactive paradigm, but instead of being an architecture, it gives you basic building blocks, Observables, to accomplish this pattern.
**[⬆ Back to Top](#table-of-contents)**
161. ### How to dispatch an action on load?
You can dispatch an action in `componentDidMount()` method and in `render()` method you can verify the data.
```javascript
class App extends Component {
componentDidMount() {
this.props.fetchData();
}
render() {
return this.props.isLoaded ? (
) : (
);
}
}
const mapStateToProps = (state) => ({
isLoaded: state.isLoaded,
});
const mapDispatchToProps = { fetchData };
export default connect(mapStateToProps, mapDispatchToProps)(App);
```
**[⬆ Back to Top](#table-of-contents)**
162. ### How to use `connect()` from React Redux?
You need to follow two steps to use your store in your container:
1. **Use `mapStateToProps()`:** It maps the state variables from your store to the props that you specify.
2. **Connect the above props to your container:** The object returned by the `mapStateToProps` function is connected to the container. You can import `connect()` from `react-redux`.
```jsx harmony
import React from "react";
import { connect } from "react-redux";
class App extends React.Component {
render() {
return
}
}
function mapStateToProps(state) {
return { containerData: state.data };
}
export default connect(mapStateToProps)(App);
```
**[⬆ Back to Top](#table-of-contents)**
163. ### How to reset state in Redux?
You need to write a _root reducer_ in your application which delegate handling the action to the reducer generated by `combineReducers()`.
For example, let us take `rootReducer()` to return the initial state after `USER_LOGOUT` action. As we know, reducers are supposed to return the initial state when they are called with `undefined` as the first argument, no matter the action.
```javascript
const appReducer = combineReducers({
/* your app's top-level reducers */
});
const rootReducer = (state, action) => {
if (action.type === "USER_LOGOUT") {
state = undefined;
}
return appReducer(state, action);
};
```
In case of using `redux-persist`, you may also need to clean your storage. `redux-persist` keeps a copy of your state in a storage engine. First, you need to import the appropriate storage engine and then, to parse the state before setting it to undefined and clean each storage state key.
```javascript
const appReducer = combineReducers({
/* your app's top-level reducers */
});
const rootReducer = (state, action) => {
if (action.type === "USER_LOGOUT") {
Object.keys(state).forEach((key) => {
storage.removeItem(`persist:${key}`);
});
state = undefined;
}
return appReducer(state, action);
};
```
**[⬆ Back to Top](#table-of-contents)**
164. ### Whats the purpose of `at` symbol in the Redux connect decorator?
The **@** symbol is in fact a JavaScript expression used to signify decorators. _Decorators_ make it possible to annotate and modify classes and properties at design time.
Let's take an example setting up Redux without and with a decorator.
- **Without decorator:**
```javascript
import React from "react";
import * as actionCreators from "./actionCreators";
import { bindActionCreators } from "redux";
import { connect } from "react-redux";
function mapStateToProps(state) {
return { todos: state.todos };
}
function mapDispatchToProps(dispatch) {
return { actions: bindActionCreators(actionCreators, dispatch) };
}
class MyApp extends React.Component {
// ...define your main app here
}
export default connect(mapStateToProps, mapDispatchToProps)(MyApp);
```
- **With decorator:**
```javascript
import React from "react";
import * as actionCreators from "./actionCreators";
import { bindActionCreators } from "redux";
import { connect } from "react-redux";
function mapStateToProps(state) {
return { todos: state.todos };
}
function mapDispatchToProps(dispatch) {
return { actions: bindActionCreators(actionCreators, dispatch) };
}
@connect(mapStateToProps, mapDispatchToProps)
export default class MyApp extends React.Component {
// ...define your main app here
}
```
The above examples are almost similar except the usage of decorator. The decorator syntax isn't built into any JavaScript runtimes yet, and is still experimental and subject to change. You can use babel for the decorators support.
**[⬆ Back to Top](#table-of-contents)**
165. ### What is the difference between React context and React Redux?
You can use **Context** in your application directly and is going to be great for passing down data to deeply nested components which what it was designed for.
Whereas **Redux** is much more powerful and provides a large number of features that the Context API doesn't provide. Also, React Redux uses context internally but it doesn't expose this fact in the public API.
**[⬆ Back to Top](#table-of-contents)**
166. ### Why are Redux state functions called reducers?
Reducers always return the accumulation of the state (based on all previous and current actions). Therefore, they act as a reducer of state. Each time a Redux reducer is called, the state and action are passed as parameters. This state is then reduced (or accumulated) based on the action, and then the next state is returned. You could _reduce_ a collection of actions and an initial state (of the store) on which to perform these actions to get the resulting final state.
**[⬆ Back to Top](#table-of-contents)**
167. ### How to make AJAX request in Redux?
You can use `redux-thunk` middleware which allows you to define async actions.
Let's take an example of fetching specific account as an AJAX call using _fetch API_:
```javascript
export function fetchAccount(id) {
return (dispatch) => {
dispatch(setLoadingAccountState()); // Show a loading spinner
fetch(`/account/${id}`, (response) => {
dispatch(doneFetchingAccount()); // Hide loading spinner
if (response.status === 200) {
dispatch(setAccount(response.json)); // Use a normal function to set the received state
} else {
dispatch(someError);
}
});
};
}
function setAccount(data) {
return { type: "SET_Account", data: data };
}
```
**[⬆ Back to Top](#table-of-contents)**
168. ### Should I keep all component's state in Redux store?
Keep your data in the Redux store, and the UI related state internally in the component.
**[⬆ Back to Top](#table-of-contents)**
169. ### What is the proper way to access Redux store?
The best way to access your store in a component is to use the `connect()` function, that creates a new component that wraps around your existing one. This pattern is called _Higher-Order Components_, and is generally the preferred way of extending a component's functionality in React. This allows you to map state and action creators to your component, and have them passed in automatically as your store updates.
Let's take an example of `` component using connect:
```javascript
import { connect } from "react-redux";
import { setVisibilityFilter } from "../actions";
import Link from "../components/Link";
const mapStateToProps = (state, ownProps) => ({
active: ownProps.filter === state.visibilityFilter,
});
const mapDispatchToProps = (dispatch, ownProps) => ({
onClick: () => dispatch(setVisibilityFilter(ownProps.filter)),
});
const FilterLink = connect(mapStateToProps, mapDispatchToProps)(Link);
export default FilterLink;
```
Due to it having quite a few performance optimizations and generally being less likely to cause bugs, the Redux developers almost always recommend using `connect()` over accessing the store directly (using context API).
```javascript
class MyComponent {
someMethod() {
doSomethingWith(this.context.store);
}
}
```
**[⬆ Back to Top](#table-of-contents)**
170. ### What is the difference between component and container in React Redux?
**Component** is a class or function component that describes the presentational part of your application.
**Container** is an informal term for a component that is connected to a Redux store. Containers _subscribe_ to Redux state updates and _dispatch_ actions, and they usually don't render DOM elements; they delegate rendering to presentational child components.
**[⬆ Back to Top](#table-of-contents)**
171. ### What is the purpose of the constants in Redux?
Constants allows you to easily find all usages of that specific functionality across the project when you use an IDE. It also prevents you from introducing silly bugs caused by typos – in which case, you will get a `ReferenceError` immediately.
Normally we will save them in a single file (`constants.js` or `actionTypes.js`).
```javascript
export const ADD_TODO = "ADD_TODO";
export const DELETE_TODO = "DELETE_TODO";
export const EDIT_TODO = "EDIT_TODO";
export const COMPLETE_TODO = "COMPLETE_TODO";
export const COMPLETE_ALL = "COMPLETE_ALL";
export const CLEAR_COMPLETED = "CLEAR_COMPLETED";
```
In Redux, you use them in two places:
1. **During action creation:**
Let's take `actions.js`:
```javascript
import { ADD_TODO } from "./actionTypes";
export function addTodo(text) {
return { type: ADD_TODO, text };
}
```
2. **In reducers:**
Let's create `reducer.js`:
```javascript
import { ADD_TODO } from "./actionTypes";
export default (state = [], action) => {
switch (action.type) {
case ADD_TODO:
return [
...state,
{
text: action.text,
completed: false,
},
];
default:
return state;
}
};
```
**[⬆ Back to Top](#table-of-contents)**
172. ### What are the different ways to write `mapDispatchToProps()`?
There are a few ways of binding _action creators_ to `dispatch()` in `mapDispatchToProps()`.
Below are the possible options:
```javascript
const mapDispatchToProps = (dispatch) => ({
action: () => dispatch(action()),
});
```
```javascript
const mapDispatchToProps = (dispatch) => ({
action: bindActionCreators(action, dispatch),
});
```
```javascript
const mapDispatchToProps = { action };
```
The third option is just a shorthand for the first one.
**[⬆ Back to Top](#table-of-contents)**
173. ### What is the use of the `ownProps` parameter in `mapStateToProps()` and `mapDispatchToProps()`?
If the `ownProps` parameter is specified, React Redux will pass the props that were passed to the component into your _connect_ functions. So, if you use a connected component:
```jsx harmony
import ConnectedComponent from "./containers/ConnectedComponent";
;
```
The `ownProps` inside your `mapStateToProps()` and `mapDispatchToProps()` functions will be an object:
```javascript
{
user: "john";
}
```
You can use this object to decide what to return from those functions.
**[⬆ Back to Top](#table-of-contents)**
174. ### How to structure Redux top level directories?
Most of the applications has several top-level directories as below:
1. **Components**: Used for _dumb_ components unaware of Redux.
2. **Containers**: Used for _smart_ components connected to Redux.
3. **Actions**: Used for all action creators, where file names correspond to part of the app.
4. **Reducers**: Used for all reducers, where files name correspond to state key.
5. **Store**: Used for store initialization.
This structure works well for small and medium size apps.
**[⬆ Back to Top](#table-of-contents)**
175. ### What is redux-saga?
`redux-saga` is a library that aims to make side effects (asynchronous things like data fetching and impure things like accessing the browser cache) in React/Redux applications easier and better.
It is available in NPM:
```console
$ npm install --save redux-saga
```
**[⬆ Back to Top](#table-of-contents)**
176. ### What is the mental model of redux-saga?
_Saga_ is like a separate thread in your application, that's solely responsible for side effects. `redux-saga` is a redux _middleware_, which means this thread can be started, paused and cancelled from the main application with normal Redux actions, it has access to the full Redux application state and it can dispatch Redux actions as well.
**[⬆ Back to Top](#table-of-contents)**
177. ### What are the differences between `call()` and `put()` in redux-saga?
Both `call()` and `put()` are effect creator functions. `call()` function is used to create effect description, which instructs middleware to call the promise. `put()` function creates an effect, which instructs middleware to dispatch an action to the store.
Let's take example of how these effects work for fetching particular user data.
```javascript
function* fetchUserSaga(action) {
// `call` function accepts rest arguments, which will be passed to `api.fetchUser` function.
// Instructing middleware to call promise, it resolved value will be assigned to `userData` variable
const userData = yield call(api.fetchUser, action.userId);
// Instructing middleware to dispatch corresponding action.
yield put({
type: "FETCH_USER_SUCCESS",
userData,
});
}
```
**[⬆ Back to Top](#table-of-contents)**
178. ### What is Redux Thunk?
_Redux Thunk_ middleware allows you to write action creators that return a function instead of an action. The thunk can be used to delay the dispatch of an action, or to dispatch only if a certain condition is met. The inner function receives the store methods `dispatch()` and `getState()` as parameters.
**[⬆ Back to Top](#table-of-contents)**
179. ### What are the differences between `redux-saga` and `redux-thunk`?
Both _Redux Thunk_ and _Redux Saga_ take care of dealing with side effects. In most of the scenarios, Thunk uses _Promises_ to deal with them, whereas Saga uses _Generators_. Thunk is simple to use and Promises are familiar to many developers, Sagas/Generators are more powerful but you will need to learn them. But both middleware can coexist, so you can start with Thunks and introduce Sagas when/if you need them.
**[⬆ Back to Top](#table-of-contents)**
180. ### What is Redux DevTools?
_Redux DevTools_ is a live-editing time travel environment for Redux with hot reloading, action replay, and customizable UI. If you don't want to bother with installing Redux DevTools and integrating it into your project, consider using Redux DevTools Extension for Chrome and Firefox.
**[⬆ Back to Top](#table-of-contents)**
181. ### What are the features of Redux DevTools?
Some of the main features of Redux DevTools are below,
1. Lets you inspect every state and action payload.
2. Lets you go back in time by _cancelling_ actions.
3. If you change the reducer code, each _staged_ action will be re-evaluated.
4. If the reducers throw, you will see during which action this happened, and what the error was.
5. With `persistState()` store enhancer, you can persist debug sessions across page reloads.
**[⬆ Back to Top](#table-of-contents)**
182. ### What are Redux selectors and why to use them?
_Selectors_ are functions that take Redux state as an argument and return some data to pass to the component.
For example, to get user details from the state:
```javascript
const getUserData = (state) => state.user.data;
```
These selectors have two main benefits,
1. The selector can compute derived data, allowing Redux to store the minimal possible state
2. The selector is not recomputed unless one of its arguments changes
**[⬆ Back to Top](#table-of-contents)**
183. ### What is Redux Form?
_Redux Form_ works with React and Redux to enable a form in React to use Redux to store all of its state. Redux Form can be used with raw HTML5 inputs, but it also works very well with common UI frameworks like Material UI, React Widgets and React Bootstrap.
**[⬆ Back to Top](#table-of-contents)**
184. ### What are the main features of Redux Form?
Some of the main features of Redux Form are:
1. Field values persistence via Redux store.
2. Validation (sync/async) and submission.
3. Formatting, parsing and normalization of field values.
**[⬆ Back to Top](#table-of-contents)**
185. ### How to add multiple middlewares to Redux?
You can use `applyMiddleware()`.
For example, you can add `redux-thunk` and `logger` passing them as arguments to `applyMiddleware()`:
```javascript
import { createStore, applyMiddleware } from "redux";
const createStoreWithMiddleware = applyMiddleware(
ReduxThunk,
logger
)(createStore);
```
**[⬆ Back to Top](#table-of-contents)**
186. ### How to set initial state in Redux?
You need to pass initial state as second argument to createStore:
```javascript
const rootReducer = combineReducers({
todos: todos,
visibilityFilter: visibilityFilter,
});
const initialState = {
todos: [{ id: 123, name: "example", completed: false }],
};
const store = createStore(rootReducer, initialState);
```
**[⬆ Back to Top](#table-of-contents)**
187. ### How Relay is different from Redux?
Relay is similar to Redux in that they both use a single store. The main difference is that relay only manages state originated from the server, and all access to the state is used via _GraphQL_ queries (for reading data) and mutations (for changing data). Relay caches the data for you and optimizes data fetching for you, by fetching only changed data and nothing more.
188. ### What is an action in Redux?
_Actions_ are plain JavaScript objects or payloads of information that send data from your application to your store. They are the only source of information for the store. Actions must have a type property that indicates the type of action being performed.
For example, let's take an action which represents adding a new todo item:
```
{
type: ADD_TODO,
text: 'Add todo item'
}
```
**[⬆ Back to Top](#table-of-contents)**
## React Native
**[⬆ Back to Top](#table-of-contents)**
188. ### What is the difference between React Native and React?
**React** is a JavaScript library, supporting both front end web and being run on the server, for building user interfaces and web applications.
**React Native** is a mobile framework that compiles to native app components, allowing you to build native mobile applications (iOS, Android, and Windows) in JavaScript that allows you to use React to build your components, and implements React under the hood.
**[⬆ Back to Top](#table-of-contents)**
189. ### How to test React Native apps?
React Native can be tested only in mobile simulators like iOS and Android. You can run the app in your mobile using expo app (https://expo.io) Where it syncs using QR code, your mobile and computer should be in same wireless network.
**[⬆ Back to Top](#table-of-contents)**
190. ### How to do logging in React Native?
You can use `console.log`, `console.warn`, etc. As of React Native v0.29 you can simply run the following to see logs in the console:
```
$ react-native log-ios
$ react-native log-android
```
**[⬆ Back to Top](#table-of-contents)**
191. ### How to debug your React Native?
Follow the below steps to debug React Native app:
1. Run your application in the iOS simulator.
2. Press `Command + D` and a webpage should open up at `http://localhost:8081/debugger-ui`.
3. Enable _Pause On Caught Exceptions_ for a better debugging experience.
4. Press `Command + Option + I` to open the Chrome Developer tools, or open it via `View` -> `Developer` -> `Developer Tools`.
5. You should now be able to debug as you normally would.
## React supported libraries & Integration
**[⬆ Back to Top](#table-of-contents)**
192. ### What is reselect and how it works?
_Reselect_ is a **selector library** (for Redux) which uses _memoization_ concept. It was originally written to compute derived data from Redux-like applications state, but it can't be tied to any architecture or library.
Reselect keeps a copy of the last inputs/outputs of the last call, and recomputes the result only if one of the inputs changes. If the the same inputs are provided twice in a row, Reselect returns the cached output. It's memoization and cache are fully customizable.
**[⬆ Back to Top](#table-of-contents)**
193. ### What is Flow?
_Flow_ is a _static type checker_ designed to find type errors in JavaScript. Flow types can express much more fine-grained distinctions than traditional type systems. For example, Flow helps you catch errors involving `null`, unlike most type systems.
**[⬆ Back to Top](#table-of-contents)**
194. ### What is the difference between Flow and PropTypes?
Flow is a _static analysis tool_ (static checker) which uses a superset of the language, allowing you to add type annotations to all of your code and catch an entire class of bugs at compile time.
PropTypes is a _basic type checker_ (runtime checker) which has been patched onto React. It can't check anything other than the types of the props being passed to a given component. If you want more flexible typechecking for your entire project Flow/TypeScript are appropriate choices.
**[⬆ Back to Top](#table-of-contents)**
195. ### How to use Font Awesome icons in React?
The below steps followed to include Font Awesome in React:
1. Install `font-awesome`:
```console
$ npm install --save font-awesome
```
2. Import `font-awesome` in your `index.js` file:
```javascript
import "font-awesome/css/font-awesome.min.css";
```
3. Add Font Awesome classes in `className`:
```javascript
render() {
return
}
```
**[⬆ Back to Top](#table-of-contents)**
196. ### What is React Dev Tools?
_React Developer Tools_ let you inspect the component hierarchy, including component props and state. It exists both as a browser extension (for Chrome and Firefox), and as a standalone app (works with other environments including Safari, IE, and React Native).
The official extensions available for different browsers or environments.
1. **Chrome extension**
2. **Firefox extension**
3. **Standalone app** (Safari, React Native, etc)
**[⬆ Back to Top](#table-of-contents)**
197. ### Why is DevTools not loading in Chrome for local files?
If you opened a local HTML file in your browser (`file://...`) then you must first open _Chrome Extensions_ and check `Allow access to file URLs`.
**[⬆ Back to Top](#table-of-contents)**
198. ### How to use Polymer in React?
You need to follow below steps to use Polymer in React,
1. Create a Polymer element:
```jsx harmony
;
Polymer({
is: "calendar-element",
ready: function () {
this.textContent = "I am a calendar";
},
});
```
2. Create the Polymer component HTML tag by importing it in a HTML document, e.g. import it in the `index.html` of your React application:
```html
```
3. Use that element in the JSX file:
```javascript
import React from "react";
class MyComponent extends React.Component {
render() {
return ;
}
}
export default MyComponent;
```
**[⬆ Back to Top](#table-of-contents)**
199. ### What are the advantages of React over Vue.js?
React has the following advantages over Vue.js:
1. Gives more flexibility in large apps developing.
2. Easier to test.
3. Suitable for mobile apps creating.
4. More information and solutions available.
**Note:** The above list of advantages are purely opinionated and it vary based on the professional experience. But they are helpful as base parameters.
**[⬆ Back to Top](#table-of-contents)**
200. ### What is the difference between React and Angular?
Let's see the difference between React and Angular in a table format.
| React | Angular |
| ------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------- |
| React is a library and has only the View layer | Angular is a framework and has complete MVC functionality |
| React handles rendering on the server side | AngularJS renders only on the client side but Angular 2 and above renders on the server side |
| React uses JSX that looks like HTML in JS which can be confusing | Angular follows the template approach for HTML, which makes code shorter and easy to understand |
| React Native, which is a React type to build mobile applications are faster and more stable | Ionic, Angular's mobile native app is relatively less stable and slower |
| In React, data flows only in one way and hence debugging is easy | In Angular, data flows both way i.e it has two-way data binding between children and parent and hence debugging is often difficult |
**Note:** The above list of differences are purely opinionated and it vary based on the professional experience. But they are helpful as base parameters.
**[⬆ Back to Top](#table-of-contents)**
201. ### Why React tab is not showing up in DevTools?
When the page loads, _React DevTools_ sets a global named `__REACT_DEVTOOLS_GLOBAL_HOOK__`, then React communicates with that hook during initialization. If the website is not using React or if React fails to communicate with DevTools then it won't show up the tab.
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202. ### What are Styled Components?
`styled-components` is a JavaScript library for styling React applications. It removes the mapping between styles and components, and lets you write actual CSS augmented with JavaScript.
**[⬆ Back to Top](#table-of-contents)**
203. ### Give an example of Styled Components?
Lets create `` and `` components with specific styles for each.
```javascript
import React from "react";
import styled from "styled-components";
// Create a component that renders an
which is centered, red and sized at 1.5em
const Title = styled.h1`
font-size: 1.5em;
text-align: center;
color: palevioletred;
`;
// Create a component that renders a with some padding and a papayawhip background
const Wrapper = styled.section`
padding: 4em;
background: papayawhip;
`;
```
These two variables, `Title` and `Wrapper`, are now components that you can render just like any other react component.
```jsx harmony
{"Lets start first styled component!"}
```
**[⬆ Back to Top](#table-of-contents)**
204. ### What is Relay?
Relay is a JavaScript framework for providing a data layer and client-server communication to web applications using the React view layer.
**[⬆ Back to Top](#table-of-contents)**
205. ### How to use TypeScript in `create-react-app` application?
Starting from [email protected] or higher, there is a built-in support for typescript. i.e, `create-react-app` now supports typescript natively. You can just pass `--typescript` option as below
```bash
npx create-react-app my-app --typescript
# or
yarn create react-app my-app --typescript
```
But for lower versions of react scripts, just supply `--scripts-version` option as `react-scripts-ts` while you create a new project. `react-scripts-ts` is a set of adjustments to take the standard `create-react-app` project pipeline and bring TypeScript into the mix.
Now the project layout should look like the following:
```
my-app/
├─ .gitignore
├─ images.d.ts
├─ node_modules/
├─ public/
├─ src/
│ └─ ...
├─ package.json
├─ tsconfig.json
├─ tsconfig.prod.json
├─ tsconfig.test.json
└─ tslint.json
```
## Miscellaneous
**[⬆ Back to Top](#table-of-contents)**
206. ### What are the main features of Reselect library?
Let's see the main features of Reselect library,
1. Selectors can compute derived data, allowing Redux to store the minimal possible state.
2. Selectors are efficient. A selector is not recomputed unless one of its arguments changes.
3. Selectors are composable. They can be used as input to other selectors.
207. #### Give an example of Reselect usage?
Let's take calculations and different amounts of a shipment order with the simplified usage of Reselect:
```javascript
import { createSelector } from "reselect";
const shopItemsSelector = (state) => state.shop.items;
const taxPercentSelector = (state) => state.shop.taxPercent;
const subtotalSelector = createSelector(shopItemsSelector, (items) =>
items.reduce((acc, item) => acc + item.value, 0)
);
const taxSelector = createSelector(
subtotalSelector,
taxPercentSelector,
(subtotal, taxPercent) => subtotal * (taxPercent / 100)
);
export const totalSelector = createSelector(
subtotalSelector,
taxSelector,
(subtotal, tax) => ({ total: subtotal + tax })
);
let exampleState = {
shop: {
taxPercent: 8,
items: [
{ name: "apple", value: 1.2 },
{ name: "orange", value: 0.95 },
],
},
};
console.log(subtotalSelector(exampleState)); // 2.15
console.log(taxSelector(exampleState)); // 0.172
console.log(totalSelector(exampleState)); // { total: 2.322 }
```
**[⬆ Back to Top](#table-of-contents)**
209. ### Does the statics object work with ES6 classes in React?
No, `statics` only works with `React.createClass()`:
```javascript
someComponent = React.createClass({
statics: {
someMethod: function () {
// ..
},
},
});
```
But you can write statics inside ES6+ classes as below,
```javascript
class Component extends React.Component {
static propTypes = {
// ...
};
static someMethod() {
// ...
}
}
```
or writing them outside class as below,
```javascript
class Component extends React.Component {
....
}
Component.propTypes = {...}
Component.someMethod = function(){....}
```
**[⬆ Back to Top](#table-of-contents)**
210. ### Can Redux only be used with React?
Redux can be used as a data store for any UI layer. The most common usage is with React and React Native, but there are bindings available for Angular, Angular 2, Vue, Mithril, and more. Redux simply provides a subscription mechanism which can be used by any other code.
**[⬆ Back to Top](#table-of-contents)**
211. ### Do you need to have a particular build tool to use Redux?
Redux is originally written in ES6 and transpiled for production into ES5 with Webpack and Babel. You should be able to use it regardless of your JavaScript build process. Redux also offers a UMD build that can be used directly without any build process at all.
**[⬆ Back to Top](#table-of-contents)**
212. ### How Redux Form `initialValues` get updated from state?
You need to add `enableReinitialize : true` setting.
```javascript
const InitializeFromStateForm = reduxForm({
form: "initializeFromState",
enableReinitialize: true,
})(UserEdit);
```
If your `initialValues` prop gets updated, your form will update too.
**[⬆ Back to Top](#table-of-contents)**
213. ### How React PropTypes allow different types for one prop?
You can use `oneOfType()` method of `PropTypes`.
For example, the height property can be defined with either `string` or `number` type as below:
```javascript
Component.propTypes = {
size: PropTypes.oneOfType([PropTypes.string, PropTypes.number]),
};
```
**[⬆ Back to Top](#table-of-contents)**
214. ### Can I import an SVG file as react component?
You can import SVG directly as component instead of loading it as a file. This feature is available with `[email protected]` and higher.
```jsx harmony
import { ReactComponent as Logo } from "./logo.svg";
const App = () => (
{/* Logo is an actual react component */}
);
```
**Note**: Don't forget about the curly braces in the import.
**[⬆ Back to Top](#table-of-contents)**
215. ### Why are inline ref callbacks or functions not recommended?
If the ref callback is defined as an inline function, it will get called twice during updates, first with null and then again with the DOM element. This is because a new instance of the function is created with each render, so React needs to clear the old ref and set up the new one.
```jsx
class UserForm extends Component {
handleSubmit = () => {
console.log("Input Value is: ", this.input.value);
};
render() {
return (
(this.input = input)} /> //
Access DOM input in handle submit
Submit
);
}
}
```
But our expectation is for the ref callback to get called once, when the component mounts. One quick fix is to use the ES7 class property syntax to define the function
```jsx
class UserForm extends Component {
handleSubmit = () => {
console.log("Input Value is: ", this.input.value);
};
setSearchInput = (input) => {
this.input = input;
};
render() {
return (
// Access DOM input
in handle submit
Submit
);
}
}
```
**Note:** In React v16.3,
**[⬆ Back to Top](#table-of-contents)**
216. ### What is render hijacking in react?
The concept of render hijacking is the ability to control what a component will output from another component. It means that you decorate your component by wrapping it into a Higher-Order component. By wrapping, you can inject additional props or make other changes, which can cause changing logic of rendering. It does not actually enable hijacking, but by using HOC you make your component behave differently.
**[⬆ Back to Top](#table-of-contents)**
217. ### What are HOC factory implementations?
There are two main ways of implementing HOCs in React.
1. Props Proxy (PP) and
2. Inheritance Inversion (II).
But they follow different approaches for manipulating the _WrappedComponent_.
**Props Proxy**
In this approach, the render method of the HOC returns a React Element of the type of the WrappedComponent. We also pass through the props that the HOC receives, hence the name **Props Proxy**.
```jsx
function ppHOC(WrappedComponent) {
return class PP extends React.Component {
render() {
return ;
}
};
}
```
**Inheritance Inversion**
In this approach, the returned HOC class (Enhancer) extends the WrappedComponent. It is called Inheritance Inversion because instead of the WrappedComponent extending some Enhancer class, it is passively extended by the Enhancer. In this way the relationship between them seems **inverse**.
```jsx
function iiHOC(WrappedComponent) {
return class Enhancer extends WrappedComponent {
render() {
return super.render();
}
};
}
```
**[⬆ Back to Top](#table-of-contents)**
218. ### How to pass numbers to React component?
You should be passing the numbers via curly braces({}) where as strings in quotes
```jsx
React.render(
,
document.getElementById("container")
);
```
**[⬆ Back to Top](#table-of-contents)**
219. ### Do I need to keep all my state into Redux? Should I ever use react internal state?
It is up to the developer's decision, i.e., it is developer's job to determine what kinds of state make up your application, and where each piece of state should live. Some users prefer to keep every single piece of data in Redux, to maintain a fully serializable and controlled version of their application at all times. Others prefer to keep non-critical or UI state, such as “is this dropdown currently open”, inside a component's internal state.
Below are the thumb rules to determine what kind of data should be put into Redux
1. Do other parts of the application care about this data?
2. Do you need to be able to create further derived data based on this original data?
3. Is the same data being used to drive multiple components?
4. Is there value to you in being able to restore this state to a given point in time (ie, time travel debugging)?
5. Do you want to cache the data (i.e, use what's in state if it's already there instead of re-requesting it)?
**[⬆ Back to Top](#table-of-contents)**
220. ### What is the purpose of registerServiceWorker in React?
React creates a service worker for you without any configuration by default. The service worker is a web API that helps you cache your assets and other files so that when the user is offline or on a slow network, he/she can still see results on the screen, as such, it helps you build a better user experience, that's what you should know about service worker for now. It's all about adding offline capabilities to your site.
```jsx
import React from "react";
import ReactDOM from "react-dom";
import App from "./App";
import registerServiceWorker from "./registerServiceWorker";
ReactDOM.render(, document.getElementById("root"));
registerServiceWorker();
```
**[⬆ Back to Top](#table-of-contents)**
221. ### What is React memo function?
Class components can be restricted from re-rendering when their input props are the same using **PureComponent or shouldComponentUpdate**. Now you can do the same with function components by wrapping them in **React.memo**.
```jsx
const MyComponent = React.memo(function MyComponent(props) {
/* only rerenders if props change */
});
```
**[⬆ Back to Top](#table-of-contents)**
222. ### What is React lazy function?
The `React.lazy` function lets you render a dynamic import as a regular component. It will automatically load the bundle containing the `OtherComponent` when the component gets rendered. This must return a Promise which resolves to a module with a default export containing a React component.
```jsx
const OtherComponent = React.lazy(() => import("./OtherComponent"));
function MyComponent() {
return (
);
}
```
**Note:**
`React.lazy` and `Suspense` is not yet available for server-side rendering. If you want to do code-splitting in a server rendered app, we still recommend React Loadable.
**[⬆ Back to Top](#table-of-contents)**
223. ### How to prevent unnecessary updates using setState?
You can compare the current value of the state with an existing state value and decide whether to rerender the page or not. If the values are the same then you need to return **null** to stop re-rendering otherwise return the latest state value.
For example, the user profile information is conditionally rendered as follows,
```jsx
getUserProfile = (user) => {
const latestAddress = user.address;
this.setState((state) => {
if (state.address === latestAddress) {
return null;
} else {
return { title: latestAddress };
}
});
};
```
**[⬆ Back to Top](#table-of-contents)**
224. ### How do you render Array, Strings and Numbers in React 16 Version?
**Arrays**: Unlike older releases, you don't need to make sure **render** method return a single element in React16. You are able to return multiple sibling elements without a wrapping element by returning an array.
For example, let us take the below list of developers,
```jsx
const ReactJSDevs = () => {
return [
];
};
```
You can also merge this array of items in another array component.
```jsx
const JSDevs = () => {
return (
- Brad
- Brodge
- Brandon
);
};
```
**Strings and Numbers:** You can also return string and number type from the render method.
```jsx
render() {
return 'Welcome to ReactJS questions';
}
// Number
render() {
return 2018;
}
```
**[⬆ Back to Top](#table-of-contents)**
225. ### How to use class field declarations syntax in React classes?
React Class Components can be made much more concise using the class field declarations. You can initialize the local state without using the constructor and declare class methods by using arrow functions without the extra need to bind them.
Let's take a counter example to demonstrate class field declarations for state without using constructor and methods without binding,
```jsx
class Counter extends Component {
state = { value: 0 };
handleIncrement = () => {
this.setState((prevState) => ({
value: prevState.value + 1,
}));
};
handleDecrement = () => {
this.setState((prevState) => ({
value: prevState.value - 1,
}));
};
render() {
return (
{this.state.value}
+
-
);
}
}
```
**[⬆ Back to Top](#table-of-contents)**
226. ### What are hooks?
Hooks is a special JavaScript function that allows you use state and other React features without writing a class. This pattern has been introduced as a new feature in React 16.8 and helped to isolate the stateful logic from the components.
Let's see an example of useState hook:
```jsx
import { useState } from "react";
function Example() {
// Declare a new state variable, which we'll call "count"
const [count, setCount] = useState(0);
return (
<>
You clicked {count} times
setCount(count + 1)}>Click me
>
);
}
```
**Note:** Hooks can be used inside an existing function component without rewriting the component.
**[⬆ Back to Top](#table-of-contents)**
227. ### What rules need to be followed for hooks?
You need to follow two rules in order to use hooks,
1. **Call Hooks only at the top level of your react functions:** You shouldn’t call Hooks inside loops, conditions, or nested functions. This will ensure that Hooks are called in the same order each time a component renders and it preserves the state of Hooks between multiple useState and useEffect calls.
2. **Call Hooks from React Functions only:** You shouldn’t call Hooks from regular JavaScript functions. Instead, you should call them from either function components or custom hooks.
The eslint plugin named **eslint-plugin-react-hooks** can be used to enforce these two rules.
**[⬆ Back to Top](#table-of-contents)**
228. ### How to ensure hooks followed the rules in your project?
React team released an ESLint plugin called **eslint-plugin-react-hooks** that enforces these two rules. You can add this plugin to your project using the below command,
```javascript
npm install eslint-plugin-react-hooks@next
```
And apply the below config in your ESLint config file,
```javascript
// Your ESLint configuration
{
"plugins": [
// ...
"react-hooks"
],
"rules": {
// ...
"react-hooks/rules-of-hooks": "error"
}
}
```
For example, the linter enforce proper naming convention for hooks. If you rename your custom hooks which as prefix "use" to something else then linter won't allow you to call built-in hooks such as useState, useEffect etc inside of your custom hook anymore.
**Note:** This plugin is intended to use in Create React App by default.
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229. ### What are the differences between Flux and Redux?
Below are the major differences between Flux and Redux
| Flux | Redux |
| ---------------------------------------------- | ------------------------------------------ |
| State is mutable | State is immutable |
| The Store contains both state and change logic | The Store and change logic are separate |
| There are multiple stores exist | There is only one store exist |
| All the stores are disconnected and flat | Single store with hierarchical reducers |
| It has a singleton dispatcher | There is no concept of dispatcher |
| React components subscribe to the store | Container components uses connect function |
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230. ### What are the benefits of React Router V4?
Below are the main benefits of React Router V4 module,
1. In React Router v4(version 4), the API is completely about components. A router can be visualized as a single component(``) which wraps specific child router components(``).
2. You don't need to manually set history. The router module will take care history by wrapping routes with `` component.
3. The application size is reduced by adding only the specific router module(Web, core, or native)
**[⬆ Back to Top](#table-of-contents)**
231. ### Can you describe about componentDidCatch lifecycle method signature?
The **componentDidCatch** lifecycle method is invoked after an error has been thrown by a descendant component. The method receives two parameters,
1. error: - The error object which was thrown
2. info: - An object with a componentStack key contains the information about which component threw the error.
The method structure would be as follows
```javascript
componentDidCatch(error, info);
```
**[⬆ Back to Top](#table-of-contents)**
232. ### In which scenarios error boundaries do not catch errors?
Below are the cases in which error boundaries doesn't work,
1. Inside Event handlers
2. Asynchronous code using **setTimeout or requestAnimationFrame** callbacks
3. During Server side rendering
4. When errors thrown in the error boundary code itself
**[⬆ Back to Top](#table-of-contents)**
233. ### Why do you not need error boundaries for event handlers?
Error boundaries do not catch errors inside event handlers.
React doesn’t need error boundaries to recover from errors in event handlers. Unlike the render method and lifecycle methods, the event handlers don’t happen during rendering. So if they throw, React still knows what to display on the screen.
If you need to catch an error inside an event handler, use the regular JavaScript try / catch statement:
```javascript
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.state = { error: null };
this.handleClick = this.handleClick.bind(this);
}
handleClick() {
try {
// Do something that could throw
} catch (error) {
this.setState({ error });
}
}
render() {
if (this.state.error) {
return
Caught an error.
;}
return Click Me;
}
}
```
Note that the above example is demonstrating regular JavaScript behavior and doesn’t use error boundaries.
**[⬆ Back to Top](#table-of-contents)**
234. ### What is the difference between try catch block and error boundaries?
Try catch block works with imperative code whereas error boundaries are meant for declarative code to render on the screen.
For example, the try catch block used for below imperative code
```javascript
try {
showButton();
} catch (error) {
// ...
}
```
Whereas error boundaries wrap declarative code as below,
```javascript
```
So if an error occurs in a **componentDidUpdate** method caused by a **setState** somewhere deep in the tree, it will still correctly propagate to the closest error boundary.
**[⬆ Back to Top](#table-of-contents)**
235. ### What is the behavior of uncaught errors in react 16?
In React 16, errors that were not caught by any error boundary will result in unmounting of the whole React component tree. The reason behind this decision is that it is worse to leave corrupted UI in place than to completely remove it. For example, it is worse for a payments app to display a wrong amount than to render nothing.
**[⬆ Back to Top](#table-of-contents)**
236. ### What is the proper placement for error boundaries?
The granularity of error boundaries usage is up to the developer based on project needs. You can follow either of these approaches,
1. You can wrap top-level route components to display a generic error message for the entire application.
2. You can also wrap individual components in an error boundary to protect them from crashing the rest of the application.
**[⬆ Back to Top](#table-of-contents)**
237. ### What is the benefit of component stack trace from error boundary?
Apart from error messages and javascript stack, React16 will display the component stack trace with file names and line numbers using error boundary concept.
For example, BuggyCounter component displays the component stack trace as below,
**[⬆ Back to Top](#table-of-contents)**
238. ### What is the required method to be defined for a class component?
The `render()` method is the only required method in a class component. i.e, All methods other than render method are optional for a class component.
**[⬆ Back to Top](#table-of-contents)**
239. ### What are the possible return types of render method?
Below are the list of following types used and return from render method,
1. **React elements:** Elements that instruct React to render a DOM node. It includes html elements such as `
` and user defined elements.2. **Arrays and fragments:** Return multiple elements to render as Arrays and Fragments to wrap multiple elements
3. **Portals:** Render children into a different DOM subtree.
4. **String and numbers:** Render both Strings and Numbers as text nodes in the DOM
5. **Booleans or null:** Doesn't render anything but these types are used to conditionally render content.
**[⬆ Back to Top](#table-of-contents)**
240. ### What is the main purpose of constructor?
The constructor is mainly used for two purposes,
1. To initialize local state by assigning object to this.state
2. For binding event handler methods to the instance
For example, the below code covers both the above cases,
```javascript
constructor(props) {
super(props);
// Don't call this.setState() here!
this.state = { counter: 0 };
this.handleClick = this.handleClick.bind(this);
}
```
**[⬆ Back to Top](#table-of-contents)**
241. ### Is it mandatory to define constructor for React component?
No, it is not mandatory. i.e, If you don’t initialize state and you don’t bind methods, you don’t need to implement a constructor for your React component.
**[⬆ Back to Top](#table-of-contents)**
242. ### What are default props?
The _defaultProps_ can be defined as a property on the component to set the default values for the props. These default props are used when props not supplied(i.e., undefined props), but not for null props. That means, If you provide null value then it remains null value.
For example, let us create color default prop for the button component,
```javascript
function MyButton {
// ...
}
MyButton.defaultProps = {
color: "red",
};
```
If `props.color` is not provided then it will set the default value to 'red'. i.e, Whenever you try to access the color prop it uses the default value
```javascript
render() {
return ; // props.color will contain red value
}
```
**[⬆ Back to Top](#table-of-contents)**
243. ### Why should not call setState in componentWillUnmount?
You should not call `setState()` in `componentWillUnmount()` because once a component instance is unmounted, it will never be mounted again.
**[⬆ Back to Top](#table-of-contents)**
244. ### What is the purpose of getDerivedStateFromError?
This lifecycle method is invoked after an error has been thrown by a descendant component. It receives the error that was thrown as a parameter and should return a value to update state.
The signature of the lifecycle method is as follows,
```javascript
static getDerivedStateFromError(error)
```
Let us take error boundary use case with the above lifecycle method for demonstration purpose,
```javascript
class ErrorBoundary extends React.Component {
constructor(props) {
super(props);
this.state = { hasError: false };
}
static getDerivedStateFromError(error) {
// Update state so the next render will show the fallback UI.
return { hasError: true };
}
render() {
if (this.state.hasError) {
// You can render any custom fallback UI
return
Something went wrong.
;}
return this.props.children;
}
}
```
**[⬆ Back to Top](#table-of-contents)**
245. ### What is the methods order when component re-rendered?
An update can be caused by changes to props or state. The below methods are called in the following order when a component is being re-rendered.
1. static getDerivedStateFromProps()
2. shouldComponentUpdate()
3. render()
4. getSnapshotBeforeUpdate()
5. componentDidUpdate()
**[⬆ Back to Top](#table-of-contents)**
246. ### What are the methods invoked during error handling?
Below methods are called when there is an error during rendering, in a lifecycle method, or in the constructor of any child component.
1. static getDerivedStateFromError()
2. componentDidCatch()
**[⬆ Back to Top](#table-of-contents)**
247. ### What is the purpose of displayName class property?
The displayName string is used in debugging messages. Usually, you don’t need to set it explicitly because it’s inferred from the name of the function or class that defines the component. You might want to set it explicitly if you want to display a different name for debugging purposes or when you create a higher-order component.
For example, To ease debugging, choose a display name that communicates that it’s the result of a withSubscription HOC.
```javascript
function withSubscription(WrappedComponent) {
class WithSubscription extends React.Component {
/* ... */
}
WithSubscription.displayName = `WithSubscription(${getDisplayName(
WrappedComponent
)})`;
return WithSubscription;
}
function getDisplayName(WrappedComponent) {
return (
WrappedComponent.displayName || WrappedComponent.name || "Component"
);
}
```
**[⬆ Back to Top](#table-of-contents)**
248. ### What is the browser support for react applications?
React supports all popular browsers, including Internet Explorer 9 and above, although some polyfills are required for older browsers such as IE 9 and IE 10. If you use **es5-shim and es5-sham** polyfill then it even support old browsers that doesn't support ES5 methods.
**[⬆ Back to Top](#table-of-contents)**
249. ### What is the purpose of unmountComponentAtNode method?
This method is available from react-dom package and it removes a mounted React component from the DOM and clean up its event handlers and state. If no component was mounted in the container, calling this function does nothing. Returns true if a component was unmounted and false if there was no component to unmount.
The method signature would be as follows,
```javascript
ReactDOM.unmountComponentAtNode(container);
```
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250. ### What is code-splitting?
Code-Splitting is a feature supported by bundlers like Webpack and Browserify which can create multiple bundles that can be dynamically loaded at runtime. The react project supports code splitting via dynamic import() feature.
For example, in the below code snippets, it will make moduleA.js and all its unique dependencies as a separate chunk that only loads after the user clicks the 'Load' button.
**moduleA.js**
```javascript
const moduleA = "Hello";
export { moduleA };
```
**App.js**
```javascript
import React, { Component } from "react";
class App extends Component {
handleClick = () => {
import("./moduleA")
.then(({ moduleA }) => {
// Use moduleA
})
.catch((err) => {
// Handle failure
});
};
render() {
return (
Load
);
}
}
export default App;
```
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251. ### What is the benefit of strict mode?
The will be helpful in the below cases
1. Identifying components with **unsafe lifecycle methods**.
2. Warning about **legacy string ref** API usage.
3. Detecting unexpected **side effects**.
4. Detecting **legacy context** API.
5. Warning about deprecated findDOMNode usage
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252. ### What are Keyed Fragments?
The Fragments declared with the explicit syntax may have keys. The general use case is mapping a collection to an array of fragments as below,
```javascript
function Glossary(props) {
return (
- {item.term}
- {item.description}
{props.items.map((item) => (
// Without the `key`, React will fire a key warning
))}
);
}
```
**Note:** key is the only attribute that can be passed to Fragment. In the future, there might be a support for additional attributes, such as event handlers.
**[⬆ Back to Top](#table-of-contents)**
253. ### Does React support all HTML attributes?
As of React 16, both standard or custom DOM attributes are fully supported. Since React components often take both custom and DOM-related props, React uses the camelCase convention just like the DOM APIs.
Let us take few props with respect to standard HTML attributes,
```javascript
// Just like node.className DOM API
// Just like node.readOnly DOM API
```
These props work similarly to the corresponding HTML attributes, with the exception of the special cases. It also support all SVG attributes.
**[⬆ Back to Top](#table-of-contents)**
254. ### What are the limitations with HOCs?
Higher-order components come with a few caveats apart from its benefits. Below are the few listed in an order,
1. **Don’t use HOCs inside the render method:**
It is not recommended to apply a HOC to a component within the render method of a component.
```javascript
render() {
// A new version of EnhancedComponent is created on every render
// EnhancedComponent1 !== EnhancedComponent2
const EnhancedComponent = enhance(MyComponent);
// That causes the entire subtree to unmount/remount each time!
return ;
}
```
The above code impacts on performance by remounting a component that causes the state of that component and all of its children to be lost. Instead, apply HOCs outside the component definition so that the resulting component is created only once.
2. **Static methods must be copied over:**
When you apply a HOC to a component the new component does not have any of the static methods of the original component
```javascript
// Define a static method
WrappedComponent.staticMethod = function () {
/*...*/
};
// Now apply a HOC
const EnhancedComponent = enhance(WrappedComponent);
// The enhanced component has no static method
typeof EnhancedComponent.staticMethod === "undefined"; // true
```
You can overcome this by copying the methods onto the container before returning it,
```javascript
function enhance(WrappedComponent) {
class Enhance extends React.Component {
/*...*/
}
// Must know exactly which method(s) to copy :(
Enhance.staticMethod = WrappedComponent.staticMethod;
return Enhance;
}
```
3. **Refs aren’t passed through:**
For HOCs you need to pass through all props to the wrapped component but this does not work for refs. This is because ref is not really a prop similar to key. In this case you need to use the React.forwardRef API
**[⬆ Back to Top](#table-of-contents)**
255. ### How to debug forwardRefs in DevTools?
**React.forwardRef** accepts a render function as parameter and DevTools uses this function to determine what to display for the ref forwarding component.
For example, If you don't name the render function or not using displayName property then it will appear as ”ForwardRef” in the DevTools,
```javascript
const WrappedComponent = React.forwardRef((props, ref) => {
return ;
});
```
But If you name the render function then it will appear as **”ForwardRef(myFunction)”**
```javascript
const WrappedComponent = React.forwardRef(function myFunction(props, ref) {
return ;
});
```
As an alternative, You can also set displayName property for forwardRef function,
```javascript
function logProps(Component) {
class LogProps extends React.Component {
// ...
}
function forwardRef(props, ref) {
return ;
}
// Give this component a more helpful display name in DevTools.
// e.g. "ForwardRef(logProps(MyComponent))"
const name = Component.displayName || Component.name;
forwardRef.displayName = `logProps(${name})`;
return React.forwardRef(forwardRef);
}
```
**[⬆ Back to Top](#table-of-contents)**
256. ### When component props defaults to true?
If you pass no value for a prop, it defaults to true. This behavior is available so that it matches the behavior of HTML.
For example, below expressions are equivalent,
```javascript
```
**Note:** It is not recommended to use this approach because it can be confused with the ES6 object shorthand (example, `{name}` which is short for `{name: name}`)
**[⬆ Back to Top](#table-of-contents)**
257. ### What is NextJS and major features of it?
Next.js is a popular and lightweight framework for static and server‑rendered applications built with React. It also provides styling and routing solutions. Below are the major features provided by NextJS,
1. Server-rendered by default
2. Automatic code splitting for faster page loads
3. Simple client-side routing (page based)
4. Webpack-based dev environment which supports (HMR)
5. Able to implement with Express or any other Node.js HTTP server
6. Customizable with your own Babel and Webpack configurations
**[⬆ Back to Top](#table-of-contents)**
258. ### How do you pass an event handler to a component?
You can pass event handlers and other functions as props to child components. It can be used in child component as below,
```html
```
**[⬆ Back to Top](#table-of-contents)**
259. ### Is it good to use arrow functions in render methods?
Yes, You can use. It is often the easiest way to pass parameters to callback functions. But you need to optimize the performance while using it.
```javascript
class Foo extends Component {
handleClick() {
console.log("Click happened");
}
render() {
return this.handleClick()}>Click Me;
}
}
```
**Note:** Using an arrow function in render method creates a new function each time the component renders, which may have performance implications
**[⬆ Back to Top](#table-of-contents)**
260. ### How to prevent a function from being called multiple times?
If you use an event handler such as **onClick or onScroll** and want to prevent the callback from being fired too quickly, then you can limit the rate at which callback is executed. This can be achieved in the below possible ways,
1. **Throttling:** Changes based on a time based frequency. For example, it can be used using \_.throttle lodash function
2. **Debouncing:** Publish changes after a period of inactivity. For example, it can be used using \_.debounce lodash function
3. **RequestAnimationFrame throttling:** Changes based on requestAnimationFrame. For example, it can be used using raf-schd lodash function
**[⬆ Back to Top](#table-of-contents)**
261. ### How JSX prevents Injection Attacks?
React DOM escapes any values embedded in JSX before rendering them. Thus it ensures that you can never inject anything that’s not explicitly written in your application. Everything is converted to a string before being rendered.
For example, you can embed user input as below,
```javascript
const name = response.potentiallyMaliciousInput;
const element =
{name}
;```
This way you can prevent XSS(Cross-site-scripting) attacks in the application.
**[⬆ Back to Top](#table-of-contents)**
262. ### How do you update rendered elements?
You can update UI(represented by rendered element) by passing the newly created element to ReactDOM's render method.
For example, lets take a ticking clock example, where it updates the time by calling render method multiple times,
```javascript
function tick() {
const element = (
Hello, world!
It is {new Date().toLocaleTimeString()}.
);
ReactDOM.render(element, document.getElementById("root"));
}
setInterval(tick, 1000);
```
**[⬆ Back to Top](#table-of-contents)**
263. ### How do you say that props are readonly?
When you declare a component as a function or a class, it must never modify its own props.
Let us take a below capital function,
```javascript
function capital(amount, interest) {
return amount + interest;
}
```
The above function is called “pure” because it does not attempt to change their inputs, and always return the same result for the same inputs. Hence, React has a single rule saying "All React components must act like pure functions with respect to their props."
**[⬆ Back to Top](#table-of-contents)**
264. ### How do you say that state updates are merged?
When you call setState() in the component, React merges the object you provide into the current state.
For example, let us take a facebook user with posts and comments details as state variables,
```javascript
constructor(props) {
super(props);
this.state = {
posts: [],
comments: []
};
}
```
Now you can update them independently with separate `setState()` calls as below,
```javascript
componentDidMount() {
fetchPosts().then(response => {
this.setState({
posts: response.posts
});
});
fetchComments().then(response => {
this.setState({
comments: response.comments
});
});
}
```
As mentioned in the above code snippets, `this.setState({comments})` updates only comments variable without modifying or replacing `posts` variable.
**[⬆ Back to Top](#table-of-contents)**
265. ### How do you pass arguments to an event handler?
During iterations or loops, it is common to pass an extra parameter to an event handler. This can be achieved through arrow functions or bind method.
Let us take an example of user details updated in a grid,
```javascript
this.updateUser(userId, e)}>Update User details
Update User details
```
In the both approaches, the synthetic argument `e` is passed as a second argument. You need to pass it explicitly for arrow functions and it will be passed automatically for `bind` method.
**[⬆ Back to Top](#table-of-contents)**
266. ### How to prevent component from rendering?
You can prevent component from rendering by returning null based on specific condition. This way it can conditionally render component.
```javascript
function Greeting(props) {
if (!props.loggedIn) {
return null;
}
return
}
```
```javascript
class User extends React.Component {
constructor(props) {
super(props);
this.state = {loggedIn: false, name: 'John'};
}
render() {
return (
//Prevent component render if it is not loggedIn
);
}
```
In the above example, the `greeting` component skips its rendering section by applying condition and returning null value.
**[⬆ Back to Top](#table-of-contents)**
267. ### What are the conditions to safely use the index as a key?
There are three conditions to make sure, it is safe use the index as a key.
1. The list and items are static– they are not computed and do not change
2. The items in the list have no ids
3. The list is never reordered or filtered.
**[⬆ Back to Top](#table-of-contents)**
268. ### Should keys be globally unique?
The keys used within arrays should be unique among their siblings but they don’t need to be globally unique. i.e, You can use the same keys with two different arrays.
For example, the below `Book` component uses two arrays with different arrays,
```javascript
function Book(props) {
const index = (
- {page.title}
{props.pages.map((page) => (
))}
);
const content = props.pages.map((page) => (
{page.title}
{page.content}
{page.pageNumber}
));
return (
{index}
{content}
);
}
```
**[⬆ Back to Top](#table-of-contents)**
269. ### What is the popular choice for form handling?
`Formik` is a form library for react which provides solutions such as validation, keeping track of the visited fields, and handling form submission.
In detail, You can categorize them as follows,
1. Getting values in and out of form state
2. Validation and error messages
3. Handling form submission
It is used to create a scalable, performant, form helper with a minimal API to solve annoying stuff.
**[⬆ Back to Top](#table-of-contents)**
270. ### What are the advantages of formik over redux form library?
Below are the main reasons to recommend formik over redux form library,
1. The form state is inherently short-term and local, so tracking it in Redux (or any kind of Flux library) is unnecessary.
2. Redux-Form calls your entire top-level Redux reducer multiple times ON EVERY SINGLE KEYSTROKE. This way it increases input latency for large apps.
3. Redux-Form is 22.5 kB minified gzipped whereas Formik is 12.7 kB
**[⬆ Back to Top](#table-of-contents)**
271. ### Why are you not required to use inheritance?
In React, it is recommended to use composition over inheritance to reuse code between components. Both Props and composition give you all the flexibility you need to customize a component’s look and behavior explicitly and safely.
Whereas, If you want to reuse non-UI functionality between components, it is suggested to extract it into a separate JavaScript module. Later components import it and use that function, object, or class, without extending it.
**[⬆ Back to Top](#table-of-contents)**
272. ### Can I use web components in react application?
Yes, you can use web components in a react application. Even though many developers won't use this combination, it may require especially if you are using third-party UI components that are written using Web Components.
For example, let us use `Vaadin` date picker web component as below,
```javascript
import React, { Component } from "react";
import "./App.css";
import "@vaadin/vaadin-date-picker";
class App extends Component {
render() {
return (
);
}
}
export default App;
```
**[⬆ Back to Top](#table-of-contents)**
273. ### What is dynamic import?
You can achieve code-splitting in your app using dynamic import.
Let's take an example of addition,
1. **Normal Import**
```javascript
import { add } from "./math";
console.log(add(10, 20));
```
2. **Dynamic Import**
```javascript
import("./math").then((math) => {
console.log(math.add(10, 20));
});
```
**[⬆ Back to Top](#table-of-contents)**
274. ### What are loadable components?
If you want to do code-splitting in a server rendered app, it is recommend to use Loadable Components because React.lazy and Suspense is not yet available for server-side rendering. Loadable lets you render a dynamic import as a regular component.
Lets take an example,
```javascript
import loadable from "@loadable/component";
const OtherComponent = loadable(() => import("./OtherComponent"));
function MyComponent() {
return (
);
}
```
Now OtherComponent will be loaded in a separated bundle
**[⬆ Back to Top](#table-of-contents)**
275. ### What is suspense component?
If the module containing the dynamic import is not yet loaded by the time parent component renders, you must show some fallback content while you’re waiting for it to load using a loading indicator. This can be done using **Suspense** component.
For example, the below code uses suspense component,
```javascript
const OtherComponent = React.lazy(() => import("./OtherComponent"));
function MyComponent() {
return (
Loading...
);
}
```
As mentioned in the above code, Suspense is wrapped above the lazy component.
**[⬆ Back to Top](#table-of-contents)**
276. ### What is route based code splitting?
One of the best place to do code splitting is with routes. The entire page is going to re-render at once so users are unlikely to interact with other elements in the page at the same time. Due to this, the user experience won't be disturbed.
Let us take an example of route based website using libraries like React Router with React.lazy,
```javascript
import { BrowserRouter as Router, Route, Switch } from "react-router-dom";
import React, { Suspense, lazy } from "react";
const Home = lazy(() => import("./routes/Home"));
const About = lazy(() => import("./routes/About"));
const App = () => (
Loading...
);
```
In the above code, the code splitting will happen at each route level.
**[⬆ Back to Top](#table-of-contents)**
277. ### Give an example on How to use context?
**Context** is designed to share data that can be considered **global** for a tree of React components.
For example, in the code below lets manually thread through a “theme” prop in order to style the Button component.
```javascript
//Lets create a context with a default theme value "luna"
const ThemeContext = React.createContext("luna");
// Create App component where it uses provider to pass theme value in the tree
class App extends React.Component {
render() {
return (
);
}
}
// A middle component where you don't need to pass theme prop anymore
function Toolbar(props) {
return (
);
}
// Lets read theme value in the button component to use
class ThemedButton extends React.Component {
static contextType = ThemeContext;
render() {
return ;
}
}
```
**[⬆ Back to Top](#table-of-contents)**
278. ### What is the purpose of default value in context?
The defaultValue argument is only used when a component does not have a matching Provider above it in the tree. This can be helpful for testing components in isolation without wrapping them.
Below code snippet provides default theme value as Luna.
```javascript
const MyContext = React.createContext(defaultValue);
```
**[⬆ Back to Top](#table-of-contents)**
279. ### How do you use contextType?
ContextType is used to consume the context object. The contextType property can be used in two ways,
1. **contextType as property of class:**
The contextType property on a class can be assigned a Context object created by React.createContext(). After that, you can consume the nearest current value of that Context type using this.context in any of the lifecycle methods and render function.
Lets assign contextType property on MyClass as below,
```javascript
class MyClass extends React.Component {
componentDidMount() {
let value = this.context;
/* perform a side-effect at mount using the value of MyContext */
}
componentDidUpdate() {
let value = this.context;
/* ... */
}
componentWillUnmount() {
let value = this.context;
/* ... */
}
render() {
let value = this.context;
/* render something based on the value of MyContext */
}
}
MyClass.contextType = MyContext;
```
2. **Static field**
You can use a static class field to initialize your contextType using public class field syntax.
```javascript
class MyClass extends React.Component {
static contextType = MyContext;
render() {
let value = this.context;
/* render something based on the value */
}
}
```
**[⬆ Back to Top](#table-of-contents)**
280. ### What is a consumer?
A Consumer is a React component that subscribes to context changes. It requires a function as a child which receives current context value as argument and returns a react node. The value argument passed to the function will be equal to the value prop of the closest Provider for this context above in the tree.
Lets take a simple example,
```javascript
{value => /* render something based on the context value */}
```
**[⬆ Back to Top](#table-of-contents)**
281. ### How do you solve performance corner cases while using context?
The context uses reference identity to determine when to re-render, there are some gotchas that could trigger unintentional renders in consumers when a provider’s parent re-renders.
For example, the code below will re-render all consumers every time the Provider re-renders because a new object is always created for value.
```javascript
class App extends React.Component {
render() {
return (
);
}
}
```
This can be solved by lifting up the value to parent state,
```javascript
class App extends React.Component {
constructor(props) {
super(props);
this.state = {
value: { something: "something" },
};
}
render() {
return (
);
}
}
```
**[⬆ Back to Top](#table-of-contents)**
282. ### What is the purpose of forward ref in HOCs?
Refs will not get passed through because ref is not a prop. It is handled differently by React just like **key**. If you add a ref to a HOC, the ref will refer to the outermost container component, not the wrapped component. In this case, you can use Forward Ref API. For example, we can explicitly forward refs to the inner FancyButton component using the React.forwardRef API.
The below HOC logs all props,
```javascript
function logProps(Component) {
class LogProps extends React.Component {
componentDidUpdate(prevProps) {
console.log("old props:", prevProps);
console.log("new props:", this.props);
}
render() {
const { forwardedRef, ...rest } = this.props;
// Assign the custom prop "forwardedRef" as a ref
return ;
}
}
return React.forwardRef((props, ref) => {
return ;
});
}
```
Let's use this HOC to log all props that get passed to our “fancy button” component,
```javascript
class FancyButton extends React.Component {
focus() {
// ...
}
// ...
}
export default logProps(FancyButton);
```
Now let's create a ref and pass it to FancyButton component. In this case, you can set focus to button element.
```javascript
import FancyButton from "./FancyButton";
const ref = React.createRef();
ref.current.focus();
;
```
**[⬆ Back to Top](#table-of-contents)**
283. ### Is ref argument available for all functions or class components?
Regular function or class components don’t receive the ref argument, and ref is not available in props either. The second ref argument only exists when you define a component with React.forwardRef call.
**[⬆ Back to Top](#table-of-contents)**
284. ### Why do you need additional care for component libraries while using forward refs?
When you start using forwardRef in a component library, you should treat it as a breaking change and release a new major version of your library. This is because your library likely has a different behavior such as what refs get assigned to, and what types are exported. These changes can break apps and other libraries that depend on the old behavior.
**[⬆ Back to Top](#table-of-contents)**
285. ### How to create react class components without ES6?
If you don’t use ES6 then you may need to use the create-react-class module instead. For default props, you need to define getDefaultProps() as a function on the passed object. Whereas for initial state, you have to provide a separate getInitialState method that returns the initial state.
```javascript
var Greeting = createReactClass({
getDefaultProps: function () {
return {
name: "Jhohn",
};
},
getInitialState: function () {
return { message: this.props.message };
},
handleClick: function () {
console.log(this.state.message);
},
render: function () {
return
Hello, {this.props.name}
;},
});
```
**Note:** If you use createReactClass then auto binding is available for all methods. i.e, You don't need to use `.bind(this)` with in constructor for event handlers.
**[⬆ Back to Top](#table-of-contents)**
286. ### Is it possible to use react without JSX?
Yes, JSX is not mandatory for using React. Actually it is convenient when you don’t want to set up compilation in your build environment. Each JSX element is just syntactic sugar for calling `React.createElement(component, props, ...children)`.
For example, let us take a greeting example with JSX,
```javascript
class Greeting extends React.Component {
render() {
return
}
}
ReactDOM.render(
,
document.getElementById("root")
);
```
You can write the same code without JSX as below,
```javascript
class Greeting extends React.Component {
render() {
return React.createElement("div", null, `Hello ${this.props.message}`);
}
}
ReactDOM.render(
React.createElement(Greeting, { message: "World" }, null),
document.getElementById("root")
);
```
**[⬆ Back to Top](#table-of-contents)**
287. ### What is diffing algorithm?
React needs to use algorithms to find out how to efficiently update the UI to match the most recent tree. The diffing algorithms is generating the minimum number of operations to transform one tree into another. However, the algorithms have a complexity in the order of O(n³) where n is the number of elements in the tree.
In this case, displaying 1000 elements would require in the order of one billion comparisons. This is far too expensive. Instead, React implements a heuristic O(n) algorithm based on two assumptions:
1. Two elements of different types will produce different trees.
2. The developer can hint at which child elements may be stable across different renders with a key prop.
**[⬆ Back to Top](#table-of-contents)**
288. ### What are the rules covered by diffing algorithm?
When diffing two trees, React first compares the two root elements. The behavior is different depending on the types of the root elements. It covers the below rules during reconciliation algorithm,
1. **Elements Of Different Types:**
Whenever the root elements have different types, React will tear down the old tree and build the new tree from scratch. For example, elements to , or from to of different types lead a full rebuild.
2. **DOM Elements Of The Same Type:**
When comparing two React DOM elements of the same type, React looks at the attributes of both, keeps the same underlying DOM node, and only updates the changed attributes. Lets take an example with same DOM elements except className attribute,
```javascript
```
3. **Component Elements Of The Same Type:**
When a component updates, the instance stays the same, so that state is maintained across renders. React updates the props of the underlying component instance to match the new element, and calls componentWillReceiveProps() and componentWillUpdate() on the underlying instance. After that, the render() method is called and the diff algorithm recurses on the previous result and the new result.
4. **Recursing On Children:**
when recursing on the children of a DOM node, React just iterates over both lists of children at the same time and generates a mutation whenever there’s a difference. For example, when adding an element at the end of the children, converting between these two trees works well.
```javascript
- first
- second
- first
- second
- third
```
5. **Handling keys:**
React supports a key attribute. When children have keys, React uses the key to match children in the original tree with children in the subsequent tree. For example, adding a key can make the tree conversion efficient,
```javascript
- Duke
- Villanova
- Connecticut
- Duke
- Villanova
```
**[⬆ Back to Top](#table-of-contents)**
289. ### When do you need to use refs?
There are few use cases to go for refs,
1. Managing focus, text selection, or media playback.
2. Triggering imperative animations.
3. Integrating with third-party DOM libraries.
**[⬆ Back to Top](#table-of-contents)**
290. ### Must prop be named as render for render props?
Even though the pattern named render props, you don’t have to use a prop named render to use this pattern. i.e, Any prop that is a function that a component uses to know what to render is technically a “render prop”. Lets take an example with the children prop for render props,
```javascript
(
The mouse position is {mouse.x}, {mouse.y}
)}
/>
```
Actually children prop doesn’t need to be named in the list of “attributes” in JSX element. Instead, you can keep it directly inside element,
```javascript
{(mouse) => (
The mouse position is {mouse.x}, {mouse.y}
)}
```
While using this above technique(without any name), explicitly state that children should be a function in your propTypes.
```javascript
Mouse.propTypes = {
children: PropTypes.func.isRequired,
};
```
**[⬆ Back to Top](#table-of-contents)**
291. ### What are the problems of using render props with pure components?
If you create a function inside a render method, it negates the purpose of pure component. Because the shallow prop comparison will always return false for new props, and each render in this case will generate a new value for the render prop. You can solve this issue by defining the render function as instance method.
**[⬆ Back to Top](#table-of-contents)**
292. ### How do you create HOC using render props?
You can implement most higher-order components (HOC) using a regular component with a render prop. For example, if you would prefer to have a withMouse HOC instead of a component, you could easily create one using a regular with a render prop.
```javascript
function withMouse(Component) {
return class extends React.Component {
render() {
return (
}
/>
);
}
};
}
```
This way render props gives the flexibility of using either pattern.
**[⬆ Back to Top](#table-of-contents)**
293. ### What is windowing technique?
Windowing is a technique that only renders a small subset of your rows at any given time, and can dramatically reduce the time it takes to re-render the components as well as the number of DOM nodes created. If your application renders long lists of data then this technique is recommended. Both react-window and react-virtualized are popular windowing libraries which provides several reusable components for displaying lists, grids, and tabular data.
**[⬆ Back to Top](#table-of-contents)**
294. ### How do you print falsy values in JSX?
The falsy values such as false, null, undefined, and true are valid children but they don't render anything. If you still want to display them then you need to convert it to string. Let's take an example on how to convert to a string,
```javascript
```
**[⬆ Back to Top](#table-of-contents)**
295. ### What is the typical use case of portals?
React portals are very useful when a parent component has overflow: hidden or has properties that affect the stacking context (e.g. z-index, position, opacity) and you need to visually “break out” of its container.
For example, dialogs, global message notifications, hovercards, and tooltips.
**[⬆ Back to Top](#table-of-contents)**
296. ### How do you set default value for uncontrolled component?
In React, the value attribute on form elements will override the value in the DOM. With an uncontrolled component, you might want React to specify the initial value, but leave subsequent updates uncontrolled. To handle this case, you can specify a **defaultValue** attribute instead of **value**.
```javascript
render() {
return (
User Name:
);
}
```
The same applies for `select` and `textArea` inputs. But you need to use **defaultChecked** for `checkbox` and `radio` inputs.
**[⬆ Back to Top](#table-of-contents)**
297. ### What is your favorite React stack?
Even though the tech stack varies from developer to developer, the most popular stack is used in react boilerplate project code. It mainly uses Redux and redux-saga for state management and asynchronous side-effects, react-router for routing purpose, styled-components for styling react components, axios for invoking REST api, and other supported stack such as webpack, reselect, ESNext, Babel.
You can clone the project https://github.com/react-boilerplate/react-boilerplate and start working on any new react project.
**[⬆ Back to Top](#table-of-contents)**
298. ### What is the difference between Real DOM and Virtual DOM?
Below are the main differences between Real DOM and Virtual DOM,
| Real DOM | Virtual DOM |
| ------------------------------------ | ------------------------------------ |
| Updates are slow | Updates are fast |
| DOM manipulation is very expensive. | DOM manipulation is very easy |
| You can update HTML directly. | You Can’t directly update HTML |
| It causes too much of memory wastage | There is no memory wastage |
| Creates a new DOM if element updates | It updates the JSX if element update |
**[⬆ Back to Top](#table-of-contents)**
299. ### How to add Bootstrap to a react application?
Bootstrap can be added to your React app in a three possible ways,
1. Using the Bootstrap CDN:
This is the easiest way to add bootstrap. Add both bootstrap CSS and JS resources in a head tag.
2. Bootstrap as Dependency:
If you are using a build tool or a module bundler such as Webpack, then this is the preferred option for adding Bootstrap to your React application
```javascript
npm install bootstrap
```
3. React Bootstrap Package:
In this case, you can add Bootstrap to our React app is by using a package that has rebuilt Bootstrap components to work particularly as React components. Below packages are popular in this category,
1. react-bootstrap
2. reactstrap
**[⬆ Back to Top](#table-of-contents)**
300. ### Can you list down top websites or applications using react as front end framework?
Below are the `top 10 websites` using React as their front-end framework,
1. Facebook
2. Uber
3. Instagram
4. WhatsApp
5. Khan Academy
6. Airbnb
7. Dropbox
8. Flipboard
9. Netflix
10. PayPal
**[⬆ Back to Top](#table-of-contents)**
301. ### Is it recommended to use CSS In JS technique in React?
React does not have any opinion about how styles are defined but if you are a beginner then good starting point is to define your styles in a separate \*.css file as usual and refer to them using className. This functionality is not part of React but came from third-party libraries. But If you want to try a different approach(CSS-In-JS) then styled-components library is a good option.
**[⬆ Back to Top](#table-of-contents)**
302. ### Do I need to rewrite all my class components with hooks?
No. But you can try Hooks in a few components(or new components) without rewriting any existing code. Because there are no plans to remove classes in ReactJS.
**[⬆ Back to Top](#table-of-contents)**
303. ### How to fetch data with React Hooks?
The effect hook called `useEffect` can be used to fetch data from an API and to set the data in the local state of the component with the useState hook’s update function.
Here is an example of fetching a list of react articles from an API using fetch.
```javascript
import React from "react";
function App() {
const [data, setData] = React.useState({ hits: [] });
React.useEffect(() => {
fetch("http://hn.algolia.com/api/v1/search?query=react")
.then(response => response.json())
.then(data => setData(data))
}, []);
return (
-
{item.title}
{data.hits.map((item) => (
))}
);
}
export default App;
```
A popular way to simplify this is by using the library axios.
We provided an empty array as second argument to the useEffect hook to avoid activating it on component updates. This way, it only fetches on component mount.
**[⬆ Back to Top](#table-of-contents)**
304. ### Is Hooks cover all use cases for classes?
Hooks doesn't cover all use cases of classes but there is a plan to add them soon. Currently there are no Hook equivalents to the uncommon **getSnapshotBeforeUpdate** and **componentDidCatch** lifecycles yet.
**[⬆ Back to Top](#table-of-contents)**
305. ### What is the stable release for hooks support?
React includes a stable implementation of React Hooks in 16.8 release for below packages
1. React DOM
2. React DOM Server
3. React Test Renderer
4. React Shallow Renderer
**[⬆ Back to Top](#table-of-contents)**
306. ### Why do we use array destructuring (square brackets notation) in `useState`?
When we declare a state variable with `useState`, it returns a pair — an array with two items. The first item is the current value, and the second is a function that updates the value. Using [0] and [1] to access them is a bit confusing because they have a specific meaning. This is why we use array destructuring instead.
For example, the array index access would look as follows:
```javascript
var userStateVariable = useState("userProfile"); // Returns an array pair
var user = userStateVariable[0]; // Access first item
var setUser = userStateVariable[1]; // Access second item
```
Whereas with array destructuring the variables can be accessed as follows:
```javascript
const [user, setUser] = useState("userProfile");
```
**[⬆ Back to Top](#table-of-contents)**
307. ### What are the sources used for introducing hooks?
Hooks got the ideas from several different sources. Below are some of them,
1. Previous experiments with functional APIs in the react-future repository
2. Community experiments with render prop APIs such as Reactions Component
3. State variables and state cells in DisplayScript.
4. Subscriptions in Rx.
5. Reducer components in ReasonReact.
**[⬆ Back to Top](#table-of-contents)**
308. ### How do you access imperative API of web components?
Web Components often expose an imperative API to implement its functions. You will need to use a **ref** to interact with the DOM node directly if you want to access imperative API of a web component. But if you are using third-party Web Components, the best solution is to write a React component that behaves as a **wrapper** for your Web Component.
**[⬆ Back to Top](#table-of-contents)**
309. ### What is formik?
Formik is a small react form library that helps you with the three major problems,
1. Getting values in and out of form state
2. Validation and error messages
3. Handling form submission
**[⬆ Back to Top](#table-of-contents)**
310. ### What are typical middleware choices for handling asynchronous calls in Redux?
Some of the popular middleware choices for handling asynchronous calls in Redux eco system are `Redux Thunk, Redux Promise, Redux Saga`.
**[⬆ Back to Top](#table-of-contents)**
311. ### Do browsers understand JSX code?
No, browsers can't understand JSX code. You need a transpiler to convert your JSX to regular Javascript that browsers can understand. The most widely used transpiler right now is Babel.
**[⬆ Back to Top](#table-of-contents)**
312. ### Describe about data flow in react?
React implements one-way reactive data flow using props which reduce boilerplate and is easier to understand than traditional two-way data binding.
**[⬆ Back to Top](#table-of-contents)**
313. ### What is react scripts?
The `react-scripts` package is a set of scripts from the create-react-app starter pack which helps you kick off projects without configuring. The `react-scripts start` command sets up the development environment and starts a server, as well as hot module reloading.
**[⬆ Back to Top](#table-of-contents)**
314. ### What are the features of create react app?
Below are the list of some of the features provided by create react app.
1. React, JSX, ES6, Typescript and Flow syntax support.
2. Autoprefixed CSS
3. CSS Reset/Normalize
4. A live development server
5. A fast interactive unit test runner with built-in support for coverage reporting
6. A build script to bundle JS, CSS, and images for production, with hashes and sourcemaps
7. An offline-first service worker and a web app manifest, meeting all the Progressive Web App criteria.
**[⬆ Back to Top](#table-of-contents)**
315. ### What is the purpose of renderToNodeStream method?
The `ReactDOMServer#renderToNodeStream` method is used to generate HTML on the server and send the markup down on the initial request for faster page loads. It also helps search engines to crawl your pages easily for SEO purposes.
**Note:** Remember this method is not available in the browser but only server.
**[⬆ Back to Top](#table-of-contents)**
316. ### What is MobX?
MobX is a simple, scalable and battle tested state management solution for applying functional reactive programming (TFRP). For reactJs application, you need to install below packages,
```bash
npm install mobx --save
npm install mobx-react --save
```
**[⬆ Back to Top](#table-of-contents)**
317. ### What are the differences between Redux and MobX?
Below are the main differences between Redux and MobX,
| Topic | Redux | MobX |
| ------------- | ------------------------------------------------------------- | ---------------------------------------------------------------------- |
| Definition | It is a javascript library for managing the application state | It is a library for reactively managing the state of your applications |
| Programming | It is mainly written in ES6 | It is written in JavaScript(ES5) |
| Data Store | There is only one large store exist for data storage | There is more than one store for storage |
| Usage | Mainly used for large and complex applications | Used for simple applications |
| Performance | Need to be improved | Provides better performance |
| How it stores | Uses JS Object to store | Uses observable to store the data |
**[⬆ Back to Top](#table-of-contents)**
318. ### Should I learn ES6 before learning ReactJS?
No, you don’t have to learn es2015/es6 to learn react. But you may find many resources or React ecosystem uses ES6 extensively. Let's see some of the frequently used ES6 features,
1. **Destructuring:** To get props and use them in a component
```javascript
// in es 5
var someData = this.props.someData;
var dispatch = this.props.dispatch;
// in es6
const { someData, dispatch } = this.props;
```
2. Spread operator: Helps in passing props down into a component
```javascript
// in es 5
// in es6
```
3. Arrow functions: Makes compact syntax
```javascript
// es 5
var users = usersList.map(function (user) {
return
});
// es 6
const users = usersList.map((user) =>
```
**[⬆ Back to Top](#table-of-contents)**
319. ### What is Concurrent Rendering?
The Concurrent rendering makes React apps to be more responsive by rendering component trees without blocking the main UI thread. It allows React to interrupt a long-running render to handle a high-priority event. i.e, When you enabled concurrent Mode, React will keep an eye on other tasks that need to be done, and if there's something with a higher priority it will pause what it is currently rendering and let the other task finish first. You can enable this in two ways,
```javascript
// 1. Part of an app by wrapping with ConcurrentMode
;
// 2. Whole app using createRoot
ReactDOM.unstable_createRoot(domNode).render();
```
**[⬆ Back to Top](#table-of-contents)**
320. ### What is the difference between async mode and concurrent mode?
Both refers the same thing. Previously concurrent Mode being referred to as "Async Mode" by React team. The name has been changed to highlight React’s ability to perform work on different priority levels. So it avoids the confusion from other approaches to Async Rendering.
**[⬆ Back to Top](#table-of-contents)**
321. ### Can I use javascript urls in react16.9?
Yes, you can use javascript: URLs but it will log a warning in the console. Because URLs starting with javascript: are dangerous by including unsanitized output in a tag like `` and create a security hole.
```javascript
const companyProfile = {
website: "javascript: alert('Your website is hacked')",
};
// It will log a warning
More details;
```
Remember that the future versions will throw an error for javascript URLs.
**[⬆ Back to Top](#table-of-contents)**
322. ### What is the purpose of eslint plugin for hooks?
The ESLint plugin enforces rules of Hooks to avoid bugs. It assumes that any function starting with ”use” and a capital letter right after it is a Hook. In particular, the rule enforces that,
1. Calls to Hooks are either inside a PascalCase function (assumed to be a component) or another useSomething function (assumed to be a custom Hook).
2. Hooks are called in the same order on every render.
**[⬆ Back to Top](#table-of-contents)**
323. ### What is the difference between Imperative and Declarative in React?
Imagine a simple UI component, such as a "Like" button. When you tap it, it turns blue if it was previously grey, and grey if it was previously blue.
The imperative way of doing this would be:
```javascript
if (user.likes()) {
if (hasBlue()) {
removeBlue();
addGrey();
} else {
removeGrey();
addBlue();
}
}
```
Basically, you have to check what is currently on the screen and handle all the changes necessary to redraw it with the current state, including undoing the changes from the previous state. You can imagine how complex this could be in a real-world scenario.
In contrast, the declarative approach would be:
```javascript
if (this.state.liked) {
return ;
} else {
return ;
}
```
Because the declarative approach separates concerns, this part of it only needs to handle how the UI should look in a sepecific state, and is therefore much simpler to understand.
**[⬆ Back to Top](#table-of-contents)**
324. ### What are the benefits of using typescript with reactjs?
Below are some of the benefits of using typescript with Reactjs,
1. It is possible to use latest JavaScript features
2. Use of interfaces for complex type definitions
3. IDEs such as VS Code was made for TypeScript
4. Avoid bugs with the ease of readability and Validation
**[⬆ Back to Top](#table-of-contents)**
325. ### How do you make sure that user remains authenticated on page refresh while using Context API State Management?
When a user logs in and reload, to persist the state generally we add the load user action in the useEffect hooks in the main App.js. While using Redux, loadUser action can be easily accessed.
**App.js**
```js
import { loadUser } from "../actions/auth";
store.dispatch(loadUser());
```
- But while using **Context API**, to access context in App.js, wrap the AuthState in index.js so that App.js can access the auth context. Now whenever the page reloads, no matter what route you are on, the user will be authenticated as **loadUser** action will be triggered on each re-render.
**index.js**
```js
import React from "react";
import ReactDOM from "react-dom";
import App from "./App";
import AuthState from "./context/auth/AuthState";
ReactDOM.render(
,
document.getElementById("root")
);
```
**App.js**
```js
const authContext = useContext(AuthContext);
const { loadUser } = authContext;
useEffect(() => {
loadUser();
}, []);
```
**loadUser**
```js
const loadUser = async () => {
const token = sessionStorage.getItem("token");
if (!token) {
dispatch({
type: ERROR,
});
}
setAuthToken(token);
try {
const res = await axios("/api/auth");
dispatch({
type: USER_LOADED,
payload: res.data.data,
});
} catch (err) {
console.error(err);
}
};
```
**[⬆ Back to Top](#table-of-contents)**
326. ### What are the benefits of new JSX transform?
There are three major benefits of new JSX transform,
1. It is possible to use JSX without importing React packages
2. The compiled output might improve the bundle size in a small amount
3. The future improvements provides the flexibility to reduce the number of concepts to learn React.
**[⬆ Back to Top](#table-of-contents)**
327. ### How is the new JSX transform different from old transform??
The new JSX transform doesn’t require React to be in scope. i.e, You don't need to import React package for simple scenarios.
Let's take an example to look at the main differences between the old and the new transform,
**Old Transform:**
```js
import React from "react";
function App() {
return
Good morning!!
;}
```
Now JSX transform convert the above code into regular JavaScript as below,
```js
import React from "react";
function App() {
return React.createElement("h1", null, "Good morning!!");
}
```
**New Transform:**
The new JSX transform doesn't require any React imports
```js
function App() {
return
Good morning!!
;}
```
Under the hood JSX transform compiles to below code
```js
import { jsx as _jsx } from "react/jsx-runtime";
function App() {
return _jsx("h1", { children: "Good morning!!" });
}
```
**Note:** You still need to import React to use Hooks.
**[⬆ Back to Top](#table-of-contents)**
328. ### How do you get redux scaffolding using create-react-app?
Redux team has provided official redux+js or redux+typescript templates for create-react-app project. The generated project setup includes,
1. Redux Toolkit and React-Redux dependencies
2. Create and configure Redux store
3. React-Redux `` passing the store to React components
4. Small "counter" example to demo how to add redux logic and React-Redux hooks API to interact with the store from components
The below commands need to be executed along with template option as below,
5. **Javascript template:**
```js
npx create-react-app my-app --template redux
```
2. **Typescript template:**
```js
npx create-react-app my-app --template redux-typescript
```
**[⬆ Back to Top](#table-of-contents)**
329. ### What are React Server components?
React Server Component is a way to write React component that gets rendered in the server-side with the purpose of improving React app performance. These components allow us to load components from the backend.
**Note:** React Server Components is still under development and not recommended for production yet.
**[⬆ Back to Top](#table-of-contents)**
330. ### What is prop drilling?
Prop Drilling is the process by which you pass data from one component of the React Component tree to another by going through other components that do not need the data but only help in passing it around.
**[⬆ Back to Top](#table-of-contents)**
331. ### What is state mutation and how to prevent it?
`State mutation` happens when you try to update the state of a component without actually using `setState` function. This can happen when you are trying to do some computations using a state variable and unknowingly save the result in the same state variable. This is the main reason why it is advised to return new instances of state variables from the reducers by using Object.assign({}, ...) or spread syntax.
This can cause unknown issues in the UI as the value of the state variable got updated without telling React to check what all components were being affected from this update and it can cause UI bugs.
Ex:
```javascript
class A extends React.component {
constructor(props) {
super(props);
this.state = {
loading: false
}
}
componentDidMount() {
let { loading } = this.state;
loading = (() => true)(); // Trying to perform an operation and directly saving in a state variable
}
```
**How to prevent it:** Make sure your state variables are immutable by either enforcing immutability by using plugins like Immutable.js, always using `setState` to make updates, and returning new instances in reducers when sending updated state values.
**[⬆ Back to Top](#table-of-contents)**
332. ### What is the difference between useState and useRef hook?
1. useState causes components to re-render after state updates whereas useRef doesn’t cause a component to re-render when the value or state changes.
Essentially, useRef is like a “box” that can hold a mutable value in its (.current) property.
2. useState allows us to update the state inside components. While useRef allows referencing DOM elements.
**[⬆ Back to Top](#table-of-contents)**
333. ### What is a wrapper component?
A wrapper in React is a component that wraps or surrounds another component or group of components. It can be used for a variety of purposes such as adding additional functionality, styling, or layout to the wrapped components.
For example, consider a simple component that displays a message:
```javascript
const Message = ({ text }) => {
return
{text}
;};
```
We can create a wrapper component that will add a border to the message component:
```javascript
const MessageWrapper = (props) => {
return (
);
};
```
Now we can use the MessageWrapper component instead of the Message component and the message will be displayed with a border:
```javascript
```
Wrapper component can also accept its own props and pass them down to the wrapped component, for example, we can create a wrapper component that will add a title to the message component:
```javascript
const MessageWrapperWithTitle = ({title, ...props}) => {
return (
{title}
);
};
```
Now we can use the MessageWrapperWithTitle component and pass title props:
```javascript
```
This way, the wrapper component can add additional functionality, styling, or layout to the wrapped component while keeping the wrapped component simple and reusable.
**[⬆ Back to Top](#table-of-contents)**
334. ### What are the differences between useEffect and useLayoutEffect hooks?
useEffect and useLayoutEffect are both React hooks that can be used to synchronize a component with an external system, such as a browser API or a third-party library. However, there are some key differences between the two:
- Timing: useEffect runs after the browser has finished painting, while useLayoutEffect runs synchronously before the browser paints. This means that useLayoutEffect can be used to measure and update layout in a way that feels more synchronous to the user.
- Browser Paint: useEffect allows browser to paint the changes before running the effect, hence it may cause some visual flicker. useLayoutEffect synchronously runs the effect before browser paints and hence it will avoid visual flicker.
- Execution Order: The order in which multiple useEffect hooks are executed is determined by React and may not be predictable. However, the order in which multiple useLayoutEffect hooks are executed is determined by the order in which they were called.
- Error handling: useEffect has a built-in mechanism for handling errors that occur during the execution of the effect, so that it does not crash the entire application. useLayoutEffect does not have this mechanism, and errors that occur during the execution of the effect will crash the entire application.
In general, it's recommended to use useEffect as much as possible, because it is more performant and less prone to errors. useLayoutEffect should only be used when you need to measure or update layout, and you can't achieve the same result using useEffect.
**[⬆ Back to Top](#table-of-contents)**
335. ### What are the differences between Functional and Class Components?
There are two different ways to create components in ReactJS. The main differences are listed down as below,
## 1. Syntax:
The classs components uses ES6 classes to create the components. It uses `render` function to display the HTML content in the webpage.
The syntax for class component looks like as below.
```js
class App extends Reacts.Component {
render(){
return
This is a class component
}}
```
**Note:** The **Pascal Case** is the recommended approach to provide naming to a component.
Functional component has been improved over the years with some added features like Hooks. Here is a syntax for functional component.
```js
function App(){
return
Hello, I'm a function component
}
```
## 2. State:
State contains information or data about a component which may change over time.
In class component, you can update the state when a user interacts with it or server updates the data using the `setState()` method. The initial state is going to be assigned in the `Constructor( ) `method using the the ` this.state` object and it is possible to different data types in the `this.state` object such as string, boolean, numbers, etc.
**A simple example showing how we use the setState() and constructor()**
```js
class App extends Component {
constructor() {
super();
this.state = {
message: "This is a class component",
};
}
updateMessage() {
this.setState({t
message: "Updating the class component",
});
}
render() {
return (
<>
{this.state.message}
{
this.updateMessage();
}}>
Click!!
>
);
}
}
```
You not use state in functional components because it was only supported in class components. But over the years hooks have been implemented in functional component which enable to use state in functional component too.
The `useState()` hook can used to implement state in funcitonal component. It returns an array with two items: the first item is current state and the next one is a function (setState) that updates the value of the current state.
Let's see an example to demonstrate the state in functional components,
```js
function App() {
const [message, setMessage] = useState("This is a functional component");
const updateMessage = () => {
setCountry("Updating the functional component");
};
return (
{message}
Click me!!
);
}
```
## 4. Props:
Props are referred to as "properties". The props are passed into react component just like arguments passed to a function. In otherwords, they are similar to HTML attributes.
The props are accessible in child class component using `this.props` as shown in below example,
```js
class Child extends React.Component {
render() {
return
This is a functional component and component name is {this.props.name}
;}
}
class Parent extends React.Component {
render() {
return (
);
}
}
```
Props in functional components are similar to that of the class components but the difference is the absence of 'this' keyword.
```js
function Child(props) {
return
This is a child component and the component name is{props.name}
;}
function Parent() {
return (
);
}
```
**[⬆ Back to Top](#table-of-contents)**
336. ### Why does strict mode render twice in React?
StrictMode renders components twice in development mode(not production) in order to detect any problems with your code and warn you about those problems. This is used to detect accidental side effects in the render phase. If you used `create-react-app` development tool then it automatically enables StrictMode by default.
```js
ReactDOM.render(
{App}
,
document.getElementById('root')
);
```
If you want to disable this behavior then you can remove `strict` mode.
```js
ReactDOM.render(
{App},
document.getElementById('root')
);
```
To detect side effects the following functions are invoked twice:
1. Class component constructor, render, and shouldComponentUpdate methods
2. Class component static getDerivedStateFromProps method
3. Function component bodies
4. State updater functions
5. Functions passed to useState, useMemo, or useReducer (any Hook)
**[⬆ Back to Top](#table-of-contents)**
## Disclaimer
The questions provided in this repository are the summary of frequently asked questions across numerous companies. We cannot guarantee that these questions will actually be asked during your interview process, nor should you focus on memorizing all of them. The primary purpose is for you to get a sense of what some companies might ask — do not get discouraged if you don't know the answer to all of them — that is ok!
Good luck with your interview 😊
---
## JavaScript Interview Questions
### Table of Contents
| No. | Questions |
| --- | ------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| 1 | [What are the possible ways to create objects in JavaScript](#what-are-the-possible-ways-to-create-objects-in-javascript) |
| 2 | [What is a prototype chain](#what-is-a-prototype-chain) |
| 3 | [What is the difference between Call, Apply and Bind](#what-is-the-difference-between-call-apply-and-bind) |
| 4 | [What is JSON and its common operations](#what-is-json-and-its-common-operations) |
| 5 | [What is the purpose of the array slice method](#what-is-the-purpose-of-the-array-slice-method) |
| 6 | [What is the purpose of the array splice method](#what-is-the-purpose-of-the-array-splice-method) |
| 7 | [What is the difference between slice and splice](#what-is-the-difference-between-slice-and-splice) |
| 8 | [How do you compare an Object with a Map](#how-do-you-compare-object-and-map) |
| 9 | [What is the difference between == and === operators](#what-is-the-difference-between--and--operators) |
| 10 | [What are lambda or arrow functions](#what-are-lambda-or-arrow-functions) |
| 11 | [What is a first class function](#what-is-a-first-class-function) |
| 12 | [What is a first order function](#what-is-a-first-order-function) |
| 13 | [What is a higher order function](#what-is-a-higher-order-function) |
| 14 | [What is a unary function](#what-is-a-unary-function) |
| 15 | [What is the currying function](#what-is-the-currying-function) |
| 16 | [What is a pure function](#what-is-a-pure-function) |
| 17 | [What is the purpose of the let keyword](#what-is-the-purpose-of-the-let-keyword) |
| 18 | [What is the difference between let and var](#what-is-the-difference-between-let-and-var) |
| 19 | [What is the reason to choose the name let as a keyword](#what-is-the-reason-to-choose-the-name-let-as-a-keyword) |
| 20 | [How do you redeclare variables in switch block without an error](#how-do-you-redeclare-variables-in-switch-block-without-an-error) |
| 21 | [What is the Temporal Dead Zone](#what-is-the-temporal-dead-zone) |
| 22 | [What is IIFE(Immediately Invoked Function Expression)](#what-is-iifeimmediately-invoked-function-expression) |
| 23 | [How do you decode or encode a URL in JavaScript?](#how-do-you-decode-or-encode-a-url-in-javascript) |
| 24 | [What is memoization](#what-is-memoization) |
| 25 | [What is Hoisting](#what-is-hoisting) |
| 26 | [What are classes in ES6](#what-are-classes-in-es6) |
| 27 | [What are closures](#what-are-closures) |
| 28 | [What are modules](#what-are-modules) |
| 29 | [Why do you need modules](#why-do-you-need-modules) |
| 30 | [What is scope in javascript](#what-is-scope-in-javascript) |
| 31 | [What is a service worker](#what-is-a-service-worker) |
| 32 | [How do you manipulate DOM using a service worker](#how-do-you-manipulate-dom-using-a-service-worker) |
| 33 | [How do you reuse information across service worker restarts](#how-do-you-reuse-information-across-service-worker-restarts) |
| 34 | [What is IndexedDB](#what-is-indexeddb) |
| 35 | [What is web storage](#what-is-web-storage) |
| 36 | [What is a post message](#what-is-a-post-message) |
| 37 | [What is a cookie](#what-is-a-cookie) |
| 38 | [Why do you need a Cookie](#why-do-you-need-a-cookie) |
| 39 | [What are the options in a cookie](#what-are-the-options-in-a-cookie) |
| 40 | [How do you delete a cookie](#how-do-you-delete-a-cookie) |
| 41 | [What are the differences between cookie, local storage and session storage](#What-are-the-differences-between-cookie-local-storage-and-session-storage) |
| 42 | [What is the main difference between localStorage and sessionStorage](#what-is-the-main-difference-between-localstorage-and-sessionstorage) |
| 43 | [How do you access web storage](#how-do-you-access-web-storage) |
| 44 | [What are the methods available on session storage](#what-are-the-methods-available-on-session-storage) |
| 45 | [What is a storage event and its event handler](#what-is-a-storage-event-and-its-event-handler) |
| 46 | [Why do you need web storage](#why-do-you-need-web-storage) |
| 47 | [How do you check web storage browser support](#how-do-you-check-web-storage-browser-support) |
| 48 | [How do you check web workers browser support](#how-do-you-check-web-workers-browser-support) |
| 49 | [Give an example of a web worker](#give-an-example-of-a-web-worker) |
| 50 | [What are the restrictions of web workers on DOM](#what-are-the-restrictions-of-web-workers-on-dom) |
| 51 | [What is a promise](#what-is-a-promise) |
| 52 | [Why do you need a promise](#why-do-you-need-a-promise) |
| 53 | [What are the three states of promise](#what-are-the-three-states-of-promise) |
| 54 | [What is a callback function](#what-is-a-callback-function) |
| 55 | [Why do we need callbacks](#why-do-we-need-callbacks) |
| 56 | [What is a callback hell](#what-is-a-callback-hell) |
| 57 | [What are server-sent events](#what-are-server-sent-events) |
| 58 | [How do you receive server-sent event notifications](#how-do-you-receive-server-sent-event-notifications) |
| 59 | [How do you check browser support for server-sent events](#how-do-you-check-browser-support-for-server-sent-events) |
| 60 | [What are the events available for server sent events](#what-are-the-events-available-for-server-sent-events) |
| 61 | [What are the main rules of promise](#what-are-the-main-rules-of-promise) |
| 62 | [What is callback in callback](#what-is-callback-in-callback) |
| 63 | [What is promise chaining](#what-is-promise-chaining) |
| 64 | [What is promise.all](#what-is-promiseall) |
| 65 | [What is the purpose of the race method in promise](#what-is-the-purpose-of-the-race-method-in-promise) |
| 66 | [What is a strict mode in javascript](#what-is-a-strict-mode-in-javascript) |
| 67 | [Why do you need strict mode](#why-do-you-need-strict-mode) |
| 68 | [How do you declare strict mode](#how-do-you-declare-strict-mode) |
| 69 | [What is the purpose of double exclamation](#what-is-the-purpose-of-double-exclamation) |
| 70 | [What is the purpose of the delete operator](#what-is-the-purpose-of-the-delete-operator) |
| 71 | [What is typeof operator](#what-is-typeof-operator) |
| 72 | [What is undefined property](#what-is-undefined-property) |
| 73 | [What is null value](#what-is-null-value) |
| 74 | [What is the difference between null and undefined](#what-is-the-difference-between-null-and-undefined) |
| 75 | [What is eval](#What-is-eval) |
| 76 | [What is the difference between window and document](#what-is-the-difference-between-window-and-document) |
| 77 | [How do you access history in javascript](#how-do-you-access-history-in-javascript) |
| 78 | [How do you detect caps lock key turned on or not](#how-do-you-detect-caps-lock-key-turned-on-or-not) |
| 79 | [What is isNaN](#what-is-isnan) |
| 80 | [What are the differences between undeclared and undefined variables](#what-are-the-differences-between-undeclared-and-undefined-variables) |
| 81 | [What are global variables](#what-are-global-variables) |
| 82 | [What are the problems with global variables](#what-are-the-problems-with-global-variables) |
| 83 | [What is NaN property](#what-is-nan-property) |
| 84 | [What is the purpose of isFinite function](#what-is-the-purpose-of-isfinite-function) |
| 85 | [What is an event flow](#what-is-an-event-flow) |
| 86 | [What is event bubbling](#what-is-event-bubbling) |
| 87 | [What is event capturing](#what-is-event-capturing) |
| 88 | [How do you submit a form using JavaScript](#how-do-you-submit-a-form-using-javascript) |
| 89 | [How do you find operating system details](#how-do-you-find-operating-system-details) |
| 90 | [What is the difference between document load and DOMContentLoaded events](#what-is-the-difference-between-document-load-and-domcontentloaded-events) |
| 91 | [What is the difference between native, host and user objects](#what-is-the-difference-between-native-host-and-user-objects) |
| 92 | [What are the tools or techniques used for debugging JavaScript code](#what-are-the-tools-or-techniques-used-for-debugging-javascript-code) |
| 93 | [What are the pros and cons of promises over callbacks](#what-are-the-pros-and-cons-of-promises-over-callbacks) |
| 94 | [What is the difference between an attribute and a property](#what-is-the-difference-between-an-attribute-and-a-property) |
| 95 | [What is same-origin policy](#what-is-same-origin-policy) |
| 96 | [What is the purpose of void 0](#what-is-the-purpose-of-void-0) |
| 97 | [Is JavaScript a compiled or interpreted language](#is-javascript-a-compiled-or-interpreted-language) |
| 98 | [Is JavaScript a case-sensitive language](#is-javascript-a-case-sensitive-language) |
| 99 | [Is there any relation between Java and JavaScript](#is-there-any-relation-between-java-and-javascript) |
| 100 | [What are events](#what-are-events) |
| 101 | [Who created javascript](#who-created-javascript) |
| 102 | [What is the use of preventDefault method](#what-is-the-use-of-preventdefault-method) |
| 103 | [What is the use of stopPropagation method](#what-is-the-use-of-stoppropagation-method) |
| 104 | [What are the steps involved in return false usage](#what-are-the-steps-involved-in-return-false-usage) |
| 105 | [What is BOM](#what-is-bom) |
| 106 | [What is the use of setTimeout](#what-is-the-use-of-settimeout) |
| 107 | [What is the use of setInterval](#what-is-the-use-of-setinterval) |
| 108 | [Why is JavaScript treated as Single threaded](#why-is-javascript-treated-as-single-threaded) |
| 109 | [What is an event delegation](#what-is-an-event-delegation) |
| 110 | [What is ECMAScript](#what-is-ecmascript) |
| 111 | [What is JSON](#what-is-json) |
| 112 | [What are the syntax rules of JSON](#what-are-the-syntax-rules-of-json) |
| 113 | [What is the purpose JSON stringify](#what-is-the-purpose-json-stringify) |
| 114 | [How do you parse JSON string](#how-do-you-parse-json-string) |
| 115 | [Why do you need JSON](#why-do-you-need-json) |
| 116 | [What are PWAs](#what-are-pwas) |
| 117 | [What is the purpose of clearTimeout method](#what-is-the-purpose-of-cleartimeout-method) |
| 118 | [What is the purpose of clearInterval method](#what-is-the-purpose-of-clearinterval-method) |
| 119 | [How do you redirect new page in javascript](#how-do-you-redirect-new-page-in-javascript) |
| 120 | [How do you check whether a string contains a substring](#how-do-you-check-whether-a-string-contains-a-substring) |
| 121 | [How do you validate an email in javascript](#how-do-you-validate-an-email-in-javascript) |
| 122 | [How do you get the current url with javascript](#how-do-you-get-the-current-url-with-javascript) |
| 123 | [What are the various url properties of location object](#what-are-the-various-url-properties-of-location-object) |
| 124 | [How do get query string values in javascript](#how-do-get-query-string-values-in-javascript) |
| 125 | [How do you check if a key exists in an object](#how-do-you-check-if-a-key-exists-in-an-object) |
| 126 | [How do you loop through or enumerate javascript object](#how-do-you-loop-through-or-enumerate-javascript-object) |
| 127 | [How do you test for an empty object](#how-do-you-test-for-an-empty-object) |
| 128 | [What is an arguments object](#what-is-an-arguments-object) |
| 129 | [How do you convert the first letter of a string to uppercase](#how-do-you-make-first-letter-of-the-string-in-an-uppercase) |
| 130 | [What are the pros and cons of for loop](#what-are-the-pros-and-cons-of-for-loop) |
| 131 | [How do you display the current date in javascript](#how-do-you-display-the-current-date-in-javascript) |
| 132 | [How do you compare two date objects](#how-do-you-compare-two-date-objects) |
| 133 | [How do you check if a string starts with another string](#how-do-you-check-if-a-string-starts-with-another-string) |
| 134 | [How do you trim a string in javascript](#how-do-you-trim-a-string-in-javascript) |
| 135 | [How do you add a key value pair in javascript](#how-do-you-add-a-key-value-pair-in-javascript) |
| 136 | [Is the '!--' notation represents a special operator](#is-the----notation-represents-a-special-operator) |
| 137 | [How do you assign default values to variables](#how-do-you-assign-default-values-to-variables) |
| 138 | [How do you define multiline strings](#how-do-you-define-multiline-strings) |
| 139 | [What is an app shell model](#what-is-an-app-shell-model) |
| 140 | [Can we define properties for functions](#can-we-define-properties-for-functions) |
| 141 | [What is the way to find the number of parameters expected by a function](#what-is-the-way-to-find-the-number-of-parameters-expected-by-a-function) |
| 142 | [What is a polyfill](#what-is-a-polyfill) |
| 143 | [What are break and continue statements](#what-are-break-and-continue-statements) |
| 144 | [What are js labels](#what-are-js-labels) |
| 145 | [What are the benefits of keeping declarations at the top](#what-are-the-benefits-of-keeping-declarations-at-the-top) |
| 146 | [What are the benefits of initializing variables](#what-are-the-benefits-of-initializing-variables) |
| 147 | [What are the recommendations to create new object](#what-are-the-recommendations-to-create-new-object) |
| 148 | [How do you define JSON arrays](#how-do-you-define-json-arrays) |
| 149 | [How do you generate random integers](#how-do-you-generate-random-integers) |
| 150 | [Can you write a random integers function to print integers with in a range](#can-you-write-a-random-integers-function-to-print-integers-with-in-a-range) |
| 151 | [What is tree shaking](#what-is-tree-shaking) |
| 152 | [What is the need of tree shaking](#what-is-the-need-of-tree-shaking) |
| 153 | [Is it recommended to use eval](#is-it-recommended-to-use-eval) |
| 154 | [What is a Regular Expression](#what-is-a-regular-expression) |
| 155 | [What are the string methods available in Regular expression](#what-are-the-string-methods-available-in-regular-expression) |
| 156 | [What are modifiers in regular expression](#what-are-modifiers-in-regular-expression) |
| 157 | [What are regular expression patterns](#what-are-regular-expression-patterns) |
| 158 | [What is a RegExp object](#what-is-a-regexp-object) |
| 159 | [How do you search a string for a pattern](#how-do-you-search-a-string-for-a-pattern) |
| 160 | [What is the purpose of exec method](#what-is-the-purpose-of-exec-method) |
| 161 | [How do you change the style of a HTML element](#how-do-you-change-the-style-of-a-html-element) |
| 162 | [What would be the result of 1+2+'3'](#what-would-be-the-result-of-123) |
| 163 | [What is a debugger statement](#what-is-a-debugger-statement) |
| 164 | [What is the purpose of breakpoints in debugging](#what-is-the-purpose-of-breakpoints-in-debugging) |
| 165 | [Can I use reserved words as identifiers](#can-i-use-reserved-words-as-identifiers) |
| 166 | [How do you detect a mobile browser](#how-do-you-detect-a-mobile-browser) |
| 167 | [How do you detect a mobile browser without regexp](#how-do-you-detect-a-mobile-browser-without-regexp) |
| 168 | [How do you get the image width and height using JS](#how-do-you-get-the-image-width-and-height-using-js) |
| 169 | [How do you make synchronous HTTP request](#how-do-you-make-synchronous-http-request) |
| 170 | [How do you make asynchronous HTTP request](#how-do-you-make-asynchronous-http-request) |
| 171 | [How do you convert date to another timezone in javascript](#how-do-you-convert-date-to-another-timezone-in-javascript) |
| 172 | [What are the properties used to get size of window](#what-are-the-properties-used-to-get-size-of-window) |
| 173 | [What is a conditional operator in javascript](#what-is-a-conditional-operator-in-javascript) |
| 174 | [Can you apply chaining on conditional operator](#Can-you-apply-chaining-on-conditional-operator) |
| 175 | [What are the ways to execute javascript after page load](#what-are-the-ways-to-execute-javascript-after-page-load) |
| 176 | [What is the difference between proto and prototype](#what-is-the-difference-between-proto-and-prototype) |
| 177 | [Give an example where do you really need semicolon](#give-an-example-where-do-you-really-need-semicolon) |
| 178 | [What is a freeze method](#what-is-a-freeze-method) |
| 179 | [What is the purpose of freeze method](#what-is-the-purpose-of-freeze-method) |
| 180 | [Why do I need to use freeze method](#why-do-i-need-to-use-freeze-method) |
| 181 | [How do you detect a browser language preference](#how-do-you-detect-a-browser-language-preference) |
| 182 | [How to convert string to title case with javascript](#how-to-convert-string-to-title-case-with-javascript) |
| 183 | [How do you detect javascript disabled in the page](#how-do-you-detect-javascript-disabled-in-the-page) |
| 184 | [What are various operators supported by javascript](#what-are-various-operators-supported-by-javascript) |
| 185 | [What is a rest parameter](#what-is-a-rest-parameter) |
| 186 | [What happens if you do not use rest parameter as a last argument](#what-happens-if-you-do-not-use-rest-parameter-as-a-last-argument) |
| 187 | [What are the bitwise operators available in javascript](#what-are-the-bitwise-operators-available-in-javascript) |
| 188 | [What is a spread operator](#what-is-a-spread-operator) |
| 189 | [How do you determine whether object is frozen or not](#how-do-you-determine-whether-object-is-frozen-or-not) |
| 190 | [How do you determine two values same or not using object](#how-do-you-determine-two-values-same-or-not-using-object) |
| 191 | [What is the purpose of using object is method](#what-is-the-purpose-of-using-object-is-method) |
| 192 | [How do you copy properties from one object to other](#how-do-you-copy-properties-from-one-object-to-other) |
| 193 | [What are the applications of assign method](#what-are-the-applications-of-assign-method) |
| 194 | [What is a proxy object](#what-is-a-proxy-object) |
| 195 | [What is the purpose of seal method](#what-is-the-purpose-of-seal-method) |
| 196 | [What are the applications of seal method](#what-are-the-applications-of-seal-method) |
| 197 | [What are the differences between freeze and seal methods](#what-are-the-differences-between-freeze-and-seal-methods) |
| 198 | [How do you determine if an object is sealed or not](#how-do-you-determine-if-an-object-is-sealed-or-not) |
| 199 | [How do you get enumerable key and value pairs](#how-do-you-get-enumerable-key-and-value-pairs) |
| 200 | [What is the main difference between Object.values and Object.entries method](#what-is-the-main-difference-between-objectvalues-and-objectentries-method) |
| 201 | [How can you get the list of keys of any object](#how-can-you-get-the-list-of-keys-of-any-object) |
| 202 | [How do you create an object with prototype](#how-do-you-create-an-object-with-prototype) |
| 203 | [What is a WeakSet](#what-is-a-weakset) |
| 204 | [What are the differences between a WeakSet and a Set](#what-are-the-differences-between-weakset-and-set) |
| 205 | [List down the collection of methods available on WeakSet](#list-down-the-collection-of-methods-available-on-weakset) |
| 206 | [What is a WeakMap](#what-is-a-weakmap) |
| 207 | [What are the differences between a WeakMap and a Map](#what-are-the-differences-between-weakmap-and-map) |
| 208 | [List down the collection of methods available on WeakMap](#list-down-the-collection-of-methods-available-on-weakmap) |
| 209 | [What is the purpose of uneval](#what-is-the-purpose-of-uneval) |
| 210 | [How do you encode an URL](#how-do-you-encode-an-url) |
| 211 | [How do you decode an URL](#how-do-you-decode-an-url) |
| 212 | [How do you print the contents of web page](#how-do-you-print-the-contents-of-web-page) |
| 213 | [What is the difference between uneval and eval](#what-is-the-difference-between-uneval-and-eval) |
| 214 | [What is an anonymous function](#what-is-an-anonymous-function) |
| 215 | [What is the precedence order between local and global variables](#what-is-the-precedence-order-between-local-and-global-variables) |
| 216 | [What are javascript accessors](#what-are-javascript-accessors) |
| 217 | [How do you define property on Object constructor](#how-do-you-define-property-on-object-constructor) |
| 218 | [What is the difference between get and defineProperty](#what-is-the-difference-between-get-and-defineproperty) |
| 219 | [What are the advantages of Getters and Setters](#what-are-the-advantages-of-getters-and-setters) |
| 220 | [Can I add getters and setters using defineProperty method](#can-i-add-getters-and-setters-using-defineproperty-method) |
| 221 | [What is the purpose of switch-case](#what-is-the-purpose-of-switch-case) |
| 222 | [What are the conventions to be followed for the usage of switch case](#what-are-the-conventions-to-be-followed-for-the-usage-of-switch-case) |
| 223 | [What are primitive data types](#what-are-primitive-data-types) |
| 224 | [What are the different ways to access object properties](#what-are-the-different-ways-to-access-object-properties) |
| 225 | [What are the function parameter rules](#what-are-the-function-parameter-rules) |
| 226 | [What is an error object](#what-is-an-error-object) |
| 227 | [When you get a syntax error](#when-you-get-a-syntax-error) |
| 228 | [What are the different error names from error object](#what-are-the-different-error-names-from-error-object) |
| 229 | [What are the various statements in error handling](#what-are-the-various-statements-in-error-handling) |
| 230 | [What are the two types of loops in javascript](#what-are-the-two-types-of-loops-in-javascript) |
| 231 | [What is nodejs](#what-is-nodejs) |
| 232 | [What is an Intl object](#what-is-an-intl-object) |
| 233 | [How do you perform language specific date and time formatting](#how-do-you-perform-language-specific-date-and-time-formatting) |
| 234 | [What is an Iterator](#what-is-an-iterator) |
| 235 | [How does the synchronous iteration work](#how-does-synchronous-iteration-works) |
| 236 | [What is an event loop](#what-is-an-event-loop) |
| 237 | [What is call stack](#what-is-call-stack) |
| 238 | [What is an event queue](#what-is-an-event-queue) |
| 239 | [What is a decorator](#what-is-a-decorator) |
| 240 | [What are the properties of Intl object](#what-are-the-properties-of-intl-object) |
| 241 | [What is an Unary operator](#what-is-an-unary-operator) |
| 242 | [How do you sort elements in an array](#how-do-you-sort-elements-in-an-array) |
| 243 | [What is the purpose of compareFunction while sorting arrays](#what-is-the-purpose-of-comparefunction-while-sorting-arrays) |
| 244 | [How do you reversing an array](#how-do-you-reversing-an-array) |
| 245 | [How do you find min and max value in an array](#how-do-you-find-min-and-max-value-in-an-array) |
| 246 | [How do you find min and max values without Math functions](#how-do-you-find-min-and-max-values-without-math-functions) |
| 247 | [What is an empty statement and purpose of it](#what-is-an-empty-statement-and-purpose-of-it) |
| 248 | [How do you get metadata of a module](#how-do-you-get-metadata-of-a-module) |
| 249 | [What is a comma operator](#what-is-a-comma-operator) |
| 250 | [What is the advantage of a comma operator](#what-is-the-advantage-of-a-comma-operator) |
| 251 | [What is typescript](#what-is-typescript) |
| 252 | [What are the differences between javascript and typescript](#what-are-the-differences-between-javascript-and-typescript) |
| 253 | [What are the advantages of typescript over javascript](#what-are-the-advantages-of-typescript-over-javascript) |
| 254 | [What is an object initializer](#what-is-an-object-initializer) |
| 255 | [What is a constructor method](#what-is-a-constructor-method) |
| 256 | [What happens if you write constructor more than once in a class](#what-happens-if-you-write-constructor-more-than-once-in-a-class) |
| 257 | [How do you call the constructor of a parent class](#how-do-you-call-the-constructor-of-a-parent-class) |
| 258 | [How do you get the prototype of an object](#how-do-you-get-the-prototype-of-an-object) |
| 259 | [What happens If I pass string type for getPrototype method](#what-happens-if-i-pass-string-type-for-getprototype-method) |
| 260 | [How do you set prototype of one object to another](#how-do-you-set-prototype-of-one-object-to-another) |
| 261 | [How do you check whether an object can be extendable or not](#how-do-you-check-whether-an-object-can-be-extendable-or-not) |
| 262 | [How do you prevent an object to extend](#how-do-you-prevent-an-object-to-extend) |
| 263 | [What are the different ways to make an object non-extensible](#what-are-the-different-ways-to-make-an-object-non-extensible) |
| 264 | [How do you define multiple properties on an object](#how-do-you-define-multiple-properties-on-an-object) |
| 265 | [What is MEAN in javascript](#what-is-mean-in-javascript) |
| 266 | [What Is Obfuscation in javascript](#what-is-obfuscation-in-javascript) |
| 267 | [Why do you need Obfuscation](#why-do-you-need-obfuscation) |
| 268 | [What is Minification](#what-is-minification) |
| 269 | [What are the advantages of minification](#what-are-the-advantages-of-minification) |
| 270 | [What are the differences between Obfuscation and Encryption](#what-are-the-differences-between-obfuscation-and-encryption) |
| 271 | [What are the common tools used for minification](#what-are-the-common-tools-used-for-minification) |
| 272 | [How do you perform form validation using javascript](#how-do-you-perform-form-validation-using-javascript) |
| 273 | [How do you perform form validation without javascript](#how-do-you-perform-form-validation-without-javascript) |
| 274 | [What are the DOM methods available for constraint validation](#what-are-the-dom-methods-available-for-constraint-validation) |
| 275 | [What are the available constraint validation DOM properties](#what-are-the-available-constraint-validation-dom-properties) |
| 276 | [What are the list of validity properties](#what-are-the-list-of-validity-properties) |
| 277 | [Give an example usage of rangeOverflow property](#give-an-example-usage-of-rangeoverflow-property) |
| 278 | [Is enums feature available in javascript](#is-enums-feature-available-in-javascript) |
| 279 | [What is an enum](#What-is-an-enum) |
| 280 | [How do you list all properties of an object](#how-do-you-list-all-properties-of-an-object) |
| 281 | [How do you get property descriptors of an object](#how-do-you-get-property-descriptors-of-an-object) |
| 282 | [What are the attributes provided by a property descriptor](#what-are-the-attributes-provided-by-a-property-descriptor) |
| 283 | [How do you extend classes](#how-do-you-extend-classes) |
| 284 | [How do I modify the url without reloading the page](#how-do-i-modify-the-url-without-reloading-the-page) |
| 285 | [How do you check whether an array includes a particular value or not](#how-do-you-check-whether-an-array-includes-a-particular-value-or-not) |
| 286 | [How do you compare scalar arrays](#how-do-you-compare-scalar-arrays) |
| 287 | [How to get the value from get parameters](#how-to-get-the-value-from-get-parameters) |
| 288 | [How do you print numbers with commas as thousand separators](#how-do-you-print-numbers-with-commas-as-thousand-separators) |
| 289 | [What is the difference between java and javascript](#what-is-the-difference-between-java-and-javascript) |
| 290 | [Does javascript supports namespace](#does-javascript-supports-namespace) |
| 291 | [How do you declare namespace](#how-do-you-declare-namespace) |
| 292 | [How do you invoke javascript code in an iframe from parent page](#how-do-you-invoke-javascript-code-in-an-iframe-from-parent-page) |
| 293 | [How do get the timezone offset from date](#how-do-get-the-timezone-offset-from-date) |
| 294 | [How do you load CSS and JS files dynamically](#how-do-you-load-css-and-js-files-dynamically) |
| 295 | [What are the different methods to find HTML elements in DOM](#what-are-the-different-methods-to-find-html-elements-in-dom) |
| 296 | [What is jQuery](#what-is-jquery) |
| 297 | [What is V8 JavaScript engine](#what-is-v8-javascript-engine) |
| 298 | [Why do we call javascript as dynamic language](#why-do-we-call-javascript-as-dynamic-language) |
| 299 | [What is a void operator](#what-is-a-void-operator) |
| 300 | [How to set the cursor to wait](#how-to-set-the-cursor-to-wait) |
| 301 | [How do you create an infinite loop](#how-do-you-create-an-infinite-loop) |
| 302 | [Why do you need to avoid with statement](#why-do-you-need-to-avoid-with-statement) |
| 303 | [What is the output of below for loops](#what-is-the-output-of-below-for-loops) |
| 304 | [List down some of the features of ES6](#list-down-some-of-the-features-of-es6) |
| 305 | [What is ES6](#what-is-es6) |
| 306 | [Can I redeclare let and const variables](#can-I-redeclare-let-and-const-variables) |
| 307 | [Is const variable makes the value immutable](#is-const-variable-makes-the-value-immutable) |
| 308 | [What are default parameters](#what-are-default-parameters) |
| 309 | [What are template literals](#what-are-template-literals) |
| 310 | [How do you write multi-line strings in template literals](#how-do-you-write-multi-line-strings-in-template-literals) |
| 311 | [What are nesting templates](#what-are-nesting-templates) |
| 312 | [What are tagged templates](#what-are-tagged-templates) |
| 313 | [What are raw strings](#what-are-raw-strings) |
| 314 | [What is destructuring assignment](#what-is-destructuring-assignment) |
| 315 | [What are default values in destructuring assignment](#what-are-default-values-in-destructuring-assignment) |
| 316 | [How do you swap variables in destructuring assignment](#how-do-you-swap-variables-in-destructuring-assignment) |
| 317 | [What are enhanced object literals](#what-are-enhanced-object-literals) |
| 318 | [What are dynamic imports](#what-are-dynamic-imports) |
| 319 | [What are the use cases for dynamic imports](#what-are-the-use-cases-for-dynamic-imports) |
| 320 | [What are typed arrays](#what-are-typed-arrays) |
| 321 | [What are the advantages of module loaders](#what-are-the-advantages-of-module-loaders) |
| 322 | [What is collation](#what-is-collation) |
| 323 | [What is for...of statement](#what-is-forof-statement) |
| 324 | [What is the output of below spread operator array](#what-is-the-output-of-below-spread-operator-array) |
| 325 | [Is PostMessage secure](#is-postmessage-secure) |
| 326 | [What are the problems with postmessage target origin as wildcard](#what-are-the-problems-with-postmessage-target-origin-as-wildcard) |
| 327 | [How do you avoid receiving postMessages from attackers](#how-do-you-avoid-receiving-postmessages-from-attackers) |
| 328 | [Can I avoid using postMessages completely](#can-i-avoid-using-postmessages-completely) |
| 329 | [Is postMessages synchronous](#is-postmessages-synchronous) |
| 330 | [What paradigm is Javascript](#what-paradigm-is-javascript) |
| 331 | [What is the difference between internal and external javascript](#what-is-the-difference-between-internal-and-external-javascript) |
| 332 | [Is JavaScript faster than server side script](#is-javascript-faster-than-server-side-script) |
| 333 | [How do you get the status of a checkbox](#how-do-you-get-the-status-of-a-checkbox) |
| 334 | [What is the purpose of double tilde operator](#what-is-the-purpose-of-double-tilde-operator) |
| 335 | [How do you convert character to ASCII code](#how-do-you-convert-character-to-ascii-code) |
| 336 | [What is ArrayBuffer](#what-is-arraybuffer) |
| 337 | [What is the output of below string expression](#what-is-the-output-of-below-string-expression) |
| 338 | [What is the purpose of Error object](#what-is-the-purpose-of-error-object) |
| 339 | [What is the purpose of EvalError object](#what-is-the-purpose-of-evalerror-object) |
| 340 | [What are the list of cases error thrown from non-strict mode to strict mode](#what-are-the-list-of-cases-error-thrown-from-non-strict-mode-to-strict-mode) |
| 341 | [Do all objects have prototypes](#do-all-objects-have-prototypes) |
| 342 | [What is the difference between a parameter and an argument](#what-is-the-difference-between-a-parameter-and-an-argument) |
| 343 | [What is the purpose of some method in arrays](#what-is-the-purpose-of-some-method-in-arrays) |
| 344 | [How do you combine two or more arrays](#how-do-you-combine-two-or-more-arrays) |
| 345 | [What is the difference between Shallow and Deep copy](#what-is-the-difference-between-shallow-and-deep-copy) |
| 346 | [How do you create specific number of copies of a string](#how-do-you-create-specific-number-of-copies-of-a-string) |
| 347 | [How do you return all matching strings against a regular expression](#how-do-you-return-all-matching-strings-against-a-regular-expression) |
| 348 | [How do you trim a string at the beginning or ending](#how-do-you-trim-a-string-at-the-beginning-or-ending) |
| 349 | [What is the output of below console statement with unary operator](#what-is-the-output-of-below-console-statement-with-unary-operator) |
| 350 | [Does javascript uses mixins](#does-javascript-uses-mixins) |
| 351 | [What is a thunk function](#what-is-a-thunk-function) |
| 352 | [What are asynchronous thunks](#what-are-asynchronous-thunks) |
| 353 | [What is the output of below function calls](#what-is-the-output-of-below-function-calls) |
| 354 | [How to remove all line breaks from a string](#how-to-remove-all-line-breaks-from-a-string) |
| 355 | [What is the difference between reflow and repaint](#what-is-the-difference-between-reflow-and-repaint) |
| 356 | [What happens with negating an array](#what-happens-with-negating-an-array) |
| 357 | [What happens if we add two arrays](#what-happens-if-we-add-two-arrays) |
| 358 | [What is the output of prepend additive operator on falsy values](#what-is-the-output-of-prepend-additive-operator-on-falsy-values) |
| 359 | [How do you create self string using special characters](#how-do-you-create-self-string-using-special-characters) |
| 360 | [How do you remove falsy values from an array](#how-do-you-remove-falsy-values-from-an-array) |
| 361 | [How do you get unique values of an array](#how-do-you-get-unique-values-of-an-array) |
| 362 | [What is destructuring aliases](#what-is-destructuring-aliases) |
| 363 | [How do you map the array values without using map method](#how-do-you-map-the-array-values-without-using-map-method) |
| 364 | [How do you empty an array](#how-do-you-empty-an-array) |
| 365 | [How do you round numbers to certain decimals](#how-do-you-rounding-numbers-to-certain-decimals) |
| 366 | [What is the easiest way to convert an array to an object](#what-is-the-easiest-way-to-convert-an-array-to-an-object) |
| 367 | [How do you create an array with some data](#how-do-you-create-an-array-with-some-data) |
| 368 | [What are the placeholders from console object](#what-are-the-placeholders-from-console-object) |
| 369 | [Is it possible to add CSS to console messages](#is-it-possible-to-add-css-to-console-messages) |
| 370 | [What is the purpose of dir method of console object](#what-is-the-purpose-of-dir-method-of-console-object) |
| 371 | [Is it possible to debug HTML elements in console](#is-it-possible-to-debug-html-elements-in-console) |
| 372 | [How do you display data in a tabular format using console object](#how-do-you-display-data-in-a-tabular-format-using-console-object) |
| 373 | [How do you verify that an argument is a Number or not](#how-do-you-verify-that-an-argument-is-a-number-or-not) |
| 374 | [How do you create copy to clipboard button](#how-do-you-create-copy-to-clipboard-button) |
| 375 | [What is the shortcut to get timestamp](#what-is-the-shortcut-to-get-timestamp) |
| 376 | [How do you flattening multi dimensional arrays](#how-do-you-flattening-multi-dimensional-arrays) |
| 377 | [What is the easiest multi condition checking](#what-is-the-easiest-multi-condition-checking) |
| 378 | [How do you capture browser back button](#how-do-you-capture-browser-back-button) |
| 379 | [How do you disable right click in the web page](#how-do-you-disable-right-click-in-the-web-page) |
| 380 | [What are wrapper objects](#what-are-wrapper-objects) |
| 381 | [What is AJAX](#what-is-ajax) |
| 382 | [What are the different ways to deal with Asynchronous Code](#what-are-the-different-ways-to-deal-with-asynchronous-code) |
| 383 | [How to cancel a fetch request](#how-to-cancel-a-fetch-request) |
| 384 | [What is web speech API](#what-is-web-speech-api) |
| 385 | [What is minimum timeout throttling](#what-is-minimum-timeout-throttling) |
| 386 | [How do you implement zero timeout in modern browsers](#how-do-you-implement-zero-timeout-in-modern-browsers) |
| 387 | [What are tasks in event loop](#what-are-tasks-in-event-loop) |
| 388 | [What is microtask](#what-is-microtask) |
| 389 | [What are different event loops](#what-are-different-event-loops) |
| 390 | [What is the purpose of queueMicrotask](#what-is-the-purpose-of-queuemicrotask) |
| 391 | [How do you use javascript libraries in typescript file](#how-do-you-use-javascript-libraries-in-typescript-file) |
| 392 | [What are the differences between promises and observables](#what-are-the-differences-between-promises-and-observables) |
| 393 | [What is heap](#what-is-heap) |
| 394 | [What is an event table](#what-is-an-event-table) |
| 395 | [What is a microTask queue](#what-is-a-microtask-queue) |
| 396 | [What is the difference between shim and polyfill](#what-is-the-difference-between-shim-and-polyfill) |
| 397 | [How do you detect primitive or non primitive value type](#how-do-you-detect-primitive-or-non-primitive-value-type) |
| 398 | [What is babel](#what-is-babel) |
| 399 | [Is Node.js completely single threaded](#is-nodejs-completely-single-threaded) |
| 400 | [What are the common use cases of observables](#what-are-the-common-use-cases-of-observables) |
| 401 | [What is RxJS](#what-is-rxjs) |
| 402 | [What is the difference between Function constructor and function declaration](#what-is-the-difference-between-function-constructor-and-function-declaration) |
| 403 | [What is a Short circuit condition](#what-is-a-short-circuit-condition) |
| 404 | [What is the easiest way to resize an array](#what-is-the-easiest-way-to-resize-an-array) |
| 405 | [What is an observable](#what-is-an-observable) |
| 406 | [What is the difference between function and class declarations](#what-is-the-difference-between-function-and-class-declarations) |
| 407 | [What is an async function](#what-is-an-async-function) |
| 408 | [How do you prevent promises swallowing errors](#how-do-you-prevent-promises-swallowing-errors) |
| 409 | [What is deno](#what-is-deno) |
| 410 | [How do you make an object iterable in javascript](#how-do-you-make-an-object-iterable-in-javascript) |
| 411 | [What is a Proper Tail Call](#what-is-a-proper-tail-call) |
| 412 | [How do you check an object is a promise or not](#how-do-you-check-an-object-is-a-promise-or-not) |
| 413 | [How to detect if a function is called as constructor](#how-to-detect-if-a-function-is-called-as-constructor) |
| 414 | [What are the differences between arguments object and rest parameter](#what-are-the-differences-between-arguments-object-and-rest-parameter) |
| 415 | [What are the differences between spread operator and rest parameter](#what-are-the-differences-between-spread-operator-and-rest-parameter) |
| 416 | [What are the different kinds of generators](#what-are-the-different-kinds-of-generators) |
| 417 | [What are the built-in iterables](#what-are-the-built-in-iterables) |
| 418 | [What are the differences between for...of and for...in statements](#what-are-the-differences-between-forof-and-forin-statements) |
| 419 | [How do you define instance and non-instance properties](#how-do-you-define-instance-and-non-instance-properties) |
| 420 | [What is the difference between isNaN and Number.isNaN?](#what-is-the-difference-between-isnan-and-numberisnan) |
| 421 | [How to invoke an IIFE without any extra brackets?](#how-to-invoke-an-iife-without-any-extra-brackets) |
| 422 | [Is that possible to use expressions in switch cases?](#is-that-possible-to-use-expressions-in-switch-cases) |
| 423 | [What is the easiest way to ignore promise errors?](#what-is-the-easiest-way-to-ignore-promise-errors) |
| 424 | [How do style the console output using CSS?](#how-do-style-the-console-output-using-css) |
| 425 | [What is nullish coalescing operator (??)?](#what-is-nullish-coalescing-operator) |
| 426 | [How do you group and nest console output?](#how-do-you-group-and-nest-console-output) |
| 427 | [What is the difference between dense and sparse arrays?](#what-is-the-difference-between-dense-and-sparse-arrays) |
| 428 | [What are the different ways to create sparse arrays?](#what-are-the-different-ways-to-create-sparse-arrays) |
| 429 | [What is the difference between setTimeout, setImmediate and process.nextTick?](#what-is-the-difference-between-settimeout-setimmediate-and-processnexttick) |
| 430 | [How do you reverse an array without modifying original array?](#how-do-you-reverse-an-array-without-modifying-original-array) |
| 431 | [How do you create custom HTML element?](#how-do-you-create-custom-html-element) |
| 432 | [What is global execution context?](#what-is-global-execution-context) |
| 433 | [What is function execution context?](#what-is-function-execution-context) |
| 434 | [What is debouncing?](#what-is-debouncing) |
| 435 | [What is throttling?](#what-is-throttling) |
| 436 | [What is optional chaining?](#what-is-optional-chaining) |
| 437 | [What is an environment record?](#what-is-an-environment-record) |
| 438 | [How to verify if a variable is an array?](#how-to-verify-if-a-variable-is-an-array) |
| 439 | [What is pass by value and pass by reference?](#what-is-pass-by-value-and-pass-by-reference) |
| 440 | [What are the differences between primitives and non-primitives?](#what-are-the-differences-between-primitives-and-non-primitives) |
| 441 | [What are hidden classes?](#what-are-hidden-classes) |
| 442 | [What is inline caching?](#what-is-inline-caching) |
| 443 | [How do you create your own bind method using either call or apply method?](#how-do-you-create-your-own-bind-method-using-either-call-or-apply-method) |
| 444 | [What are the differences between pure and impure functions?](#what-are-the-differences-between-pure-and-impure-functions?) |
| 445 | [What is referential transparency?](#what-is-referential-transparency) |
| 446 | [What are the possible side-effects in javascript?](#what-are-the-possible-side-effects-in-javascript) |
| 447 | [What are compose and pipe functions?](#what-are-compose-and-pipe-functions) |
| 448 | [What is module pattern?](#what-is-module-pattern) |
| 449 | [What is Function Composition?](#what-is-function-composition) |
| 450 | [How to use await outside of async function prior to ES2022?](#how-to-use-await-outside-of-async-function-prior-to-es2022) |
1. ### What are the possible ways to create objects in JavaScript
There are many ways to create objects in javascript as below
1. **Object constructor:**
The simplest way to create an empty object is using the Object constructor. Currently this approach is not recommended.
```javascript
var object = new Object();
```
The `Object()` is a built-in constructor function so "new" keyword is not required. the above can be written as:
```javascript
var object = Object();
```
2. **Object's create method:**
The create method of Object creates a new object by passing the prototype object as a parameter
```javascript
var object = Object.create(null);
```
3. **Object literal syntax:**
The object literal syntax (or object initializer), is a comma-separated set of name-value pairs wrapped in curly braces.
```javascript
var object = {
name: "Sudheer",
age: 34
};
Object literal property values can be of any data type, including array, function, and nested object.
```
**Note:** This is an easiest way to create an object
4. **Function constructor:**
Create any function and apply the new operator to create object instances,
```javascript
function Person(name) {
this.name = name;
this.age = 21;
}
var object = new Person("Sudheer");
```
5. **Function constructor with prototype:**
This is similar to function constructor but it uses prototype for their properties and methods,
```javascript
function Person() {}
Person.prototype.name = "Sudheer";
var object = new Person();
```
This is equivalent to an instance created with an object create method with a function prototype and then call that function with an instance and parameters as arguments.
```javascript
function func() {}
new func(x, y, z);
```
**(OR)**
```javascript
// Create a new instance using function prototype.
var newInstance = Object.create(func.prototype)
// Call the function
var result = func.call(newInstance, x, y, z),
// If the result is a non-null object then use it otherwise just use the new instance.
console.log(result && typeof result === 'object' ? result : newInstance);
```
6. **ES6 Class syntax:**
ES6 introduces class feature to create the objects
```javascript
class Person {
constructor(name) {
this.name = name;
}
}
var object = new Person("Sudheer");
```
7. **Singleton pattern:**
A Singleton is an object which can only be instantiated one time. Repeated calls to its constructor return the same instance and this way one can ensure that they don't accidentally create multiple instances.
```javascript
var object = new (function () {
this.name = "Sudheer";
})();
```
**[⬆ Back to Top](#table-of-contents)**
2. ### What is a prototype chain
**Prototype chaining** is used to build new types of objects based on existing ones. It is similar to inheritance in a class based language.
The prototype on object instance is available through **Object.getPrototypeOf(object)** or **\_\_proto\_\_** property whereas prototype on constructors function is available through **Object.prototype**.
**[⬆ Back to Top](#table-of-contents)**
3. ### What is the difference between Call, Apply and Bind
The difference between Call, Apply and Bind can be explained with below examples,
**Call:** The call() method invokes a function with a given `this` value and arguments provided one by one
```javascript
var employee1 = { firstName: "John", lastName: "Rodson" };
var employee2 = { firstName: "Jimmy", lastName: "Baily" };
function invite(greeting1, greeting2) {
console.log(
greeting1 + " " + this.firstName + " " + this.lastName + ", " + greeting2
);
}
invite.call(employee1, "Hello", "How are you?"); // Hello John Rodson, How are you?
invite.call(employee2, "Hello", "How are you?"); // Hello Jimmy Baily, How are you?
```
**Apply:** Invokes the function with a given `this` value and allows you to pass in arguments as an array
```javascript
var employee1 = { firstName: "John", lastName: "Rodson" };
var employee2 = { firstName: "Jimmy", lastName: "Baily" };
function invite(greeting1, greeting2) {
console.log(
greeting1 + " " + this.firstName + " " + this.lastName + ", " + greeting2
);
}
invite.apply(employee1, ["Hello", "How are you?"]); // Hello John Rodson, How are you?
invite.apply(employee2, ["Hello", "How are you?"]); // Hello Jimmy Baily, How are you?
```
**bind:** returns a new function, allowing you to pass any number of arguments
```javascript
var employee1 = { firstName: "John", lastName: "Rodson" };
var employee2 = { firstName: "Jimmy", lastName: "Baily" };
function invite(greeting1, greeting2) {
console.log(
greeting1 + " " + this.firstName + " " + this.lastName + ", " + greeting2
);
}
var inviteEmployee1 = invite.bind(employee1);
var inviteEmployee2 = invite.bind(employee2);
inviteEmployee1("Hello", "How are you?"); // Hello John Rodson, How are you?
inviteEmployee2("Hello", "How are you?"); // Hello Jimmy Baily, How are you?
```
Call and apply are pretty interchangeable. Both execute the current function immediately. You need to decide whether it’s easier to send in an array or a comma separated list of arguments. You can remember by treating Call is for **comma** (separated list) and Apply is for **Array**.
Whereas Bind creates a new function that will have `this` set to the first parameter passed to bind().
**[⬆ Back to Top](#table-of-contents)**
4. ### What is JSON and its common operations
**JSON** is a text-based data format following JavaScript object syntax, which was popularized by `Douglas Crockford`. It is useful when you want to transmit data across a network and it is basically just a text file with an extension of .json, and a MIME type of application/json
**Parsing:** Converting a string to a native object
```javascript
JSON.parse(text);
```
**Stringification:** converting a native object to a string so it can be transmitted across the network
```javascript
JSON.stringify(object);
```
**[⬆ Back to Top](#table-of-contents)**
5. ### What is the purpose of the array slice method
The **slice()** method returns the selected elements in an array as a new array object. It selects the elements starting at the given start argument, and ends at the given optional end argument without including the last element. If you omit the second argument then it selects till the end.
Some of the examples of this method are,
```javascript
let arrayIntegers = [1, 2, 3, 4, 5];
let arrayIntegers1 = arrayIntegers.slice(0, 2); // returns [1,2]
let arrayIntegers2 = arrayIntegers.slice(2, 3); // returns [3]
let arrayIntegers3 = arrayIntegers.slice(4); //returns [5]
```
**Note:** Slice method won't mutate the original array but it returns the subset as a new array.
**[⬆ Back to Top](#table-of-contents)**
6. ### What is the purpose of the array splice method
The **splice()** method is used either adds/removes items to/from an array, and then returns the removed item. The first argument specifies the array position for insertion or deletion whereas the optional second argument indicates the number of elements to be deleted. Each additional argument is added to the array.
Some of the examples of this method are,
```javascript
let arrayIntegersOriginal1 = [1, 2, 3, 4, 5];
let arrayIntegersOriginal2 = [1, 2, 3, 4, 5];
let arrayIntegersOriginal3 = [1, 2, 3, 4, 5];
let arrayIntegers1 = arrayIntegersOriginal1.splice(0, 2); // returns [1, 2]; original array: [3, 4, 5]
let arrayIntegers2 = arrayIntegersOriginal2.splice(3); // returns [4, 5]; original array: [1, 2, 3]
let arrayIntegers3 = arrayIntegersOriginal3.splice(3, 1, "a", "b", "c"); //returns [4]; original array: [1, 2, 3, "a", "b", "c", 5]
```
**Note:** Splice method modifies the original array and returns the deleted array.
**[⬆ Back to Top](#table-of-contents)**
7. ### What is the difference between slice and splice
Some of the major difference in a tabular form
| Slice | Splice |
| -------------------------------------------- | ----------------------------------------------- |
| Doesn't modify the original array(immutable) | Modifies the original array(mutable) |
| Returns the subset of original array | Returns the deleted elements as array |
| Used to pick the elements from array | Used to insert or delete elements to/from array |
**[⬆ Back to Top](#table-of-contents)**
8. ### How do you compare Object and Map
**Objects** are similar to **Maps** in that both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. Due to this reason, Objects have been used as Maps historically. But there are important differences that make using a Map preferable in certain cases.
1. The keys of an Object are Strings and Symbols, whereas they can be any value for a Map, including functions, objects, and any primitive.
2. The keys in Map are ordered while keys added to Object are not. Thus, when iterating over it, a Map object returns keys in order of insertion.
3. You can get the size of a Map easily with the size property, while the number of properties in an Object must be determined manually.
4. A Map is an iterable and can thus be directly iterated, whereas iterating over an Object requires obtaining its keys in some fashion and iterating over them.
5. An Object has a prototype, so there are default keys in the map that could collide with your keys if you're not careful. As of ES5 this can be bypassed by using map = Object.create(null), but this is seldom done.
6. A Map may perform better in scenarios involving frequent addition and removal of key pairs.
**[⬆ Back to Top](#table-of-contents)**
9. ### What is the difference between == and === operators
JavaScript provides both strict(===, !==) and type-converting(==, !=) equality comparison. The strict operators take type of variable in consideration, while non-strict operators make type correction/conversion based upon values of variables. The strict operators follow the below conditions for different types,
1. Two strings are strictly equal when they have the same sequence of characters, same length, and same characters in corresponding positions.
2. Two numbers are strictly equal when they are numerically equal. i.e, Having the same number value.
There are two special cases in this,
1. NaN is not equal to anything, including NaN.
2. Positive and negative zeros are equal to one another.
3. Two Boolean operands are strictly equal if both are true or both are false.
4. Two objects are strictly equal if they refer to the same Object.
5. Null and Undefined types are not equal with ===, but equal with ==. i.e,
null===undefined --> false but null==undefined --> true
Some of the example which covers the above cases,
```javascript
0 == false // true
0 === false // false
1 == "1" // true
1 === "1" // false
null == undefined // true
null === undefined // false
'0' == false // true
'0' === false // false
[]==[] or []===[] //false, refer different objects in memory
{}=={} or {}==={} //false, refer different objects in memory
```
**[⬆ Back to Top](#table-of-contents)**
10. ### What are lambda or arrow functions
An arrow function is a shorter syntax for a function expression and does not have its own **this, arguments, super, or new.target**. These functions are best suited for non-method functions, and they cannot be used as constructors.
**[⬆ Back to Top](#table-of-contents)**
11. ### What is a first class function
In Javascript, functions are first class objects. First-class functions means when functions in that language are treated like any other variable.
For example, in such a language, a function can be passed as an argument to other functions, can be returned by another function and can be assigned as a value to a variable. For example, in the below example, handler functions assigned to a listener
```javascript
const handler = () => console.log("This is a click handler function");
document.addEventListener("click", handler);
```
**[⬆ Back to Top](#table-of-contents)**
12. ### What is a first order function
First-order function is a function that doesn’t accept another function as an argument and doesn’t return a function as its return value.
```javascript
const firstOrder = () => console.log("I am a first order function!");
```
**[⬆ Back to Top](#table-of-contents)**
13. ### What is a higher order function
Higher-order function is a function that accepts another function as an argument or returns a function as a return value or both.
```javascript
const firstOrderFunc = () =>
console.log("Hello, I am a First order function");
const higherOrder = (ReturnFirstOrderFunc) => ReturnFirstOrderFunc();
higherOrder(firstOrderFunc);
```
**[⬆ Back to Top](#table-of-contents)**
14. ### What is a unary function
Unary function (i.e. monadic) is a function that accepts exactly one argument. It stands for a single argument accepted by a function.
Let us take an example of unary function,
```javascript
const unaryFunction = (a) => console.log(a + 10); // Add 10 to the given argument and display the value
```
**[⬆ Back to Top](#table-of-contents)**
15. ### What is the currying function
Currying is the process of taking a function with multiple arguments and turning it into a sequence of functions each with only a single argument. Currying is named after a mathematician **Haskell Curry**. By applying currying, a n-ary function turns it into a unary function.
Let's take an example of n-ary function and how it turns into a currying function,
```javascript
const multiArgFunction = (a, b, c) => a + b + c;
console.log(multiArgFunction(1, 2, 3)); // 6
const curryUnaryFunction = (a) => (b) => (c) => a + b + c;
curryUnaryFunction(1); // returns a function: b => c => 1 + b + c
curryUnaryFunction(1)(2); // returns a function: c => 3 + c
curryUnaryFunction(1)(2)(3); // returns the number 6
```
Curried functions are great to improve **code reusability** and **functional composition**.
**[⬆ Back to Top](#table-of-contents)**
16. ### What is a pure function
A **Pure function** is a function where the return value is only determined by its arguments without any side effects. i.e, If you call a function with the same arguments 'n' number of times and 'n' number of places in the application then it will always return the same value.
Let's take an example to see the difference between pure and impure functions,
```javascript
//Impure
let numberArray = [];
const impureAddNumber = (number) => numberArray.push(number);
//Pure
const pureAddNumber = (number) => (argNumberArray) =>
argNumberArray.concat([number]);
//Display the results
console.log(impureAddNumber(6)); // returns 1
console.log(numberArray); // returns [6]
console.log(pureAddNumber(7)(numberArray)); // returns [6, 7]
console.log(numberArray); // returns [6]
```
As per the above code snippets, the **Push** function is impure itself by altering the array and returning a push number index independent of the parameter value. . Whereas **Concat** on the other hand takes the array and concatenates it with the other array producing a whole new array without side effects. Also, the return value is a concatenation of the previous array.
Remember that Pure functions are important as they simplify unit testing without any side effects and no need for dependency injection. They also avoid tight coupling and make it harder to break your application by not having any side effects. These principles are coming together with **Immutability** concept of ES6 by giving preference to **const** over **let** usage.
**[⬆ Back to Top](#table-of-contents)**
17. ### What is the purpose of the let keyword
The `let` statement declares a **block scope local variable**. Hence the variables defined with let keyword are limited in scope to the block, statement, or expression on which it is used. Whereas variables declared with the `var` keyword used to define a variable globally, or locally to an entire function regardless of block scope.
Let's take an example to demonstrate the usage,
```javascript
let counter = 30;
if (counter === 30) {
let counter = 31;
console.log(counter); // 31
}
console.log(counter); // 30 (because the variable in if block won't exist here)
```
**[⬆ Back to Top](#table-of-contents)**
18. ### What is the difference between let and var
You can list out the differences in a tabular format
| var | let |
| ----------------------------------------------------- | --------------------------- |
| It is been available from the beginning of JavaScript | Introduced as part of ES6 |
| It has function scope | It has block scope |
| Variables will be hoisted | Hoisted but not initialized |
Let's take an example to see the difference,
```javascript
function userDetails(username) {
if (username) {
console.log(salary); // undefined due to hoisting
console.log(age); // ReferenceError: Cannot access 'age' before initialization
let age = 30;
var salary = 10000;
}
console.log(salary); //10000 (accessible due to function scope)
console.log(age); //error: age is not defined(due to block scope)
}
userDetails("John");
```
**[⬆ Back to Top](#table-of-contents)**
19. ### What is the reason to choose the name let as a keyword
`let` is a mathematical statement that was adopted by early programming languages like **Scheme** and **Basic**. It has been borrowed from dozens of other languages that use `let` already as a traditional keyword as close to `var` as possible.
**[⬆ Back to Top](#table-of-contents)**
20. ### How do you redeclare variables in switch block without an error
If you try to redeclare variables in a `switch block` then it will cause errors because there is only one block. For example, the below code block throws a syntax error as below,
```javascript
let counter = 1;
switch (x) {
case 0:
let name;
break;
case 1:
let name; // SyntaxError for redeclaration.
break;
}
```
To avoid this error, you can create a nested block inside a case clause and create a new block scoped lexical environment.
```javascript
let counter = 1;
switch (x) {
case 0: {
let name;
break;
}
case 1: {
let name; // No SyntaxError for redeclaration.
break;
}
}
```
**[⬆ Back to Top](#table-of-contents)**
21. ### What is the Temporal Dead Zone
The Temporal Dead Zone is a behavior in JavaScript that occurs when declaring a variable with the let and const keywords, but not with var. In ECMAScript 6, accessing a `let` or `const` variable before its declaration (within its scope) causes a ReferenceError. The time span when that happens, between the creation of a variable’s binding and its declaration, is called the temporal dead zone.
Let's see this behavior with an example,
```javascript
function somemethod() {
console.log(counter1); // undefined
console.log(counter2); // ReferenceError
var counter1 = 1;
let counter2 = 2;
}
```
**[⬆ Back to Top](#table-of-contents)**
22. ### What is IIFE(Immediately Invoked Function Expression)
IIFE (Immediately Invoked Function Expression) is a JavaScript function that runs as soon as it is defined. The signature of it would be as below,
```javascript
(function () {
// logic here
})();
```
The primary reason to use an IIFE is to obtain data privacy because any variables declared within the IIFE cannot be accessed by the outside world. i.e, If you try to access variables with IIFE then it throws an error as below,
```javascript
(function () {
var message = "IIFE";
console.log(message);
})();
console.log(message); //Error: message is not defined
```
**[⬆ Back to Top](#table-of-contents)**
23. ### How do you decode or encode a URL in JavaScript?
`encodeURI()` function is used to encode an URL. This function requires a URL string as a parameter and return that encoded string.
`decodeURI()` function is used to decode an URL. This function requires an encoded URL string as parameter and return that decoded string.
**Note:** If you want to encode characters such as `/ ? : @ & = + $ #` then you need to use `encodeURIComponent()`.
```javascript
let uri = "employeeDetails?name=john&occupation=manager";
let encoded_uri = encodeURI(uri);
let decoded_uri = decodeURI(encoded_uri);
```
**[⬆ Back to Top](#table-of-contents)**
24. ### What is memoization
Memoization is a programming technique which attempts to increase a function’s performance by caching its previously computed results. Each time a memoized function is called, its parameters are used to index the cache. If the data is present, then it can be returned, without executing the entire function. Otherwise the function is executed and then the result is added to the cache.
Let's take an example of adding function with memoization,
```javascript
const memoizAddition = () => {
let cache = {};
return (value) => {
if (value in cache) {
console.log("Fetching from cache");
return cache[value]; // Here, cache.value cannot be used as property name starts with the number which is not a valid JavaScript identifier. Hence, can only be accessed using the square bracket notation.
} else {
console.log("Calculating result");
let result = value + 20;
cache[value] = result;
return result;
}
};
};
// returned function from memoizAddition
const addition = memoizAddition();
console.log(addition(20)); //output: 40 calculated
console.log(addition(20)); //output: 40 cached
```
**[⬆ Back to Top](#table-of-contents)**
25. ### What is Hoisting
Hoisting is a JavaScript mechanism where variables, function declarations and classes are moved to the top of their scope before code execution. Remember that JavaScript only hoists declarations, not initialisation.
Let's take a simple example of variable hoisting,
```javascript
console.log(message); //output : undefined
var message = "The variable Has been hoisted";
```
The above code looks like as below to the interpreter,
```javascript
var message;
console.log(message);
message = "The variable Has been hoisted";
```
In the same fashion, function declarations are hoisted too
```javascript
message("Good morning"); //Good morning
function message(name) {
console.log(name);
}
```
This hoisting makes functions to be safely used in code before they are declared.
**[⬆ Back to Top](#table-of-contents)**
26. ### What are classes in ES6
In ES6, Javascript classes are primarily syntactic sugar over JavaScript’s existing prototype-based inheritance.
For example, the prototype based inheritance written in function expression as below,
```javascript
function Bike(model, color) {
this.model = model;
this.color = color;
}
Bike.prototype.getDetails = function () {
return this.model + " bike has" + this.color + " color";
};
```
Whereas ES6 classes can be defined as an alternative
```javascript
class Bike {
constructor(color, model) {
this.color = color;
this.model = model;
}
getDetails() {
return this.model + " bike has" + this.color + " color";
}
}
```
**[⬆ Back to Top](#table-of-contents)**
27. ### What are closures
A closure is the combination of a function and the lexical environment within which that function was declared. i.e, It is an inner function that has access to the outer or enclosing function’s variables. The closure has three scope chains
1. Own scope where variables defined between its curly brackets
2. Outer function’s variables
3. Global variables
Let's take an example of closure concept,
```javascript
function Welcome(name) {
var greetingInfo = function (message) {
console.log(message + " " + name);
};
return greetingInfo;
}
var myFunction = Welcome("John");
myFunction("Welcome "); //Output: Welcome John
myFunction("Hello Mr."); //output: Hello Mr.John
```
As per the above code, the inner function(i.e, greetingInfo) has access to the variables in the outer function scope(i.e, Welcome) even after the outer function has returned.
**[⬆ Back to Top](#table-of-contents)**
28. ### What are modules
Modules refer to small units of independent, reusable code and also act as the foundation of many JavaScript design patterns. Most of the JavaScript modules export an object literal, a function, or a constructor
**[⬆ Back to Top](#table-of-contents)**
29. ### Why do you need modules
Below are the list of benefits using modules in javascript ecosystem
1. Maintainability
2. Reusability
3. Namespacing
**[⬆ Back to Top](#table-of-contents)**
30. ### What is scope in javascript
Scope is the accessibility of variables, functions, and objects in some particular part of your code during runtime. In other words, scope determines the visibility of variables and other resources in areas of your code.
**[⬆ Back to Top](#table-of-contents)**
31. ### What is a service worker
A Service worker is basically a script (JavaScript file) that runs in the background, separate from a web page and provides features that don't need a web page or user interaction. Some of the major features of service workers are Rich offline experiences(offline first web application development), periodic background syncs, push notifications, intercept and handle network requests and programmatically managing a cache of responses.
**[⬆ Back to Top](#table-of-contents)**
32. ### How do you manipulate DOM using a service worker
Service worker can't access the DOM directly. But it can communicate with the pages it controls by responding to messages sent via the `postMessage` interface, and those pages can manipulate the DOM.
**[⬆ Back to Top](#table-of-contents)**
33. ### How do you reuse information across service worker restarts
The problem with service worker is that it gets terminated when not in use, and restarted when it's next needed, so you cannot rely on global state within a service worker's `onfetch` and `onmessage` handlers. In this case, service workers will have access to IndexedDB API in order to persist and reuse across restarts.
**[⬆ Back to Top](#table-of-contents)**
34. ### What is IndexedDB
IndexedDB is a low-level API for client-side storage of larger amounts of structured data, including files/blobs. This API uses indexes to enable high-performance searches of this data.
**[⬆ Back to Top](#table-of-contents)**
35. ### What is web storage
Web storage is an API that provides a mechanism by which browsers can store key/value pairs locally within the user's browser, in a much more intuitive fashion than using cookies. The web storage provides two mechanisms for storing data on the client.
1. **Local storage:** It stores data for current origin with no expiration date.
2. **Session storage:** It stores data for one session and the data is lost when the browser tab is closed.
**[⬆ Back to Top](#table-of-contents)**
36. ### What is a post message
Post message is a method that enables cross-origin communication between Window objects.(i.e, between a page and a pop-up that it spawned, or between a page and an iframe embedded within it). Generally, scripts on different pages are allowed to access each other if and only if the pages follow same-origin policy(i.e, pages share the same protocol, port number, and host).
**[⬆ Back to Top](#table-of-contents)**
37. ### What is a Cookie
A cookie is a piece of data that is stored on your computer to be accessed by your browser. Cookies are saved as key/value pairs.
For example, you can create a cookie named username as below,
```javascript
document.cookie = "username=John";
```
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38. ### Why do you need a Cookie
Cookies are used to remember information about the user profile(such as username). It basically involves two steps,
1. When a user visits a web page, the user profile can be stored in a cookie.
2. Next time the user visits the page, the cookie remembers the user profile.
**[⬆ Back to Top](#table-of-contents)**
39. ### What are the options in a cookie
There are few below options available for a cookie,
1. By default, the cookie is deleted when the browser is closed but you can change this behavior by setting expiry date (in UTC time).
```javascript
document.cookie = "username=John; expires=Sat, 8 Jun 2019 12:00:00 UTC";
```
1. By default, the cookie belongs to a current page. But you can tell the browser what path the cookie belongs to using a path parameter.
```javascript
document.cookie = "username=John; path=/services";
```
**[⬆ Back to Top](#table-of-contents)**
40. ### How do you delete a cookie
You can delete a cookie by setting the expiry date as a passed date. You don't need to specify a cookie value in this case.
For example, you can delete a username cookie in the current page as below.
```javascript
document.cookie =
"username=; expires=Fri, 07 Jun 2019 00:00:00 UTC; path=/;";
```
**Note:** You should define the cookie path option to ensure that you delete the right cookie. Some browsers doesn't allow to delete a cookie unless you specify a path parameter.
**[⬆ Back to Top](#table-of-contents)**
41. ### What are the differences between cookie, local storage and session storage
Below are some of the differences between cookie, local storage and session storage,
| Feature | Cookie | Local storage | Session storage |
| --------------------------------- | ---------------------------------- | ---------------- | ------------------- |
| Accessed on client or server side | Both server-side & client-side | client-side only | client-side only |
| Lifetime | As configured using Expires option | until deleted | until tab is closed |
| SSL support | Supported | Not supported | Not supported |
| Maximum data size | 4KB | 5 MB | 5MB |
**[⬆ Back to Top](#table-of-contents)**
42. ### What is the main difference between localStorage and sessionStorage
LocalStorage is the same as SessionStorage but it persists the data even when the browser is closed and reopened(i.e it has no expiration time) whereas in sessionStorage data gets cleared when the page session ends.
**[⬆ Back to Top](#table-of-contents)**
43. ### How do you access web storage
The Window object implements the `WindowLocalStorage` and `WindowSessionStorage` objects which has `localStorage`(window.localStorage) and `sessionStorage`(window.sessionStorage) properties respectively. These properties create an instance of the Storage object, through which data items can be set, retrieved and removed for a specific domain and storage type (session or local).
For example, you can read and write on local storage objects as below
```javascript
localStorage.setItem("logo", document.getElementById("logo").value);
localStorage.getItem("logo");
```
**[⬆ Back to Top](#table-of-contents)**
44. ### What are the methods available on session storage
The session storage provided methods for reading, writing and clearing the session data
```javascript
// Save data to sessionStorage
sessionStorage.setItem("key", "value");
// Get saved data from sessionStorage
let data = sessionStorage.getItem("key");
// Remove saved data from sessionStorage
sessionStorage.removeItem("key");
// Remove all saved data from sessionStorage
sessionStorage.clear();
```
**[⬆ Back to Top](#table-of-contents)**
45. ### What is a storage event and its event handler
The StorageEvent is an event that fires when a storage area has been changed in the context of another document. Whereas onstorage property is an EventHandler for processing storage events.
The syntax would be as below
```javascript
window.onstorage = functionRef;
```
Let's take the example usage of onstorage event handler which logs the storage key and it's values
```javascript
window.onstorage = function (e) {
console.log(
"The " +
e.key +
" key has been changed from " +
e.oldValue +
" to " +
e.newValue +
"."
);
};
```
**[⬆ Back to Top](#table-of-contents)**
46. ### Why do you need web storage
Web storage is more secure, and large amounts of data can be stored locally, without affecting website performance. Also, the information is never transferred to the server. Hence this is a more recommended approach than Cookies.
**[⬆ Back to Top](#table-of-contents)**
47. ### How do you check web storage browser support
You need to check browser support for localStorage and sessionStorage before using web storage,
```javascript
if (typeof Storage !== "undefined") {
// Code for localStorage/sessionStorage.
} else {
// Sorry! No Web Storage support..
}
```
**[⬆ Back to Top](#table-of-contents)**
48. ### How do you check web workers browser support
You need to check browser support for web workers before using it
```javascript
if (typeof Worker !== "undefined") {
// code for Web worker support.
} else {
// Sorry! No Web Worker support..
}
```
**[⬆ Back to Top](#table-of-contents)**
49. ### Give an example of a web worker
You need to follow below steps to start using web workers for counting example
1. Create a Web Worker File: You need to write a script to increment the count value. Let's name it as counter.js
```javascript
let i = 0;
function timedCount() {
i = i + 1;
postMessage(i);
setTimeout("timedCount()", 500);
}
timedCount();
```
Here postMessage() method is used to post a message back to the HTML page
1. Create a Web Worker Object: You can create a web worker object by checking for browser support. Let's name this file as web_worker_example.js
```javascript
if (typeof w == "undefined") {
w = new Worker("counter.js");
}
```
and we can receive messages from web worker
```javascript
w.onmessage = function (event) {
document.getElementById("message").innerHTML = event.data;
};
```
1. Terminate a Web Worker:
Web workers will continue to listen for messages (even after the external script is finished) until it is terminated. You can use the terminate() method to terminate listening to the messages.
```javascript
w.terminate();
```
1. Reuse the Web Worker: If you set the worker variable to undefined you can reuse the code
```javascript
w = undefined;
```
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50. ### What are the restrictions of web workers on DOM
WebWorkers don't have access to below javascript objects since they are defined in an external files
1. Window object
2. Document object
3. Parent object
**[⬆ Back to Top](#table-of-contents)**
51. ### What is a promise
A promise is an object that may produce a single value some time in the future with either a resolved value or a reason that it’s not resolved(for example, network error). It will be in one of the 3 possible states: fulfilled, rejected, or pending.
The syntax of Promise creation looks like below,
```javascript
const promise = new Promise(function (resolve, reject) {
// promise description
});
```
The usage of a promise would be as below,
```javascript
const promise = new Promise(
(resolve) => {
setTimeout(() => {
resolve("I'm a Promise!");
}, 5000);
},
(reject) => {}
);
promise.then((value) => console.log(value));
```
The action flow of a promise will be as below,
**[⬆ Back to Top](#table-of-contents)**
52. ### Why do you need a promise
Promises are used to handle asynchronous operations. They provide an alternative approach for callbacks by reducing the callback hell and writing the cleaner code.
**[⬆ Back to Top](#table-of-contents)**
53. ### What are the three states of promise
Promises have three states:
1. **Pending:** This is an initial state of the Promise before an operation begins
2. **Fulfilled:** This state indicates that the specified operation was completed.
3. **Rejected:** This state indicates that the operation did not complete. In this case an error value will be thrown.
**[⬆ Back to Top](#table-of-contents)**
54. ### What is a callback function
A callback function is a function passed into another function as an argument. This function is invoked inside the outer function to complete an action.
Let's take a simple example of how to use callback function
```javascript
function callbackFunction(name) {
console.log("Hello " + name);
}
function outerFunction(callback) {
let name = prompt("Please enter your name.");
callback(name);
}
outerFunction(callbackFunction);
```
**[⬆ Back to Top](#table-of-contents)**
55. ### Why do we need callbacks
The callbacks are needed because javascript is an event driven language. That means instead of waiting for a response javascript will keep executing while listening for other events.
Let's take an example with the first function invoking an API call(simulated by setTimeout) and the next function which logs the message.
```javascript
function firstFunction() {
// Simulate a code delay
setTimeout(function () {
console.log("First function called");
}, 1000);
}
function secondFunction() {
console.log("Second function called");
}
firstFunction();
secondFunction();
Output;
// Second function called
// First function called
```
As observed from the output, javascript didn't wait for the response of the first function and the remaining code block got executed. So callbacks are used in a way to make sure that certain code doesn’t execute until the other code finishes execution.
**[⬆ Back to Top](#table-of-contents)**
56. ### What is a callback hell
Callback Hell is an anti-pattern with multiple nested callbacks which makes code hard to read and debug when dealing with asynchronous logic. The callback hell looks like below,
```javascript
async1(function(){
async2(function(){
async3(function(){
async4(function(){
....
});
});
});
});
```
**[⬆ Back to Top](#table-of-contents)**
57. ### What are server-sent events
Server-sent events (SSE) is a server push technology enabling a browser to receive automatic updates from a server via HTTP connection without resorting to polling. These are a one way communications channel - events flow from server to client only. This has been used in Facebook/Twitter updates, stock price updates, news feeds etc.
**[⬆ Back to Top](#table-of-contents)**
58. ### How do you receive server-sent event notifications
The EventSource object is used to receive server-sent event notifications. For example, you can receive messages from server as below,
```javascript
if (typeof EventSource !== "undefined") {
var source = new EventSource("sse_generator.js");
source.onmessage = function (event) {
document.getElementById("output").innerHTML += event.data + "
";
};
}
```
**[⬆ Back to Top](#table-of-contents)**
59. ### How do you check browser support for server-sent events
You can perform browser support for server-sent events before using it as below,
```javascript
if (typeof EventSource !== "undefined") {
// Server-sent events supported. Let's have some code here!
} else {
// No server-sent events supported
}
```
**[⬆ Back to Top](#table-of-contents)**
60. ### What are the events available for server sent events
Below are the list of events available for server sent events
| Event | Description |
|---- | ---------
| onopen | It is used when a connection to the server is opened |
| onmessage | This event is used when a message is received |
| onerror | It happens when an error occurs|
**[⬆ Back to Top](#table-of-contents)**
61. ### What are the main rules of promise
A promise must follow a specific set of rules:
1. A promise is an object that supplies a standard-compliant `.then()` method
2. A pending promise may transition into either fulfilled or rejected state
3. A fulfilled or rejected promise is settled and it must not transition into any other state.
4. Once a promise is settled, the value must not change.
**[⬆ Back to Top](#table-of-contents)**
62. ### What is callback in callback
You can nest one callback inside in another callback to execute the actions sequentially one by one. This is known as callbacks in callbacks.
```javascript
loadScript("/script1.js", function (script) {
console.log("first script is loaded");
loadScript("/script2.js", function (script) {
console.log("second script is loaded");
loadScript("/script3.js", function (script) {
console.log("third script is loaded");
// after all scripts are loaded
});
});
});
```
**[⬆ Back to Top](#table-of-contents)**
63. ### What is promise chaining
The process of executing a sequence of asynchronous tasks one after another using promises is known as Promise chaining. Let's take an example of promise chaining for calculating the final result,
```javascript
new Promise(function (resolve, reject) {
setTimeout(() => resolve(1), 1000);
})
.then(function (result) {
console.log(result); // 1
return result * 2;
})
.then(function (result) {
console.log(result); // 2
return result * 3;
})
.then(function (result) {
console.log(result); // 6
return result * 4;
});
```
In the above handlers, the result is passed to the chain of .then() handlers with the below work flow,
1. The initial promise resolves in 1 second,
2. After that `.then` handler is called by logging the result(1) and then return a promise with the value of result \* 2.
3. After that the value passed to the next `.then` handler by logging the result(2) and return a promise with result \* 3.
4. Finally the value passed to the last `.then` handler by logging the result(6) and return a promise with result \* 4.
**[⬆ Back to Top](#table-of-contents)**
64. ### What is promise.all
Promise.all is a promise that takes an array of promises as an input (an iterable), and it gets resolved when all the promises get resolved or any one of them gets rejected. For example, the syntax of promise.all method is below,
```javascript
Promise.all([Promise1, Promise2, Promise3]) .then(result) => { console.log(result) }) .catch(error => console.log(`Error in promises ${error}`))
```
**Note:** Remember that the order of the promises(output the result) is maintained as per input order.
**[⬆ Back to Top](#table-of-contents)**
65. ### What is the purpose of the race method in promise
Promise.race() method will return the promise instance which is firstly resolved or rejected. Let's take an example of race() method where promise2 is resolved first
```javascript
var promise1 = new Promise(function (resolve, reject) {
setTimeout(resolve, 500, "one");
});
var promise2 = new Promise(function (resolve, reject) {
setTimeout(resolve, 100, "two");
});
Promise.race([promise1, promise2]).then(function (value) {
console.log(value); // "two" // Both promises will resolve, but promise2 is faster
});
```
**[⬆ Back to Top](#table-of-contents)**
66. ### What is a strict mode in javascript
Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a function, in a “strict” operating context. This way it prevents certain actions from being taken and throws more exceptions. The literal expression `"use strict";` instructs the browser to use the javascript code in the Strict mode.
**[⬆ Back to Top](#table-of-contents)**
67. ### Why do you need strict mode
Strict mode is useful to write "secure" JavaScript by notifying "bad syntax" into real errors. For example, it eliminates accidentally creating a global variable by throwing an error and also throws an error for assignment to a non-writable property, a getter-only property, a non-existing property, a non-existing variable, or a non-existing object.
**[⬆ Back to Top](#table-of-contents)**
68. ### How do you declare strict mode
The strict mode is declared by adding "use strict"; to the beginning of a script or a function.
If declared at the beginning of a script, it has global scope.
```javascript
"use strict";
x = 3.14; // This will cause an error because x is not declared
```
and if you declare inside a function, it has local scope
```javascript
x = 3.14; // This will not cause an error.
myFunction();
function myFunction() {
"use strict";
y = 3.14; // This will cause an error
}
```
**[⬆ Back to Top](#table-of-contents)**
69. ### What is the purpose of double exclamation
The double exclamation or negation(!!) ensures the resulting type is a boolean. If it was falsey (e.g. 0, null, undefined, etc.), it will be false, otherwise, it will be true.
For example, you can test IE version using this expression as below,
```javascript
let isIE8 = false;
isIE8 = !!navigator.userAgent.match(/MSIE 8.0/);
console.log(isIE8); // returns true or false
```
If you don't use this expression then it returns the original value.
```javascript
console.log(navigator.userAgent.match(/MSIE 8.0/)); // returns either an Array or null
```
**Note:** The expression !! is not an operator, but it is just twice of ! operator.
**[⬆ Back to Top](#table-of-contents)**
70. ### What is the purpose of the delete operator
The delete keyword is used to delete the property as well as its value.
```javascript
var user = { name: "John", age: 20 };
delete user.age;
console.log(user); // {name: "John"}
```
**[⬆ Back to Top](#table-of-contents)**
71. ### What is typeof operator
You can use the JavaScript typeof operator to find the type of a JavaScript variable. It returns the type of a variable or an expression.
```javascript
typeof "John Abraham"; // Returns "string"
typeof (1 + 2); // Returns "number"
typeof [1, 2, 3]; // Returns "object" because all arrays are also objects
```
**[⬆ Back to Top](#table-of-contents)**
72. ### What is undefined property
The undefined property indicates that a variable has not been assigned a value, or declared but not initialized at all. The type of undefined value is undefined too.
```javascript
var user; // Value is undefined, type is undefined
console.log(typeof user); //undefined
```
Any variable can be emptied by setting the value to undefined.
```javascript
user = undefined;
```
**[⬆ Back to Top](#table-of-contents)**
73. ### What is null value
The value null represents the intentional absence of any object value. It is one of JavaScript's primitive values. The type of null value is object.
You can empty the variable by setting the value to null.
```javascript
var user = null;
console.log(typeof user); //object
```
**[⬆ Back to Top](#table-of-contents)**
74. ### What is the difference between null and undefined
Below are the main differences between null and undefined,
| Null | Undefined |
| ----------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------- |
| It is an assignment value which indicates that variable points to no object. | It is not an assignment value where a variable has been declared but has not yet been assigned a value. |
| Type of null is object | Type of undefined is undefined |
| The null value is a primitive value that represents the null, empty, or non-existent reference. | The undefined value is a primitive value used when a variable has not been assigned a value. |
| Indicates the absence of a value for a variable | Indicates absence of variable itself |
| Converted to zero (0) while performing primitive operations | Converted to NaN while performing primitive operations |
**[⬆ Back to Top](#table-of-contents)**
75. ### What is eval
The eval() function evaluates JavaScript code represented as a string. The string can be a JavaScript expression, variable, statement, or sequence of statements.
```javascript
console.log(eval("1 + 2")); // 3
```
**[⬆ Back to Top](#table-of-contents)**
76. ### What is the difference between window and document
Below are the main differences between window and document,
| Window | Document |
| ----------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------- |
| It is the root level element in any web page | It is the direct child of the window object. This is also known as Document Object Model(DOM) |
| By default window object is available implicitly in the page | You can access it via window.document or document. |
| It has methods like alert(), confirm() and properties like document, location | It provides methods like getElementById, getElementsByTagName, createElement etc |
**[⬆ Back to Top](#table-of-contents)**
77. ### How do you access history in javascript
The window.history object contains the browser's history. You can load previous and next URLs in the history using back() and next() methods.
```javascript
function goBack() {
window.history.back();
}
function goForward() {
window.history.forward();
}
```
**Note:** You can also access history without window prefix.
**[⬆ Back to Top](#table-of-contents)**
78. ### How do you detect caps lock key turned on or not
The `mouseEvent getModifierState()` is used to return a boolean value that indicates whether the specified modifier key is activated or not. The modifiers such as CapsLock, ScrollLock and NumLock are activated when they are clicked, and deactivated when they are clicked again.
Let's take an input element to detect the CapsLock on/off behavior with an example,
```html
function enterInput(e) {
var flag = e.getModifierState("CapsLock");
if (flag) {
document.getElementById("feedback").innerHTML = "CapsLock activated";
} else {
document.getElementById("feedback").innerHTML =
"CapsLock not activated";
}
}
```
**[⬆ Back to Top](#table-of-contents)**
79. ### What is isNaN
The isNaN() function is used to determine whether a value is an illegal number (Not-a-Number) or not. i.e, This function returns true if the value equates to NaN. Otherwise it returns false.
```javascript
isNaN("Hello"); //true
isNaN("100"); //false
```
**[⬆ Back to Top](#table-of-contents)**
80. ### What are the differences between undeclared and undefined variables
Below are the major differences between undeclared(not defined) and undefined variables,
| undeclared | undefined |
| ------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------- |
| These variables do not exist in a program and are not declared | These variables declared in the program but have not assigned any value |
| If you try to read the value of an undeclared variable, then a runtime error is encountered | If you try to read the value of an undefined variable, an undefined value is returned. |
**[⬆ Back to Top](#table-of-contents)**
81. ### What are global variables
Global variables are those that are available throughout the length of the code without any scope. The var keyword is used to declare a local variable but if you omit it then it will become global variable
```javascript
msg = "Hello"; // var is missing, it becomes global variable
```
**[⬆ Back to Top](#table-of-contents)**
82. ### What are the problems with global variables
The problem with global variables is the conflict of variable names of local and global scope. It is also difficult to debug and test the code that relies on global variables.
**[⬆ Back to Top](#table-of-contents)**
83. ### What is NaN property
The NaN property is a global property that represents "Not-a-Number" value. i.e, It indicates that a value is not a legal number. It is very rare to use NaN in a program but it can be used as return value for few cases
```javascript
Math.sqrt(-1);
parseInt("Hello");
```
**[⬆ Back to Top](#table-of-contents)**
84. ### What is the purpose of isFinite function
The isFinite() function is used to determine whether a number is a finite, legal number. It returns false if the value is +infinity, -infinity, or NaN (Not-a-Number), otherwise it returns true.
```javascript
isFinite(Infinity); // false
isFinite(NaN); // false
isFinite(-Infinity); // false
isFinite(100); // true
```
**[⬆ Back to Top](#table-of-contents)**
85. ### What is an event flow
Event flow is the order in which event is received on the web page. When you click an element that is nested in various other elements, before your click actually reaches its destination, or target element, it must trigger the click event for each of its parent elements first, starting at the top with the global window object.
There are two ways of event flow
1. Top to Bottom(Event Capturing)
2. Bottom to Top (Event Bubbling)
**[⬆ Back to Top](#table-of-contents)**
86. ### What is event bubbling
Event bubbling is a type of event propagation where the event first triggers on the innermost target element, and then successively triggers on the ancestors (parents) of the target element in the same nesting hierarchy till it reaches the outermost DOM element.
**[⬆ Back to Top](#table-of-contents)**
87. ### What is event capturing
Event capturing is a type of event propagation where the event is first captured by the outermost element, and then successively triggers on the descendants (children) of the target element in the same nesting hierarchy till it reaches the innermost DOM element.
**[⬆ Back to Top](#table-of-contents)**
88. ### How do you submit a form using JavaScript
You can submit a form using `document.forms[0].submit()`. All the form input's information is submitted using onsubmit event handler
```javascript
function submit() {
document.forms[0].submit();
}
```
**[⬆ Back to Top](#table-of-contents)**
89. ### How do you find operating system details
The window.navigator object contains information about the visitor's browser OS details. Some of the OS properties are available under platform property,
```javascript
console.log(navigator.platform);
```
**[⬆ Back to Top](#table-of-contents)**
90. ### What is the difference between document load and DOMContentLoaded events
The `DOMContentLoaded` event is fired when the initial HTML document has been completely loaded and parsed, without waiting for assets(stylesheets, images, and subframes) to finish loading. Whereas The load event is fired when the whole page has loaded, including all dependent resources(stylesheets, images).
**[⬆ Back to Top](#table-of-contents)**
91. ### What is the difference between native, host and user objects
`Native objects` are objects that are part of the JavaScript language defined by the ECMAScript specification. For example, String, Math, RegExp, Object, Function etc core objects defined in the ECMAScript spec.
`Host objects` are objects provided by the browser or runtime environment (Node). For example, window, XmlHttpRequest, DOM nodes etc are considered as host objects.
`User objects` are objects defined in the javascript code. For example, User objects created for profile information.
**[⬆ Back to Top](#table-of-contents)**
92. ### What are the tools or techniques used for debugging JavaScript code
You can use below tools or techniques for debugging javascript
1. Chrome Devtools
2. debugger statement
3. Good old console.log statement
**[⬆ Back to Top](#table-of-contents)**
93. ### What are the pros and cons of promises over callbacks
Below are the list of pros and cons of promises over callbacks,
**Pros:**
1. It avoids callback hell which is unreadable
2. Easy to write sequential asynchronous code with .then()
3. Easy to write parallel asynchronous code with Promise.all()
4. Solves some of the common problems of callbacks(call the callback too late, too early, many times and swallow errors/exceptions)
**Cons:**
1. It makes little complex code
2. You need to load a polyfill if ES6 is not supported
**[⬆ Back to Top](#table-of-contents)**
94. ### What is the difference between an attribute and a property
Attributes are defined on the HTML markup whereas properties are defined on the DOM. For example, the below HTML element has 2 attributes type and value,
```javascript
```
You can retrieve the attribute value as below,
```javascript
const input = document.querySelector("input");
console.log(input.getAttribute("value")); // Good morning
console.log(input.value); // Good morning
```
And after you change the value of the text field to "Good evening", it becomes like
```javascript
console.log(input.getAttribute("value")); // Good evening
console.log(input.value); // Good evening
```
**[⬆ Back to Top](#table-of-contents)**
95. ### What is same-origin policy
The same-origin policy is a policy that prevents JavaScript from making requests across domain boundaries. An origin is defined as a combination of URI scheme, hostname, and port number. If you enable this policy then it prevents a malicious script on one page from obtaining access to sensitive data on another web page using Document Object Model(DOM).
**[⬆ Back to Top](#table-of-contents)**
96. ### What is the purpose of void 0
Void(0) is used to prevent the page from refreshing. This will be helpful to eliminate the unwanted side-effect, because it will return the undefined primitive value. It is commonly used for HTML documents that use href="JavaScript:Void(0);" within an `` element. i.e, when you click a link, the browser loads a new page or refreshes the same page. But this behavior will be prevented using this expression.
For example, the below link notify the message without reloading the page
```javascript
Click Me!
```
**[⬆ Back to Top](#table-of-contents)**
97. ### Is JavaScript a compiled or interpreted language
JavaScript is an interpreted language, not a compiled language. An interpreter in the browser reads over the JavaScript code, interprets each line, and runs it. Nowadays modern browsers use a technology known as Just-In-Time (JIT) compilation, which compiles JavaScript to executable bytecode just as it is about to run.
**[⬆ Back to Top](#table-of-contents)**
98. ### Is JavaScript a case-sensitive language
Yes, JavaScript is a case sensitive language. The language keywords, variables, function & object names, and any other identifiers must always be typed with a consistent capitalization of letters.
**[⬆ Back to Top](#table-of-contents)**
99. ### Is there any relation between Java and JavaScript
No, they are entirely two different programming languages and have nothing to do with each other. But both of them are Object Oriented Programming languages and like many other languages, they follow similar syntax for basic features(if, else, for, switch, break, continue etc).
**[⬆ Back to Top](#table-of-contents)**
100. ### What are events
Events are "things" that happen to HTML elements. When JavaScript is used in HTML pages, JavaScript can `react` on these events. Some of the examples of HTML events are,
1. Web page has finished loading
2. Input field was changed
3. Button was clicked
Let's describe the behavior of click event for button element,
```javascript
function greeting() {
alert('Hello! Good morning');
}
Click me
```
**[⬆ Back to Top](#table-of-contents)**
101. ### Who created javascript
JavaScript was created by Brendan Eich in 1995 during his time at Netscape Communications. Initially it was developed under the name `Mocha`, but later the language was officially called `LiveScript` when it first shipped in beta releases of Netscape.
**[⬆ Back to Top](#table-of-contents)**
102. ### What is the use of preventDefault method
The preventDefault() method cancels the event if it is cancelable, meaning that the default action or behaviour that belongs to the event will not occur. For example, prevent form submission when clicking on submit button and prevent opening the page URL when clicking on hyperlink are some common use cases.
```javascript
document
.getElementById("link")
.addEventListener("click", function (event) {
event.preventDefault();
});
```
**Note:** Remember that not all events are cancelable.
**[⬆ Back to Top](#table-of-contents)**
103. ### What is the use of stopPropagation method
The stopPropagation method is used to stop the event from bubbling up the event chain. For example, the below nested divs with stopPropagation method prevents default event propagation when clicking on nested div(Div1)
```javascript
Click DIV1 Element
function firstFunc(event) {
alert("DIV 1");
event.stopPropagation();
}
function secondFunc() {
alert("DIV 2");
}
```
**[⬆ Back to Top](#table-of-contents)**
104. ### What are the steps involved in return false usage
The return false statement in event handlers performs the below steps,
1. First it stops the browser's default action or behaviour.
2. It prevents the event from propagating the DOM
3. Stops callback execution and returns immediately when called.
**[⬆ Back to Top](#table-of-contents)**
105. ### What is BOM
The Browser Object Model (BOM) allows JavaScript to "talk to" the browser. It consists of the objects navigator, history, screen, location and document which are children of the window. The Browser Object Model is not standardized and can change based on different browsers.
**[⬆ Back to Top](#table-of-contents)**
106. ### What is the use of setTimeout
The setTimeout() method is used to call a function or evaluate an expression after a specified number of milliseconds. For example, let's log a message after 2 seconds using setTimeout method,
```javascript
setTimeout(function () {
console.log("Good morning");
}, 2000);
```
**[⬆ Back to Top](#table-of-contents)**
107. ### What is the use of setInterval
The setInterval() method is used to call a function or evaluate an expression at specified intervals (in milliseconds). For example, let's log a message after 2 seconds using setInterval method,
```javascript
setInterval(function () {
console.log("Good morning");
}, 2000);
```
**[⬆ Back to Top](#table-of-contents)**
108. ### Why is JavaScript treated as Single threaded
JavaScript is a single-threaded language. Because the language specification does not allow the programmer to write code so that the interpreter can run parts of it in parallel in multiple threads or processes. Whereas languages like java, go, C++ can make multi-threaded and multi-process programs.
**[⬆ Back to Top](#table-of-contents)**
109. ### What is an event delegation
Event delegation is a technique for listening to events where you delegate a parent element as the listener for all of the events that happen inside it.
For example, if you wanted to detect field changes in inside a specific form, you can use event delegation technique,
```javascript
var form = document.querySelector("#registration-form");
// Listen for changes to fields inside the form
form.addEventListener(
"input",
function (event) {
// Log the field that was changed
console.log(event.target);
},
false
);
```
**[⬆ Back to Top](#table-of-contents)**
110. ### What is ECMAScript
ECMAScript is the scripting language that forms the basis of JavaScript. ECMAScript standardized by the ECMA International standards organization in the ECMA-262 and ECMA-402 specifications. The first edition of ECMAScript was released in 1997.
**[⬆ Back to Top](#table-of-contents)**
111. ### What is JSON
JSON (JavaScript Object Notation) is a lightweight format that is used for data interchanging. It is based on a subset of JavaScript language in the way objects are built in JavaScript.
**[⬆ Back to Top](#table-of-contents)**
112. ### What are the syntax rules of JSON
Below are the list of syntax rules of JSON
1. The data is in name/value pairs
2. The data is separated by commas
3. Curly braces hold objects
4. Square brackets hold arrays
**[⬆ Back to Top](#table-of-contents)**
113. ### What is the purpose JSON stringify
When sending data to a web server, the data has to be in a string format. You can achieve this by converting JSON object into a string using stringify() method.
```javascript
var userJSON = { name: "John", age: 31 };
var userString = JSON.stringify(userJSON);
console.log(userString); //"{"name":"John","age":31}"
```
**[⬆ Back to Top](#table-of-contents)**
114. ### How do you parse JSON string
When receiving the data from a web server, the data is always in a string format. But you can convert this string value to a javascript object using parse() method.
```javascript
var userString = '{"name":"John","age":31}';
var userJSON = JSON.parse(userString);
console.log(userJSON); // {name: "John", age: 31}
```
**[⬆ Back to Top](#table-of-contents)**
115. ### Why do you need JSON
When exchanging data between a browser and a server, the data can only be text. Since JSON is text only, it can easily be sent to and from a server, and used as a data format by any programming language.
**[⬆ Back to Top](#table-of-contents)**
116. ### What are PWAs
Progressive web applications (PWAs) are a type of mobile app delivered through the web, built using common web technologies including HTML, CSS and JavaScript. These PWAs are deployed to servers, accessible through URLs, and indexed by search engines.
**[⬆ Back to Top](#table-of-contents)**
117. ### What is the purpose of clearTimeout method
The clearTimeout() function is used in javascript to clear the timeout which has been set by setTimeout()function before that. i.e, The return value of setTimeout() function is stored in a variable and it’s passed into the clearTimeout() function to clear the timer.
For example, the below setTimeout method is used to display the message after 3 seconds. This timeout can be cleared by the clearTimeout() method.
```javascript
var msg;
function greeting() {
alert('Good morning');
}
function start() {
msg =setTimeout(greeting, 3000);
}
function stop() {
clearTimeout(msg);
}
```
**[⬆ Back to Top](#table-of-contents)**
118. ### What is the purpose of clearInterval method
The clearInterval() function is used in javascript to clear the interval which has been set by setInterval() function. i.e, The return value returned by setInterval() function is stored in a variable and it’s passed into the clearInterval() function to clear the interval.
For example, the below setInterval method is used to display the message for every 3 seconds. This interval can be cleared by the clearInterval() method.
```javascript
var msg;
function greeting() {
alert('Good morning');
}
function start() {
msg = setInterval(greeting, 3000);
}
function stop() {
clearInterval(msg);
}
```
**[⬆ Back to Top](#table-of-contents)**
119. ### How do you redirect new page in javascript
In vanilla javascript, you can redirect to a new page using the `location` property of window object. The syntax would be as follows,
```javascript
function redirect() {
window.location.href = "newPage.html";
}
```
**[⬆ Back to Top](#table-of-contents)**
120. ### How do you check whether a string contains a substring
There are 3 possible ways to check whether a string contains a substring or not,
1. **Using includes:** ES6 provided `String.prototype.includes` method to test a string contains a substring
```javascript
var mainString = "hello",
subString = "hell";
mainString.includes(subString);
```
1. **Using indexOf:** In an ES5 or older environment, you can use `String.prototype.indexOf` which returns the index of a substring. If the index value is not equal to -1 then it means the substring exists in the main string.
```javascript
var mainString = "hello",
subString = "hell";
mainString.indexOf(subString) !== -1;
```
1. **Using RegEx:** The advanced solution is using Regular expression's test method(`RegExp.test`), which allows for testing for against regular expressions
```javascript
var mainString = "hello",
regex = /hell/;
regex.test(mainString);
```
**[⬆ Back to Top](#table-of-contents)**
121. ### How do you validate an email in javascript
You can validate an email in javascript using regular expressions. It is recommended to do validations on the server side instead of the client side. Because the javascript can be disabled on the client side.
```javascript
function validateEmail(email) {
var re =
/^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
return re.test(String(email).toLowerCase());
}
```
**[⬆ Back to Top](#table-of-contents)**
The above regular expression accepts unicode characters.
122. ### How do you get the current url with javascript
You can use `window.location.href` expression to get the current url path and you can use the same expression for updating the URL too. You can also use `document.URL` for read-only purposes but this solution has issues in FF.
```javascript
console.log("location.href", window.location.href); // Returns full URL
```
**[⬆ Back to Top](#table-of-contents)**
123. ### What are the various url properties of location object
The below `Location` object properties can be used to access URL components of the page,
1. href - The entire URL
2. protocol - The protocol of the URL
3. host - The hostname and port of the URL
4. hostname - The hostname of the URL
5. port - The port number in the URL
6. pathname - The path name of the URL
7. search - The query portion of the URL
8. hash - The anchor portion of the URL
**[⬆ Back to Top](#table-of-contents)**
124. ### How do get query string values in javascript
You can use URLSearchParams to get query string values in javascript. Let's see an example to get the client code value from URL query string,
```javascript
const urlParams = new URLSearchParams(window.location.search);
const clientCode = urlParams.get("clientCode");
```
**[⬆ Back to Top](#table-of-contents)**
125. ### How do you check if a key exists in an object
You can check whether a key exists in an object or not using three approaches,
1. **Using in operator:** You can use the in operator whether a key exists in an object or not
```javascript
"key" in obj;
```
and If you want to check if a key doesn't exist, remember to use parenthesis,
```javascript
!("key" in obj);
```
1. **Using hasOwnProperty method:** You can use `hasOwnProperty` to particularly test for properties of the object instance (and not inherited properties)
```javascript
obj.hasOwnProperty("key"); // true
```
1. **Using undefined comparison:** If you access a non-existing property from an object, the result is undefined. Let’s compare the properties against undefined to determine the existence of the property.
```javascript
const user = {
name: "John",
};
console.log(user.name !== undefined); // true
console.log(user.nickName !== undefined); // false
```
**[⬆ Back to Top](#table-of-contents)**
126. ### How do you loop through or enumerate javascript object
You can use the `for-in` loop to loop through javascript object. You can also make sure that the key you get is an actual property of an object, and doesn't come from the prototype using `hasOwnProperty` method.
```javascript
var object = {
k1: "value1",
k2: "value2",
k3: "value3",
};
for (var key in object) {
if (object.hasOwnProperty(key)) {
console.log(key + " -> " + object[key]); // k1 -> value1 ...
}
}
```
**[⬆ Back to Top](#table-of-contents)**
127. ### How do you test for an empty object
There are different solutions based on ECMAScript versions
1. **Using Object entries(ECMA 7+):** You can use object entries length along with constructor type.
```javascript
Object.entries(obj).length === 0 && obj.constructor === Object; // Since date object length is 0, you need to check constructor check as well
```
1. **Using Object keys(ECMA 5+):** You can use object keys length along with constructor type.
```javascript
Object.keys(obj).length === 0 && obj.constructor === Object; // Since date object length is 0, you need to check constructor check as well
```
1. **Using for-in with hasOwnProperty(Pre-ECMA 5):** You can use a for-in loop along with hasOwnProperty.
```javascript
function isEmpty(obj) {
for (var prop in obj) {
if (obj.hasOwnProperty(prop)) {
return false;
}
}
return JSON.stringify(obj) === JSON.stringify({});
}
```
**[⬆ Back to Top](#table-of-contents)**
128. ### What is an arguments object
The arguments object is an Array-like object accessible inside functions that contains the values of the arguments passed to that function. For example, let's see how to use arguments object inside sum function,
```javascript
function sum() {
var total = 0;
for (var i = 0, len = arguments.length; i < len; ++i) {
total += arguments[i];
}
return total;
}
sum(1, 2, 3); // returns 6
```
**Note:** You can't apply array methods on arguments object. But you can convert into a regular array as below.
```javascript
var argsArray = Array.prototype.slice.call(arguments);
```
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129. ### How do you make first letter of the string in an uppercase
You can create a function which uses a chain of string methods such as charAt, toUpperCase and slice methods to generate a string with the first letter in uppercase.
```javascript
function capitalizeFirstLetter(string) {
return string.charAt(0).toUpperCase() + string.slice(1);
}
```
**[⬆ Back to Top](#table-of-contents)**
130. ### What are the pros and cons of for loop
The for-loop is a commonly used iteration syntax in javascript. It has both pros and cons
#### Pros
1. Works on every environment
2. You can use break and continue flow control statements
#### Cons
1. Too verbose
2. Imperative
3. You might face one-by-off errors
**[⬆ Back to Top](#table-of-contents)**
131. ### How do you display the current date in javascript
You can use `new Date()` to generate a new Date object containing the current date and time. For example, let's display the current date in mm/dd/yyyy
```javascript
var today = new Date();
var dd = String(today.getDate()).padStart(2, "0");
var mm = String(today.getMonth() + 1).padStart(2, "0"); //January is 0!
var yyyy = today.getFullYear();
today = mm + "/" + dd + "/" + yyyy;
document.write(today);
```
**[⬆ Back to Top](#table-of-contents)**
132. ### How do you compare two date objects
You need to use date.getTime() method to compare date values instead of comparison operators (==, !=, ===, and !== operators)
```javascript
var d1 = new Date();
var d2 = new Date(d1);
console.log(d1.getTime() === d2.getTime()); //True
console.log(d1 === d2); // False
```
**[⬆ Back to Top](#table-of-contents)**
133. ### How do you check if a string starts with another string
You can use ECMAScript 6's `String.prototype.startsWith()` method to check if a string starts with another string or not. But it is not yet supported in all browsers. Let's see an example to see this usage,
```javascript
"Good morning".startsWith("Good"); // true
"Good morning".startsWith("morning"); // false
```
**[⬆ Back to Top](#table-of-contents)**
134. ### How do you trim a string in javascript
JavaScript provided a trim method on string types to trim any whitespaces present at the beginning or ending of the string.
```javascript
" Hello World ".trim(); //Hello World
```
If your browser(";
}
```
The continue statement is used to "jump over" one iteration in the loop. i.e, It breaks one iteration (in the loop), if a specified condition occurs, and continues with the next iteration in the loop.
```javascript
for (i = 0; i < 10; i++) {
if (i === 5) {
continue;
}
text += "Number: " + i + "
";
}
```
**[⬆ Back to Top](#table-of-contents)**
144. ### What are js labels
The label statement allows us to name loops and blocks in JavaScript. We can then use these labels to refer back to the code later. For example, the below code with labels avoids printing the numbers when they are same,
```javascript
var i, j;
loop1: for (i = 0; i < 3; i++) {
loop2: for (j = 0; j < 3; j++) {
if (i === j) {
continue loop1;
}
console.log("i = " + i + ", j = " + j);
}
}
// Output is:
// "i = 1, j = 0"
// "i = 2, j = 0"
// "i = 2, j = 1"
```
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145. ### What are the benefits of keeping declarations at the top
It is recommended to keep all declarations at the top of each script or function. The benefits of doing this are,
1. Gives cleaner code
2. It provides a single place to look for local variables
3. Easy to avoid unwanted global variables
4. It reduces the possibility of unwanted re-declarations
**[⬆ Back to Top](#table-of-contents)**
146. ### What are the benefits of initializing variables
It is recommended to initialize variables because of the below benefits,
1. It gives cleaner code
2. It provides a single place to initialize variables
3. Avoid undefined values in the code
**[⬆ Back to Top](#table-of-contents)**
147. ### What are the recommendations to create new object
It is recommended to avoid creating new objects using `new Object()`. Instead you can initialize values based on it's type to create the objects.
1. Assign {} instead of new Object()
2. Assign "" instead of new String()
3. Assign 0 instead of new Number()
4. Assign false instead of new Boolean()
5. Assign [] instead of new Array()
6. Assign /()/ instead of new RegExp()
7. Assign function (){} instead of new Function()
You can define them as an example,
```javascript
var v1 = {};
var v2 = "";
var v3 = 0;
var v4 = false;
var v5 = [];
var v6 = /()/;
var v7 = function () {};
```
**[⬆ Back to Top](#table-of-contents)**
148. ### How do you define JSON arrays
JSON arrays are written inside square brackets and arrays contain javascript objects. For example, the JSON array of users would be as below,
```javascript
"users":[
{"firstName":"John", "lastName":"Abrahm"},
{"firstName":"Anna", "lastName":"Smith"},
{"firstName":"Shane", "lastName":"Warn"}
]
```
**[⬆ Back to Top](#table-of-contents)**
149. ### How do you generate random integers
You can use Math.random() with Math.floor() to return random integers. For example, if you want generate random integers between 1 to 10, the multiplication factor should be 10,
```javascript
Math.floor(Math.random() * 10) + 1; // returns a random integer from 1 to 10
Math.floor(Math.random() * 100) + 1; // returns a random integer from 1 to 100
```
**Note:** Math.random() returns a random number between 0 (inclusive), and 1 (exclusive)
**[⬆ Back to Top](#table-of-contents)**
150. ### Can you write a random integers function to print integers with in a range
Yes, you can create a proper random function to return a random number between min and max (both included)
```javascript
function randomInteger(min, max) {
return Math.floor(Math.random() * (max - min + 1)) + min;
}
randomInteger(1, 100); // returns a random integer from 1 to 100
randomInteger(1, 1000); // returns a random integer from 1 to 1000
```
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151. ### What is tree shaking
Tree shaking is a form of dead code elimination. It means that unused modules will not be included in the bundle during the build process and for that it relies on the static structure of ES2015 module syntax,( i.e. import and export). Initially this has been popularized by the ES2015 module bundler `rollup`.
**[⬆ Back to Top](#table-of-contents)**
152. ### What is the need of tree shaking
Tree Shaking can significantly reduce the code size in any application. i.e, The less code we send over the wire the more performant the application will be. For example, if we just want to create a “Hello World” Application using SPA frameworks then it will take around a few MBs, but by tree shaking it can bring down the size to just a few hundred KBs. Tree shaking is implemented in Rollup and Webpack bundlers.
**[⬆ Back to Top](#table-of-contents)**
153. ### Is it recommended to use eval
No, it allows arbitrary code to be run which causes a security problem. As we know that the eval() function is used to run text as code. In most of the cases, it should not be necessary to use it.
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154. ### What is a Regular Expression
A regular expression is a sequence of characters that forms a search pattern. You can use this search pattern for searching data in a text. These can be used to perform all types of text search and text replace operations. Let's see the syntax format now,
```javascript
/pattern/modifiers;
```
For example, the regular expression or search pattern with case-insensitive username would be,
```javascript
/John/i;
```
**[⬆ Back to Top](#table-of-contents)**
155. ### What are the string methods available in Regular expression
Regular Expressions has two string methods: search() and replace().
The search() method uses an expression to search for a match, and returns the position of the match.
```javascript
var msg = "Hello John";
var n = msg.search(/John/i); // 6
```
The replace() method is used to return a modified string where the pattern is replaced.
```javascript
var msg = "Hello John";
var n = msg.replace(/John/i, "Buttler"); // Hello Buttler
```
**[⬆ Back to Top](#table-of-contents)**
156. ### What are modifiers in regular expression
Modifiers can be used to perform case-insensitive and global searches. Let's list down some of the modifiers,
| Modifier | Description |
| -------- | ------------------------------------------------------- |
| i | Perform case-insensitive matching |
| g | Perform a global match rather than stops at first match |
| m | Perform multiline matching |
Let's take an example of global modifier,
```javascript
var text = "Learn JS one by one";
var pattern = /one/g;
var result = text.match(pattern); // one,one
```
**[⬆ Back to Top](#table-of-contents)**
157. ### What are regular expression patterns
Regular Expressions provide a group of patterns in order to match characters. Basically they are categorized into 3 types,
1. **Brackets:** These are used to find a range of characters.
For example, below are some use cases,
1. [abc]: Used to find any of the characters between the brackets(a,b,c)
2. [0-9]: Used to find any of the digits between the brackets
3. (a|b): Used to find any of the alternatives separated with |
2. **Metacharacters:** These are characters with a special meaning
For example, below are some use cases,
1. \\d: Used to find a digit
2. \\s: Used to find a whitespace character
3. \\b: Used to find a match at the beginning or ending of a word
3. **Quantifiers:** These are useful to define quantities
For example, below are some use cases,
1. n+: Used to find matches for any string that contains at least one n
2. n\*: Used to find matches for any string that contains zero or more occurrences of n
3. n?: Used to find matches for any string that contains zero or one occurrences of n
**[⬆ Back to Top](#table-of-contents)**
158. ### What is a RegExp object
RegExp object is a regular expression object with predefined properties and methods. Let's see the simple usage of RegExp object,
```javascript
var regexp = new RegExp("\\w+");
console.log(regexp);
// expected output: /\w+/
```
**[⬆ Back to Top](#table-of-contents)**
159. ### How do you search a string for a pattern
You can use the test() method of regular expression in order to search a string for a pattern, and return true or false depending on the result.
```javascript
var pattern = /you/;
console.log(pattern.test("How are you?")); //true
```
**[⬆ Back to Top](#table-of-contents)**
160. ### What is the purpose of exec method
The purpose of exec method is similar to test method but it executes a search for a match in a specified string and returns a result array, or null instead of returning true/false.
```javascript
var pattern = /you/;
console.log(pattern.exec("How are you?")); //["you", index: 8, input: "How are you?", groups: undefined]
```
**[⬆ Back to Top](#table-of-contents)**
161. ### How do you change the style of a HTML element
You can change inline style or classname of a HTML element using javascript
1. **Using style property:** You can modify inline style using style property
```javascript
document.getElementById("title").style.fontSize = "30px";
```
1. **Using ClassName property:** It is easy to modify element class using className property
```javascript
document.getElementById("title").className = "custom-title";
```
**[⬆ Back to Top](#table-of-contents)**
162. ### What would be the result of 1+2+'3'
The output is going to be `33`. Since `1` and `2` are numeric values, the result of the first two digits is going to be a numeric value `3`. The next digit is a string type value because of that the addition of numeric value `3` and string type value `3` is just going to be a concatenation value `33`.
**[⬆ Back to Top](#table-of-contents)**
163. ### What is a debugger statement
The debugger statement invokes any available debugging functionality, such as setting a breakpoint. If no debugging functionality is available, this statement has no effect.
For example, in the below function a debugger statement has been inserted. So
execution is paused at the debugger statement just like a breakpoint in the script source.
```javascript
function getProfile() {
// code goes here
debugger;
// code goes here
}
```
**[⬆ Back to Top](#table-of-contents)**
164. ### What is the purpose of breakpoints in debugging
You can set breakpoints in the javascript code once the debugger statement is executed and the debugger window pops up. At each breakpoint, javascript will stop executing, and let you examine the JavaScript values. After examining values, you can resume the execution of code using the play button.
**[⬆ Back to Top](#table-of-contents)**
165. ### Can I use reserved words as identifiers
No, you cannot use the reserved words as variables, labels, object or function names. Let's see one simple example,
```javascript
var else = "hello"; // Uncaught SyntaxError: Unexpected token else
```
**[⬆ Back to Top](#table-of-contents)**
166. ### How do you detect a mobile browser
You can use regex which returns a true or false value depending on whether or not the user is browsing with a mobile.
```javascript
window.mobilecheck = function () {
var mobileCheck = false;
(function (a) {
if (
/(android|bb\d+|meego).+mobile|avantgo|bada\/|blackberry|blazer|compal|elaine|fennec|hiptop|iemobile|ip(hone|od)|iris|kindle|lge |maemo|midp|mmp|mobile.+firefox|netfront|opera m(ob|in)i|palm( os)?|phone|p(ixi|re)\/|plucker|pocket|psp|series(4|6)0|symbian|treo|up\.(browser|link)|vodafone|wap|windows ce|xda|xiino/i.test(
a
) ||
/1207|6310|6590|3gso|4thp|50[1-6]i|770s|802s|a wa|abac|ac(er|oo|s\-)|ai(ko|rn)|al(av|ca|co)|amoi|an(ex|ny|yw)|aptu|ar(ch|go)|as(te|us)|attw|au(di|\-m|r |s )|avan|be(ck|ll|nq)|bi(lb|rd)|bl(ac|az)|br(e|v)w|bumb|bw\-(n|u)|c55\/|capi|ccwa|cdm\-|cell|chtm|cldc|cmd\-|co(mp|nd)|craw|da(it|ll|ng)|dbte|dc\-s|devi|dica|dmob|do(c|p)o|ds(12|\-d)|el(49|ai)|em(l2|ul)|er(ic|k0)|esl8|ez([4-7]0|os|wa|ze)|fetc|fly(\-|_)|g1 u|g560|gene|gf\-5|g\-mo|go(\.w|od)|gr(ad|un)|haie|hcit|hd\-(m|p|t)|hei\-|hi(pt|ta)|hp( i|ip)|hs\-c|ht(c(\-| |_|a|g|p|s|t)|tp)|hu(aw|tc)|i\-(20|go|ma)|i230|iac( |\-|\/)|ibro|idea|ig01|ikom|im1k|inno|ipaq|iris|ja(t|v)a|jbro|jemu|jigs|kddi|keji|kgt( |\/)|klon|kpt |kwc\-|kyo(c|k)|le(no|xi)|lg( g|\/(k|l|u)|50|54|\-[a-w])|libw|lynx|m1\-w|m3ga|m50\/|ma(te|ui|xo)|mc(01|21|ca)|m\-cr|me(rc|ri)|mi(o8|oa|ts)|mmef|mo(01|02|bi|de|do|t(\-| |o|v)|zz)|mt(50|p1|v )|mwbp|mywa|n10[0-2]|n20[2-3]|n30(0|2)|n50(0|2|5)|n7(0(0|1)|10)|ne((c|m)\-|on|tf|wf|wg|wt)|nok(6|i)|nzph|o2im|op(ti|wv)|oran|owg1|p800|pan(a|d|t)|pdxg|pg(13|\-([1-8]|c))|phil|pire|pl(ay|uc)|pn\-2|po(ck|rt|se)|prox|psio|pt\-g|qa\-a|qc(07|12|21|32|60|\-[2-7]|i\-)|qtek|r380|r600|raks|rim9|ro(ve|zo)|s55\/|sa(ge|ma|mm|ms|ny|va)|sc(01|h\-|oo|p\-)|sdk\/|se(c(\-|0|1)|47|mc|nd|ri)|sgh\-|shar|sie(\-|m)|sk\-0|sl(45|id)|sm(al|ar|b3|it|t5)|so(ft|ny)|sp(01|h\-|v\-|v )|sy(01|mb)|t2(18|50)|t6(00|10|18)|ta(gt|lk)|tcl\-|tdg\-|tel(i|m)|tim\-|t\-mo|to(pl|sh)|ts(70|m\-|m3|m5)|tx\-9|up(\.b|g1|si)|utst|v400|v750|veri|vi(rg|te)|vk(40|5[0-3]|\-v)|vm40|voda|vulc|vx(52|53|60|61|70|80|81|83|85|98)|w3c(\-| )|webc|whit|wi(g |nc|nw)|wmlb|wonu|x700|yas\-|your|zeto|zte\-/i.test(
a.substr(0, 4)
)
)
mobileCheck = true;
})(navigator.userAgent || navigator.vendor || window.opera);
return mobileCheck;
};
```
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167. ### How do you detect a mobile browser without regexp
You can detect mobile browsers by simply running through a list of devices and checking if the useragent matches anything. This is an alternative solution for RegExp usage,
```javascript
function detectmob() {
if (
navigator.userAgent.match(/Android/i) ||
navigator.userAgent.match(/webOS/i) ||
navigator.userAgent.match(/iPhone/i) ||
navigator.userAgent.match(/iPad/i) ||
navigator.userAgent.match(/iPod/i) ||
navigator.userAgent.match(/BlackBerry/i) ||
navigator.userAgent.match(/Windows Phone/i)
) {
return true;
} else {
return false;
}
}
```
**[⬆ Back to Top](#table-of-contents)**
168. ### How do you get the image width and height using JS
You can programmatically get the image and check the dimensions(width and height) using Javascript.
```javascript
var img = new Image();
img.onload = function () {
console.log(this.width + "x" + this.height);
};
img.src = "http://www.google.com/intl/en_ALL/images/logo.gif";
```
**[⬆ Back to Top](#table-of-contents)**
169. ### How do you make synchronous HTTP request
Browsers provide an XMLHttpRequest object which can be used to make synchronous HTTP requests from JavaScript
```javascript
function httpGet(theUrl) {
var xmlHttpReq = new XMLHttpRequest();
xmlHttpReq.open("GET", theUrl, false); // false for synchronous request
xmlHttpReq.send(null);
return xmlHttpReq.responseText;
}
```
**[⬆ Back to Top](#table-of-contents)**
170. ### How do you make asynchronous HTTP request
Browsers provide an XMLHttpRequest object which can be used to make asynchronous HTTP requests from JavaScript by passing the 3rd parameter as true.
```javascript
function httpGetAsync(theUrl, callback) {
var xmlHttpReq = new XMLHttpRequest();
xmlHttpReq.onreadystatechange = function () {
if (xmlHttpReq.readyState == 4 && xmlHttpReq.status == 200)
callback(xmlHttpReq.responseText);
};
xmlHttp.open("GET", theUrl, true); // true for asynchronous
xmlHttp.send(null);
}
```
**[⬆ Back to Top](#table-of-contents)**
171. ### How do you convert date to another timezone in javascript
You can use the toLocaleString() method to convert dates in one timezone to another. For example, let's convert current date to British English timezone as below,
```javascript
console.log(event.toLocaleString("en-GB", { timeZone: "UTC" })); //29/06/2019, 09:56:00
```
**[⬆ Back to Top](#table-of-contents)**
172. ### What are the properties used to get size of window
You can use innerWidth, innerHeight, clientWidth, clientHeight properties of windows, document element and document body objects to find the size of a window. Let's use them combination of these properties to calculate the size of a window or document,
```javascript
var width =
window.innerWidth ||
document.documentElement.clientWidth ||
document.body.clientWidth;
var height =
window.innerHeight ||
document.documentElement.clientHeight ||
document.body.clientHeight;
```
**[⬆ Back to Top](#table-of-contents)**
173. ### What is a conditional operator in javascript
The conditional (ternary) operator is the only JavaScript operator that takes three operands which acts as a shortcut for if statements.
```javascript
var isAuthenticated = false;
console.log(
isAuthenticated ? "Hello, welcome" : "Sorry, you are not authenticated"
); //Sorry, you are not authenticated
```
**[⬆ Back to Top](#table-of-contents)**
174. ### Can you apply chaining on conditional operator
Yes, you can apply chaining on conditional operators similar to if … else if … else if … else chain. The syntax is going to be as below,
```javascript
function traceValue(someParam) {
return condition1
? value1
: condition2
? value2
: condition3
? value3
: value4;
}
// The above conditional operator is equivalent to:
function traceValue(someParam) {
if (condition1) {
return value1;
} else if (condition2) {
return value2;
} else if (condition3) {
return value3;
} else {
return value4;
}
}
```
**[⬆ Back to Top](#table-of-contents)**
175. ### What are the ways to execute javascript after page load
You can execute javascript after page load in many different ways,
1. **window.onload:**
```javascript
window.onload = function ...
```
1. **document.onload:**
```javascript
document.onload = function ...
```
1. **body onload:**
```javascript
```
**[⬆ Back to Top](#table-of-contents)**
176. ### What is the difference between proto and prototype
The `__proto__` object is the actual object that is used in the lookup chain to resolve methods, etc. Whereas `prototype` is the object that is used to build `__proto__` when you create an object with new.
```javascript
new Employee().__proto__ === Employee.prototype;
new Employee().prototype === undefined;
```
There are few more differences,
| feature | Prototype | proto |
| ---------- | ------------------------------------------------------------ | ---------------------------------------------------------- |
| Access | All the function constructors have prototype properties. | All the objects have \_\_proto\_\_ property |
| Purpose | Used to reduce memory wastage with a single copy of function | Used in lookup chain to resolve methods, constructors etc. |
| ECMAScript | Introduced in ES6 | Introduced in ES5 |
| Usage | Frequently used | Rarely used |
**[⬆ Back to Top](#table-of-contents)**
177. ### Give an example where do you really need semicolon
It is recommended to use semicolons after every statement in JavaScript. For example, in the below case it throws an error ".. is not a function" at runtime due to missing semicolon.
```javascript
// define a function
var fn = (function () {
//...
})(
// semicolon missing at this line
// then execute some code inside a closure
function () {
//...
}
)();
```
and it will be interpreted as
```javascript
var fn = (function () {
//...
})(function () {
//...
})();
```
In this case, we are passing the second function as an argument to the first function and then trying to call the result of the first function call as a function. Hence, the second function will fail with a "... is not a function" error at runtime.
**[⬆ Back to Top](#table-of-contents)**
178. ### What is a freeze method
The **freeze()** method is used to freeze an object. Freezing an object does not allow adding new properties to an object,prevents from removing and prevents changing the enumerability, configurability, or writability of existing properties. i.e, It returns the passed object and does not create a frozen copy.
```javascript
const obj = {
prop: 100,
};
Object.freeze(obj);
obj.prop = 200; // Throws an error in strict mode
console.log(obj.prop); //100
```
Remember freezing is only applied to the top-level properties in objects but not for nested objects.
For example, let's try to freeze user object which has employment details as nested object and observe that details have been changed.
```javascript
const user = {
name: "John",
employment: {
department: "IT",
},
};
Object.freeze(user);
user.employment.department = "HR";
```
**Note:** It causes a TypeError if the argument passed is not an object.
**[⬆ Back to Top](#table-of-contents)**
179. ### What is the purpose of freeze method
Below are the main benefits of using freeze method,
1. It is used for freezing objects and arrays.
2. It is used to make an object immutable.
**[⬆ Back to Top](#table-of-contents)**
180. ### Why do I need to use freeze method
In the Object-oriented paradigm, an existing API contains certain elements that are not intended to be extended, modified, or re-used outside of their current context. Hence it works as the `final` keyword which is used in various languages.
**[⬆ Back to Top](#table-of-contents)**
181. ### How do you detect a browser language preference
You can use navigator object to detect a browser language preference as below,
```javascript
var language =
(navigator.languages && navigator.languages[0]) || // Chrome / Firefox
navigator.language || // All browsers
navigator.userLanguage; // IE <= 10
console.log(language);
```
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182. ### How to convert string to title case with javascript
Title case means that the first letter of each word is capitalized. You can convert a string to title case using the below function,
```javascript
function toTitleCase(str) {
return str.replace(/\w\S*/g, function (txt) {
return txt.charAt(0).toUpperCase() + txt.substring(1).toLowerCase();
});
}
toTitleCase("good morning john"); // Good Morning John
```
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183. ### How do you detect javascript disabled in the page
You can use the `` tag to detect javascript disabled or not. The code block inside `` gets executed when JavaScript is disabled, and is typically used to display alternative content when the page generated in JavaScript.
```javascript
// JS related code goes here
JavaScript is disabled in the page. Please click Next Page
```
**[⬆ Back to Top](#table-of-contents)**
184. ### What are various operators supported by javascript
An operator is capable of manipulating(mathematical and logical computations) a certain value or operand. There are various operators supported by JavaScript as below,
1. **Arithmetic Operators:** Includes + (Addition),– (Subtraction), \* (Multiplication), / (Division), % (Modulus), + + (Increment) and – – (Decrement)
2. **Comparison Operators:** Includes = =(Equal),!= (Not Equal), ===(Equal with type), > (Greater than),> = (Greater than or Equal to),< (Less than),<= (Less than or Equal to)
3. **Logical Operators:** Includes &&(Logical AND),||(Logical OR),!(Logical NOT)
4. **Assignment Operators:** Includes = (Assignment Operator), += (Add and Assignment Operator), – = (Subtract and Assignment Operator), \*= (Multiply and Assignment), /= (Divide and Assignment), %= (Modules and Assignment)
5. **Ternary Operators:** It includes conditional(: ?) Operator
6. **typeof Operator:** It uses to find type of variable. The syntax looks like `typeof variable`
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185. ### What is a rest parameter
Rest parameter is an improved way to handle function parameters which allows us to represent an indefinite number of arguments as an array. The syntax would be as below,
```javascript
function f(a, b, ...theArgs) {
// ...
}
```
For example, let's take a sum example to calculate on dynamic number of parameters,
```javascript
function sum(...args) {
let total = 0;
for (const i of args) {
total += i;
}
return total;
}
console.log(sum(1, 2)); //3
console.log(sum(1, 2, 3)); //6
console.log(sum(1, 2, 3, 4)); //13
console.log(sum(1, 2, 3, 4, 5)); //15
```
**Note:** Rest parameter is added in ES2015 or ES6
**[⬆ Back to Top](#table-of-contents)**
186. ### What happens if you do not use rest parameter as a last argument
The rest parameter should be the last argument, as its job is to collect all the remaining arguments into an array. For example, if you define a function like below it doesn’t make any sense and will throw an error.
```javascript
function someFunc(a,…b,c){
//You code goes here
return;
}
```
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187. ### What are the bitwise operators available in javascript
Below are the list of bitwise logical operators used in JavaScript
1. Bitwise AND ( & )
2. Bitwise OR ( | )
3. Bitwise XOR ( ^ )
4. Bitwise NOT ( ~ )
5. Left Shift ( << )
6. Sign Propagating Right Shift ( >> )
7. Zero fill Right Shift ( >>> )
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188. ### What is a spread operator
Spread operator allows iterables( arrays / objects / strings ) to be expanded into single arguments/elements. Let's take an example to see this behavior,
```javascript
function calculateSum(x, y, z) {
return x + y + z;
}
const numbers = [1, 2, 3];
console.log(calculateSum(...numbers)); // 6
```
**[⬆ Back to Top](#table-of-contents)**
189. ### How do you determine whether object is frozen or not
Object.isFrozen() method is used to determine if an object is frozen or not.An object is frozen if all of the below conditions hold true,
1. If it is not extensible.
2. If all of its properties are non-configurable.
3. If all its data properties are non-writable.
The usage is going to be as follows,
```javascript
const object = {
property: "Welcome JS world",
};
Object.freeze(object);
console.log(Object.isFrozen(object));
```
**[⬆ Back to Top](#table-of-contents)**
190. ### How do you determine two values same or not using object
The Object.is() method determines whether two values are the same value. For example, the usage with different types of values would be,
```javascript
Object.is("hello", "hello"); // true
Object.is(window, window); // true
Object.is([], []); // false
```
Two values are the same if one of the following holds:
1. both undefined
2. both null
3. both true or both false
4. both strings of the same length with the same characters in the same order
5. both the same object (means both object have same reference)
6. both numbers and
both +0
both -0
both NaN
both non-zero and both not NaN and both have the same value.
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191. ### What is the purpose of using object is method
Some of the applications of Object's `is` method are follows,
1. It is used for comparison of two strings.
2. It is used for comparison of two numbers.
3. It is used for comparing the polarity of two numbers.
4. It is used for comparison of two objects.
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192. ### How do you copy properties from one object to other
You can use the Object.assign() method which is used to copy the values and properties from one or more source objects to a target object. It returns the target object which has properties and values copied from the source objects. The syntax would be as below,
```javascript
Object.assign(target, ...sources);
```
Let's take example with one source and one target object,
```javascript
const target = { a: 1, b: 2 };
const source = { b: 3, c: 4 };
const returnedTarget = Object.assign(target, source);
console.log(target); // { a: 1, b: 3, c: 4 }
console.log(returnedTarget); // { a: 1, b: 3, c: 4 }
```
As observed in the above code, there is a common property(`b`) from source to target so it's value has been overwritten.
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193. ### What are the applications of assign method
Below are the some of main applications of Object.assign() method,
1. It is used for cloning an object.
2. It is used to merge objects with the same properties.
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194. ### What is a proxy object
The Proxy object is used to define custom behavior for fundamental operations such as property lookup, assignment, enumeration, function invocation, etc. The syntax would be as follows,
```javascript
var p = new Proxy(target, handler);
```
Let's take an example of proxy object,
```javascript
var handler = {
get: function (obj, prop) {
return prop in obj ? obj[prop] : 100;
},
};
var p = new Proxy({}, handler);
p.a = 10;
p.b = null;
console.log(p.a, p.b); // 10, null
console.log("c" in p, p.c); // false, 100
```
In the above code, it uses `get` handler which define the behavior of the proxy when an operation is performed on it
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195. ### What is the purpose of seal method
The **Object.seal()** method is used to seal an object, by preventing new properties from being added to it and marking all existing properties as non-configurable. But values of present properties can still be changed as long as they are writable. Let's see the below example to understand more about seal() method
```javascript
const object = {
property: "Welcome JS world",
};
Object.seal(object);
object.property = "Welcome to object world";
console.log(Object.isSealed(object)); // true
delete object.property; // You cannot delete when sealed
console.log(object.property); //Welcome to object world
```
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196. ### What are the applications of seal method
Below are the main applications of Object.seal() method,
1. It is used for sealing objects and arrays.
2. It is used to make an object immutable.
**[⬆ Back to Top](#table-of-contents)**
197. ### What are the differences between freeze and seal methods
If an object is frozen using the Object.freeze() method then its properties become immutable and no changes can be made in them whereas if an object is sealed using the Object.seal() method then the changes can be made in the existing properties of the object.
**[⬆ Back to Top](#table-of-contents)**
198. ### How do you determine if an object is sealed or not
The Object.isSealed() method is used to determine if an object is sealed or not. An object is sealed if all of the below conditions hold true
1. If it is not extensible.
2. If all of its properties are non-configurable.
3. If it is not removable (but not necessarily non-writable).
Let's see it in the action
```javascript
const object = {
property: "Hello, Good morning",
};
Object.seal(object); // Using seal() method to seal the object
console.log(Object.isSealed(object)); // checking whether the object is sealed or not
```
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199. ### How do you get enumerable key and value pairs
The Object.entries() method is used to return an array of a given object's own enumerable string-keyed property [key, value] pairs, in the same order as that provided by a for...in loop. Let's see the functionality of object.entries() method in an example,
```javascript
const object = {
a: "Good morning",
b: 100,
};
for (let [key, value] of Object.entries(object)) {
console.log(`${key}: ${value}`); // a: 'Good morning'
// b: 100
}
```
**Note:** The order is not guaranteed as object defined.
**[⬆ Back to Top](#table-of-contents)**
200. ### What is the main difference between Object.values and Object.entries method
The Object.values() method's behavior is similar to Object.entries() method but it returns an array of values instead [key,value] pairs.
```javascript
const object = {
a: "Good morning",
b: 100,
};
for (let value of Object.values(object)) {
console.log(`${value}`); // 'Good morning'
100;
}
```
**[⬆ Back to Top](#table-of-contents)**
201. ### How can you get the list of keys of any object
You can use the `Object.keys()` method which is used to return an array of a given object's own property names, in the same order as we get with a normal loop. For example, you can get the keys of a user object,
```javascript
const user = {
name: "John",
gender: "male",
age: 40,
};
console.log(Object.keys(user)); //['name', 'gender', 'age']
```
**[⬆ Back to Top](#table-of-contents)**
202. ### How do you create an object with prototype
The Object.create() method is used to create a new object with the specified prototype object and properties. i.e, It uses an existing object as the prototype of the newly created object. It returns a new object with the specified prototype object and properties.
```javascript
const user = {
name: "John",
printInfo: function () {
console.log(`My name is ${this.name}.`);
},
};
const admin = Object.create(user);
admin.name = "Nick"; // Remember that "name" is a property set on "admin" but not on "user" object
admin.printInfo(); // My name is Nick
```
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203. ### What is a WeakSet
WeakSet is used to store a collection of weakly(weak references) held objects. The syntax would be as follows,
```javascript
new WeakSet([iterable]);
```
Let's see the below example to explain it's behavior,
```javascript
var ws = new WeakSet();
var user = {};
ws.add(user);
ws.has(user); // true
ws.delete(user); // removes user from the set
ws.has(user); // false, user has been removed
```
**[⬆ Back to Top](#table-of-contents)**
204. ### What are the differences between WeakSet and Set
The main difference is that references to objects in Set are strong while references to objects in WeakSet are weak. i.e, An object in WeakSet can be garbage collected if there is no other reference to it.
Other differences are,
1. Sets can store any value Whereas WeakSets can store only collections of objects
2. WeakSet does not have size property unlike Set
3. WeakSet does not have methods such as clear, keys, values, entries, forEach.
4. WeakSet is not iterable.
**[⬆ Back to Top](#table-of-contents)**
205. ### List down the collection of methods available on WeakSet
Below are the list of methods available on WeakSet,
1. add(value): A new object is appended with the given value to the weakset
2. delete(value): Deletes the value from the WeakSet collection.
3. has(value): It returns true if the value is present in the WeakSet Collection, otherwise it returns false.
Let's see the functionality of all the above methods in an example,
```javascript
var weakSetObject = new WeakSet();
var firstObject = {};
var secondObject = {};
// add(value)
weakSetObject.add(firstObject);
weakSetObject.add(secondObject);
console.log(weakSetObject.has(firstObject)); //true
weakSetObject.delete(secondObject);
```
**[⬆ Back to Top](#table-of-contents)**
206. ### What is a WeakMap
The WeakMap object is a collection of key/value pairs in which the keys are weakly referenced. In this case, keys must be objects and the values can be arbitrary values. The syntax is looking like as below,
```javascript
new WeakMap([iterable]);
```
Let's see the below example to explain it's behavior,
```javascript
var ws = new WeakMap();
var user = {};
ws.set(user);
ws.has(user); // true
ws.delete(user); // removes user from the map
ws.has(user); // false, user has been removed
```
**[⬆ Back to Top](#table-of-contents)**
207. ### What are the differences between WeakMap and Map
The main difference is that references to key objects in Map are strong while references to key objects in WeakMap are weak. i.e, A key object in WeakMap can be garbage collected if there is no other reference to it.
Other differences are,
1. Maps can store any key type Whereas WeakMaps can store only collections of key objects
2. WeakMap does not have size property unlike Map
3. WeakMap does not have methods such as clear, keys, values, entries, forEach.
4. WeakMap is not iterable.
**[⬆ Back to Top](#table-of-contents)**
208. ### List down the collection of methods available on WeakMap
Below are the list of methods available on WeakMap,
1. set(key, value): Sets the value for the key in the WeakMap object. Returns the WeakMap object.
2. delete(key): Removes any value associated to the key.
3. has(key): Returns a Boolean asserting whether a value has been associated to the key in the WeakMap object or not.
4. get(key): Returns the value associated to the key, or undefined if there is none.
Let's see the functionality of all the above methods in an example,
```javascript
var weakMapObject = new WeakMap();
var firstObject = {};
var secondObject = {};
// set(key, value)
weakMapObject.set(firstObject, "John");
weakMapObject.set(secondObject, 100);
console.log(weakMapObject.has(firstObject)); //true
console.log(weakMapObject.get(firstObject)); // John
weakMapObject.delete(secondObject);
```
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209. ### What is the purpose of uneval
The uneval() is an inbuilt function which is used to create a string representation of the source code of an Object. It is a top-level function and is not associated with any object. Let's see the below example to know more about it's functionality,
```javascript
var a = 1;
uneval(a); // returns a String containing 1
uneval(function user() {}); // returns "(function user(){})"
```
**[⬆ Back to Top](#table-of-contents)**
210. ### How do you encode an URL
The encodeURI() function is used to encode complete URI which has special characters except (, / ? : @ & = + $ #) characters.
```javascript
var uri = "https://mozilla.org/?x=шеллы";
var encoded = encodeURI(uri);
console.log(encoded); // https://mozilla.org/?x=%D1%88%D0%B5%D0%BB%D0%BB%D1%8B
```
**[⬆ Back to Top](#table-of-contents)**
211. ### How do you decode an URL
The decodeURI() function is used to decode a Uniform Resource Identifier (URI) previously created by encodeURI().
```javascript
var uri = "https://mozilla.org/?x=шеллы";
var encoded = encodeURI(uri);
console.log(encoded); // https://mozilla.org/?x=%D1%88%D0%B5%D0%BB%D0%BB%D1%8B
try {
console.log(decodeURI(encoded)); // "https://mozilla.org/?x=шеллы"
} catch (e) {
// catches a malformed URI
console.error(e);
}
```
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212. ### How do you print the contents of web page
The window object provided a print() method which is used to print the contents of the current window. It opens a Print dialog box which lets you choose between various printing options. Let's see the usage of print method in an example,
```html
```
**Note:** In most browsers, it will block while the print dialog is open.
**[⬆ Back to Top](#table-of-contents)**
213. ### What is the difference between uneval and eval
The `uneval` function returns the source of a given object; whereas the `eval` function does the opposite, by evaluating that source code in a different memory area. Let's see an example to clarify the difference,
```javascript
var msg = uneval(function greeting() {
return "Hello, Good morning";
});
var greeting = eval(msg);
greeting(); // returns "Hello, Good morning"
```
**[⬆ Back to Top](#table-of-contents)**
214. ### What is an anonymous function
An anonymous function is a function without a name! Anonymous functions are commonly assigned to a variable name or used as a callback function. The syntax would be as below,
```javascript
function (optionalParameters) {
//do something
}
const myFunction = function(){ //Anonymous function assigned to a variable
//do something
};
[1, 2, 3].map(function(element){ //Anonymous function used as a callback function
//do something
});
```
Let's see the above anonymous function in an example,
```javascript
var x = function (a, b) {
return a * b;
};
var z = x(5, 10);
console.log(z); // 50
```
**[⬆ Back to Top](#table-of-contents)**
215. ### What is the precedence order between local and global variables
A local variable takes precedence over a global variable with the same name. Let's see this behavior in an example.
```javascript
var msg = "Good morning";
function greeting() {
msg = "Good Evening";
console.log(msg); // Good Evening
}
greeting();
```
**[⬆ Back to Top](#table-of-contents)**
216. ### What are javascript accessors
ECMAScript 5 introduced javascript object accessors or computed properties through getters and setters. Getters uses the `get` keyword whereas Setters uses the `set` keyword.
```javascript
var user = {
firstName: "John",
lastName: "Abraham",
language: "en",
get lang() {
return this.language;
},
set lang(lang) {
this.language = lang;
},
};
console.log(user.lang); // getter access lang as en
user.lang = "fr";
console.log(user.lang); // setter used to set lang as fr
```
**[⬆ Back to Top](#table-of-contents)**
217. ### How do you define property on Object constructor
The Object.defineProperty() static method is used to define a new property directly on an object, or modify an existing property on an object, and returns the object. Let's see an example to know how to define property,
```javascript
const newObject = {};
Object.defineProperty(newObject, "newProperty", {
value: 100,
writable: false,
});
console.log(newObject.newProperty); // 100
newObject.newProperty = 200; // It throws an error in strict mode due to writable setting
```
**[⬆ Back to Top](#table-of-contents)**
218. ### What is the difference between get and defineProperty
Both have similar results until unless you use classes. If you use `get` the property will be defined on the prototype of the object whereas using `Object.defineProperty()` the property will be defined on the instance it is applied to.
**[⬆ Back to Top](#table-of-contents)**
219. ### What are the advantages of Getters and Setters
Below are the list of benefits of Getters and Setters,
1. They provide simpler syntax
2. They are used for defining computed properties, or accessors in JS.
3. Useful to provide equivalence relation between properties and methods
4. They can provide better data quality
5. Useful for doing things behind the scenes with the encapsulated logic.
**[⬆ Back to Top](#table-of-contents)**
220. ### Can I add getters and setters using defineProperty method
Yes, You can use the `Object.defineProperty()` method to add Getters and Setters. For example, the below counter object uses increment, decrement, add and subtract properties,
```javascript
var obj = { counter: 0 };
// Define getters
Object.defineProperty(obj, "increment", {
get: function () {
this.counter++;
},
});
Object.defineProperty(obj, "decrement", {
get: function () {
this.counter--;
},
});
// Define setters
Object.defineProperty(obj, "add", {
set: function (value) {
this.counter += value;
},
});
Object.defineProperty(obj, "subtract", {
set: function (value) {
this.counter -= value;
},
});
obj.add = 10;
obj.subtract = 5;
console.log(obj.increment); //6
console.log(obj.decrement); //5
```
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221. ### What is the purpose of switch-case
The switch case statement in JavaScript is used for decision making purposes. In a few cases, using the switch case statement is going to be more convenient than if-else statements. The syntax would be as below,
```javascript
switch (expression)
{
case value1:
statement1;
break;
case value2:
statement2;
break;
.
.
case valueN:
statementN;
break;
default:
statementDefault;
}
```
The above multi-way branch statement provides an easy way to dispatch execution to different parts of code based on the value of the expression.
**[⬆ Back to Top](#table-of-contents)**
222. ### What are the conventions to be followed for the usage of switch case
Below are the list of conventions should be taken care,
1. The expression can be of type either number or string.
2. Duplicate values are not allowed for the expression.
3. The default statement is optional. If the expression passed to switch does not match with any case value then the statement within default case will be executed.
4. The break statement is used inside the switch to terminate a statement sequence.
5. The break statement is optional. But if it is omitted, the execution will continue on into the next case.
**[⬆ Back to Top](#table-of-contents)**
223. ### What are primitive data types
A primitive data type is data that has a primitive value (which has no properties or methods). There are 7 types of primitive data types.
1. string
2. number
3. boolean
4. null
5. undefined
6. bigint
7. symbol
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224. ### What are the different ways to access object properties
There are 3 possible ways for accessing the property of an object.
1. **Dot notation:** It uses dot for accessing the properties
```javascript
objectName.property;
```
1. **Square brackets notation:** It uses square brackets for property access
```javascript
objectName["property"];
```
1. **Expression notation:** It uses expression in the square brackets
```javascript
objectName[expression];
```
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225. ### What are the function parameter rules
JavaScript functions follow below rules for parameters,
1. The function definitions do not specify data types for parameters.
2. Do not perform type checking on the passed arguments.
3. Do not check the number of arguments received.
i.e, The below function follows the above rules,
```javascript
function functionName(parameter1, parameter2, parameter3) {
console.log(parameter1); // 1
}
functionName(1);
```
**[⬆ Back to Top](#table-of-contents)**
226. ### What is an error object
An error object is a built in error object that provides error information when an error occurs. It has two properties: name and message. For example, the below function logs error details,
```javascript
try {
greeting("Welcome");
} catch (err) {
console.log(err.name + "
" + err.message);
}
```
**[⬆ Back to Top](#table-of-contents)**
227. ### When you get a syntax error
A SyntaxError is thrown if you try to evaluate code with a syntax error. For example, the below missing quote for the function parameter throws a syntax error
```javascript
try {
eval("greeting('welcome)"); // Missing ' will produce an error
} catch (err) {
console.log(err.name);
}
```
**[⬆ Back to Top](#table-of-contents)**
228. ### What are the different error names from error object
There are 6 different types of error names returned from error object,
| Error Name | Description |
|---- | ---------
| EvalError | An error has occurred in the eval() function |
| RangeError | An error has occurred with a number "out of range" |
| ReferenceError | An error due to an illegal reference|
| SyntaxError | An error due to a syntax error|
| TypeError | An error due to a type error |
| URIError | An error due to encodeURI() |
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229. ### What are the various statements in error handling
Below are the list of statements used in an error handling,
1. **try:** This statement is used to test a block of code for errors
2. **catch:** This statement is used to handle the error
3. **throw:** This statement is used to create custom errors.
4. **finally:** This statement is used to execute code after try and catch regardless of the result.
**[⬆ Back to Top](#table-of-contents)**
230. ### What are the two types of loops in javascript
1. **Entry Controlled loops:** In this kind of loop type, the test condition is tested before entering the loop body. For example, For Loop and While Loop comes under this category.
2. **Exit Controlled Loops:** In this kind of loop type, the test condition is tested or evaluated at the end of the loop body. i.e, the loop body will execute at least once irrespective of test condition true or false. For example, do-while loop comes under this category.
**[⬆ Back to Top](#table-of-contents)**
231. ### What is nodejs
Node.js is a server-side platform built on Chrome's JavaScript runtime for easily building fast and scalable network applications. It is an event-based, non-blocking, asynchronous I/O runtime that uses Google's V8 JavaScript engine and libuv library.
**[⬆ Back to Top](#table-of-contents)**
232. ### What is an Intl object
The Intl object is the namespace for the ECMAScript Internationalization API, which provides language sensitive string comparison, number formatting, and date and time formatting. It provides access to several constructors and language sensitive functions.
**[⬆ Back to Top](#table-of-contents)**
233. ### How do you perform language specific date and time formatting
You can use the `Intl.DateTimeFormat` object which is a constructor for objects that enable language-sensitive date and time formatting. Let's see this behavior with an example,
```javascript
var date = new Date(Date.UTC(2019, 07, 07, 3, 0, 0));
console.log(new Intl.DateTimeFormat("en-GB").format(date)); // 07/08/2019
console.log(new Intl.DateTimeFormat("en-AU").format(date)); // 07/08/2019
```
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234. ### What is an Iterator
An iterator is an object which defines a sequence and a return value upon its termination. It implements the Iterator protocol with a `next()` method which returns an object with two properties: `value` (the next value in the sequence) and `done` (which is true if the last value in the sequence has been consumed).
**[⬆ Back to Top](#table-of-contents)**
235. ### How does synchronous iteration works
Synchronous iteration was introduced in ES6 and it works with below set of components,
**Iterable:** It is an object which can be iterated over via a method whose key is Symbol.iterator.
**Iterator:** It is an object returned by invoking `[Symbol.iterator]()` on an iterable. This iterator object wraps each iterated element in an object and returns it via `next()` method one by one.
**IteratorResult:** It is an object returned by `next()` method. The object contains two properties; the `value` property contains an iterated element and the `done` property determines whether the element is the last element or not.
Let's demonstrate synchronous iteration with an array as below,
```javascript
const iterable = ["one", "two", "three"];
const iterator = iterable[Symbol.iterator]();
console.log(iterator.next()); // { value: 'one', done: false }
console.log(iterator.next()); // { value: 'two', done: false }
console.log(iterator.next()); // { value: 'three', done: false }
console.log(iterator.next()); // { value: 'undefined, done: true }
```
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236. ### What is an event loop
The event loop is a process that continuously monitors both the call stack and the event queue and checks whether or not the call stack is empty. If the call stack is empty and there are pending events in the event queue, the event loop dequeues the event from the event queue and pushes it to the call stack. The call stack executes the event, and any additional events generated during the execution are added to the end of the event queue.
**Note:** The event loop allows Node.js to perform non-blocking I/O operations, even though JavaScript is single-threaded, by offloading operations to the system kernel whenever possible. Since most modern kernels are multi-threaded, they can handle multiple operations executing in the background.
**[⬆ Back to Top](#table-of-contents)**
237. ### What is call stack
Call Stack is a data structure for javascript interpreters to keep track of function calls(creates execution context) in the program. It has two major actions,
1. Whenever you call a function for its execution, you are pushing it to the stack.
2. Whenever the execution is completed, the function is popped out of the stack.
Let's take an example and it's state representation in a diagram format
```javascript
function hungry() {
eatFruits();
}
function eatFruits() {
return "I'm eating fruits";
}
// Invoke the `hungry` function
hungry();
```
The above code processed in a call stack as below,
1. Add the `hungry()` function to the call stack list and execute the code.
2. Add the `eatFruits()` function to the call stack list and execute the code.
3. Delete the `eatFruits()` function from our call stack list.
4. Delete the `hungry()` function from the call stack list since there are no items anymore.
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238. ### What is an event queue
The event queue follows the queue data structure. It stores async callbacks to be added to the call stack. It is also known as the Callback Queue or Macrotask Queue.
Whenever the call stack receives an async function, it is moved into the Web API. Based on the function, Web API executes it and awaits the result. Once it is finished, it moves the callback into the event queue (the callback of the promise is moved into the microtask queue).
The event loop constantly checks whether or not the call stack is empty. Once the call stack is empty and there is a callback in the event queue, the event loop moves the callback into the call stack. But if there is a callback in the microtask queue as well, it is moved first. The microtask queue has a higher priority than the event queue.
**[⬆ Back to Top](#table-of-contents)**
239. ### What is a decorator
A decorator is an expression that evaluates to a function and that takes the target, name, and decorator descriptor as arguments. Also, it optionally returns a decorator descriptor to install on the target object. Let's define admin decorator for user class at design time,
```javascript
function admin(isAdmin) {
return function(target) {
target.isAdmin = isAdmin;
}
}
@admin(true)
class User() {
}
console.log(User.isAdmin); //true
@admin(false)
class User() {
}
console.log(User.isAdmin); //false
```
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240. ### What are the properties of Intl object
Below are the list of properties available on Intl object,
1. **Collator:** These are the objects that enable language-sensitive string comparison.
2. **DateTimeFormat:** These are the objects that enable language-sensitive date and time formatting.
3. **ListFormat:** These are the objects that enable language-sensitive list formatting.
4. **NumberFormat:** Objects that enable language-sensitive number formatting.
5. **PluralRules:** Objects that enable plural-sensitive formatting and language-specific rules for plurals.
6. **RelativeTimeFormat:** Objects that enable language-sensitive relative time formatting.
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241. ### What is an Unary operator
The unary(+) operator is used to convert a variable to a number.If the variable cannot be converted, it will still become a number but with the value NaN. Let's see this behavior in an action.
```javascript
var x = "100";
var y = +x;
console.log(typeof x, typeof y); // string, number
var a = "Hello";
var b = +a;
console.log(typeof a, typeof b, b); // string, number, NaN
```
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242. ### How do you sort elements in an array
The sort() method is used to sort the elements of an array in place and returns the sorted array. The example usage would be as below,
```javascript
var months = ["Aug", "Sep", "Jan", "June"];
months.sort();
console.log(months); // ["Aug", "Jan", "June", "Sep"]
```
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243. ### What is the purpose of compareFunction while sorting arrays
The compareFunction is used to define the sort order. If omitted, the array elements are converted to strings, then sorted according to each character's Unicode code point value. Let's take an example to see the usage of compareFunction,
```javascript
let numbers = [1, 2, 5, 3, 4];
numbers.sort((a, b) => b - a);
console.log(numbers); // [5, 4, 3, 2, 1]
```
**[⬆ Back to Top](#table-of-contents)**
244. ### How do you reversing an array
You can use the reverse() method to reverse the elements in an array. This method is useful to sort an array in descending order. Let's see the usage of reverse() method in an example,
```javascript
let numbers = [1, 2, 5, 3, 4];
numbers.sort((a, b) => b - a);
numbers.reverse();
console.log(numbers); // [1, 2, 3, 4 ,5]
```
**[⬆ Back to Top](#table-of-contents)**
245. ### How do you find min and max value in an array
You can use `Math.min` and `Math.max` methods on array variables to find the minimum and maximum elements within an array. Let's create two functions to find the min and max value with in an array,
```javascript
var marks = [50, 20, 70, 60, 45, 30];
function findMin(arr) {
return Math.min.apply(null, arr);
}
function findMax(arr) {
return Math.max.apply(null, arr);
}
console.log(findMin(marks));
console.log(findMax(marks));
```
**[⬆ Back to Top](#table-of-contents)**
246. ### How do you find min and max values without Math functions
You can write functions which loop through an array comparing each value with the lowest value or highest value to find the min and max values. Let's create those functions to find min and max values,
```javascript
var marks = [50, 20, 70, 60, 45, 30];
function findMin(arr) {
var length = arr.length;
var min = Infinity;
while (length--) {
if (arr[length] < min) {
min = arr[length];
}
}
return min;
}
function findMax(arr) {
var length = arr.length;
var max = -Infinity;
while (length--) {
if (arr[length] > max) {
max = arr[length];
}
}
return max;
}
console.log(findMin(marks));
console.log(findMax(marks));
```
**[⬆ Back to Top](#table-of-contents)**
247. ### What is an empty statement and purpose of it
The empty statement is a semicolon (;) indicating that no statement will be executed, even if JavaScript syntax requires one. Since there is no action with an empty statement you might think that it's usage is quite less, but the empty statement is occasionally useful when you want to create a loop that has an empty body. For example, you can initialize an array with zero values as below,
```javascript
// Initialize an array a
for (let i = 0; i < a.length; a[i++] = 0);
```
**[⬆ Back to Top](#table-of-contents)**
248. ### How do you get metadata of a module
You can use the `import.meta` object which is a meta-property exposing context-specific meta data to a JavaScript module. It contains information about the current module, such as the module's URL. In browsers, you might get different meta data than NodeJS.
```javascript
;
console.log(import.meta); // { url: "file:///home/user/welcome-module.js" }
```
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249. ### What is a comma operator
The comma operator is used to evaluate each of its operands from left to right and returns the value of the last operand. This is totally different from comma usage within arrays, objects, and function arguments and parameters. For example, the usage for numeric expressions would be as below,
```javascript
var x = 1;
x = (x++, x);
console.log(x); // 2
```
**[⬆ Back to Top](#table-of-contents)**
250. ### What is the advantage of a comma operator
It is normally used to include multiple expressions in a location that requires a single expression. One of the common usages of this comma operator is to supply multiple parameters in a `for` loop. For example, the below for loop uses multiple expressions in a single location using comma operator,
```javascript
for (var a = 0, b =10; a <= 10; a++, b--)
```
You can also use the comma operator in a return statement where it processes before returning.
```javascript
function myFunction() {
var a = 1;
return (a += 10), a; // 11
}
```
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251. ### What is typescript
TypeScript is a typed superset of JavaScript created by Microsoft that adds optional types, classes, async/await, and many other features, and compiles to plain JavaScript. Angular built entirely in TypeScript and used as a primary language. You can install it globally as
```bash
npm install -g typescript
```
Let's see a simple example of TypeScript usage,
```typescript
function greeting(name: string): string {
return "Hello, " + name;
}
let user = "Sudheer";
console.log(greeting(user));
```
The greeting method allows only string type as argument.
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252. ### What are the differences between javascript and typescript
Below are the list of differences between javascript and typescript,
| feature | typescript | javascript |
| ------------------- | ------------------------------------- | ----------------------------------------------- |
| Language paradigm | Object oriented programming language | Scripting language |
| Typing support | Supports static typing | It has dynamic typing |
| Modules | Supported | Not supported |
| Interface | It has interfaces concept | Doesn't support interfaces |
| Optional parameters | Functions support optional parameters | No support of optional parameters for functions |
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253. ### What are the advantages of typescript over javascript
Below are some of the advantages of typescript over javascript,
1. TypeScript is able to find compile time errors at the development time only and it makes sures less runtime errors. Whereas javascript is an interpreted language.
2. TypeScript is strongly-typed or supports static typing which allows for checking type correctness at compile time. This is not available in javascript.
3. TypeScript compiler can compile the .ts files into ES3,ES4 and ES5 unlike ES6 features of javascript which may not be supported in some browsers.
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254. ### What is an object initializer
An object initializer is an expression that describes the initialization of an Object. The syntax for this expression is represented as a comma-delimited list of zero or more pairs of property names and associated values of an object, enclosed in curly braces ({}). This is also known as literal notation. It is one of the ways to create an object.
```javascript
var initObject = { a: "John", b: 50, c: {} };
console.log(initObject.a); // John
```
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255. ### What is a constructor method
The constructor method is a special method for creating and initializing an object created within a class. If you do not specify a constructor method, a default constructor is used. The example usage of constructor would be as below,
```javascript
class Employee {
constructor() {
this.name = "John";
}
}
var employeeObject = new Employee();
console.log(employeeObject.name); // John
```
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256. ### What happens if you write constructor more than once in a class
The "constructor" in a class is a special method and it should be defined only once in a class. i.e, If you write a constructor method more than once in a class it will throw a `SyntaxError` error.
```javascript
class Employee {
constructor() {
this.name = "John";
}
constructor() { // Uncaught SyntaxError: A class may only have one constructor
this.age = 30;
}
}
var employeeObject = new Employee();
console.log(employeeObject.name);
```
**[⬆ Back to Top](#table-of-contents)**
257. ### How do you call the constructor of a parent class
You can use the `super` keyword to call the constructor of a parent class. Remember that `super()` must be called before using 'this' reference. Otherwise it will cause a reference error. Let's the usage of it,
```javascript
class Square extends Rectangle {
constructor(length) {
super(length, length);
this.name = "Square";
}
get area() {
return this.width * this.height;
}
set area(value) {
this.area = value;
}
}
```
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258. ### How do you get the prototype of an object
You can use the `Object.getPrototypeOf(obj)` method to return the prototype of the specified object. i.e. The value of the internal `prototype` property. If there are no inherited properties then `null` value is returned.
```javascript
const newPrototype = {};
const newObject = Object.create(newPrototype);
console.log(Object.getPrototypeOf(newObject) === newPrototype); // true
```
**[⬆ Back to Top](#table-of-contents)**
259. ### What happens If I pass string type for getPrototype method
In ES5, it will throw a TypeError exception if the obj parameter isn't an object. Whereas in ES2015, the parameter will be coerced to an `Object`.
```javascript
// ES5
Object.getPrototypeOf("James"); // TypeError: "James" is not an object
// ES2015
Object.getPrototypeOf("James"); // String.prototype
```
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260. ### How do you set prototype of one object to another
You can use the `Object.setPrototypeOf()` method that sets the prototype (i.e., the internal `Prototype` property) of a specified object to another object or null. For example, if you want to set prototype of a square object to rectangle object would be as follows,
```javascript
Object.setPrototypeOf(Square.prototype, Rectangle.prototype);
Object.setPrototypeOf({}, null);
```
**[⬆ Back to Top](#table-of-contents)**
261. ### How do you check whether an object can be extendable or not
The `Object.isExtensible()` method is used to determine if an object is extendable or not. i.e, Whether it can have new properties added to it or not.
```javascript
const newObject = {};
console.log(Object.isExtensible(newObject)); //true
```
**Note:** By default, all the objects are extendable. i.e, The new properties can be added or modified.
**[⬆ Back to Top](#table-of-contents)**
262. ### How do you prevent an object to extend
The `Object.preventExtensions()` method is used to prevent new properties from ever being added to an object. In other words, it prevents future extensions to the object. Let's see the usage of this property,
```javascript
const newObject = {};
Object.preventExtensions(newObject); // NOT extendable
try {
Object.defineProperty(newObject, "newProperty", {
// Adding new property
value: 100,
});
} catch (e) {
console.log(e); // TypeError: Cannot define property newProperty, object is not extensible
}
```
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263. ### What are the different ways to make an object non-extensible
You can mark an object non-extensible in 3 ways,
1. Object.preventExtensions
2. Object.seal
3. Object.freeze
```javascript
var newObject = {};
Object.preventExtensions(newObject); // Prevent objects are non-extensible
Object.isExtensible(newObject); // false
var sealedObject = Object.seal({}); // Sealed objects are non-extensible
Object.isExtensible(sealedObject); // false
var frozenObject = Object.freeze({}); // Frozen objects are non-extensible
Object.isExtensible(frozenObject); // false
```
**[⬆ Back to Top](#table-of-contents)**
264. ### How do you define multiple properties on an object
The `Object.defineProperties()` method is used to define new or modify existing properties directly on an object and returning the object. Let's define multiple properties on an empty object,
```javascript
const newObject = {};
Object.defineProperties(newObject, {
newProperty1: {
value: "John",
writable: true,
},
newProperty2: {},
});
```
**[⬆ Back to Top](#table-of-contents)**
265. ### What is MEAN in javascript
The MEAN (MongoDB, Express, AngularJS, and Node.js) stack is the most popular open-source JavaScript software tech stack available for building dynamic web apps where you can write both the server-side and client-side halves of the web project entirely in JavaScript.
**[⬆ Back to Top](#table-of-contents)**
266. ### What Is Obfuscation in javascript
Obfuscation is the deliberate act of creating obfuscated javascript code(i.e, source or machine code) that is difficult for humans to understand. It is something similar to encryption, but a machine can understand the code and execute it.
Let's see the below function before Obfuscation,
```javascript
function greeting() {
console.log("Hello, welcome to JS world");
}
```
And after the code Obfuscation, it would be appeared as below,
```javascript
eval(
(function (p, a, c, k, e, d) {
e = function (c) {
return c;
};
if (!"".replace(/^/, String)) {
while (c--) {
d[c] = k[c] || c;
}
k = [
function (e) {
return d[e];
},
];
e = function () {
return "\\w+";
};
c = 1;
}
while (c--) {
if (k[c]) {
p = p.replace(new RegExp("\\b" + e(c) + "\\b", "g"), k[c]);
}
}
return p;
})(
"2 1(){0.3('4, 7 6 5 8')}",
9,
9,
"console|greeting|function|log|Hello|JS|to|welcome|world".split("|"),
0,
{}
)
);
```
**[⬆ Back to Top](#table-of-contents)**
267. ### Why do you need Obfuscation
Below are the few reasons for Obfuscation,
1. The Code size will be reduced. So data transfers between server and client will be fast.
2. It hides the business logic from outside world and protects the code from others
3. Reverse engineering is highly difficult
4. The download time will be reduced
**[⬆ Back to Top](#table-of-contents)**
268. ### What is Minification
Minification is the process of removing all unnecessary characters(empty spaces are removed) and variables will be renamed without changing it's functionality. It is also a type of obfuscation .
**[⬆ Back to Top](#table-of-contents)**
269. ### What are the advantages of minification
Normally it is recommended to use minification for heavy traffic and intensive requirements of resources. It reduces file sizes with below benefits,
1. Decreases loading times of a web page
2. Saves bandwidth usages
**[⬆ Back to Top](#table-of-contents)**
270. ### What are the differences between Obfuscation and Encryption
Below are the main differences between Obfuscation and Encryption,
| Feature | Obfuscation | Encryption |
| ------------------ | ----------------------------------------------- | ----------------------------------------------------------------------- |
| Definition | Changing the form of any data in any other form | Changing the form of information to an unreadable format by using a key |
| A key to decode | It can be decoded without any key | It is required |
| Target data format | It will be converted to a complex form | Converted into an unreadable format |
**[⬆ Back to Top](#table-of-contents)**
271. ### What are the common tools used for minification
There are many online/offline tools to minify the javascript files,
1. Google's Closure Compiler
2. UglifyJS2
3. jsmin
4. javascript-minifier.com/
5. prettydiff.com
**[⬆ Back to Top](#table-of-contents)**
272. ### How do you perform form validation using javascript
JavaScript can be used to perform HTML form validation. For example, if the form field is empty, the function needs to notify, and return false, to prevent the form being submitted.
Lets' perform user login in an html form,
```html
User name:
```
And the validation on user login is below,
```javascript
function validateForm() {
var x = document.forms["myForm"]["uname"].value;
if (x == "") {
alert("The username shouldn't be empty");
return false;
}
}
```
**[⬆ Back to Top](#table-of-contents)**
273. ### How do you perform form validation without javascript
You can perform HTML form validation automatically without using javascript. The validation enabled by applying the `required` attribute to prevent form submission when the input is empty.
```html
```
**Note:** Automatic form validation does not work in Internet Explorer 9 or earlier.
**[⬆ Back to Top](#table-of-contents)**
274. ### What are the DOM methods available for constraint validation
The below DOM methods are available for constraint validation on an invalid input,
1. checkValidity(): It returns true if an input element contains valid data.
2. setCustomValidity(): It is used to set the validationMessage property of an input element.
Let's take an user login form with DOM validations
```javascript
function myFunction() {
var userName = document.getElementById("uname");
if (!userName.checkValidity()) {
document.getElementById("message").innerHTML =
userName.validationMessage;
} else {
document.getElementById("message").innerHTML =
"Entered a valid username";
}
}
```
**[⬆ Back to Top](#table-of-contents)**
275. ### What are the available constraint validation DOM properties
Below are the list of some of the constraint validation DOM properties available,
1. validity: It provides a list of boolean properties related to the validity of an input element.
2. validationMessage: It displays the message when the validity is false.
3. willValidate: It indicates if an input element will be validated or not.
**[⬆ Back to Top](#table-of-contents)**
276. ### What are the list of validity properties
The validity property of an input element provides a set of properties related to the validity of data.
1. customError: It returns true, if a custom validity message is set.
2. patternMismatch: It returns true, if an element's value does not match its pattern attribute.
3. rangeOverflow: It returns true, if an element's value is greater than its max attribute.
4. rangeUnderflow: It returns true, if an element's value is less than its min attribute.
5. stepMismatch: It returns true, if an element's value is invalid according to step attribute.
6. tooLong: It returns true, if an element's value exceeds its maxLength attribute.
7. typeMismatch: It returns true, if an element's value is invalid according to type attribute.
8. valueMissing: It returns true, if an element with a required attribute has no value.
9. valid: It returns true, if an element's value is valid.
**[⬆ Back to Top](#table-of-contents)**
277. ### Give an example usage of rangeOverflow property
If an element's value is greater than its max attribute then rangeOverflow property returns true. For example, the below form submission throws an error if the value is more than 100,
```html
OK
```
```javascript
function myOverflowFunction() {
if (document.getElementById("age").validity.rangeOverflow) {
alert("The mentioned age is not allowed");
}
}
```
**[⬆ Back to Top](#table-of-contents)**
278. ### Is enums feature available in javascript
No, javascript does not natively support enums. But there are different kinds of solutions to simulate them even though they may not provide exact equivalents. For example, you can use freeze or seal on object,
```javascript
var DaysEnum = Object.freeze({"monday":1, "tuesday":2, "wednesday":3, ...})
```
**[⬆ Back to Top](#table-of-contents)**
279. ### What is an enum
An enum is a type restricting variables to one value from a predefined set of constants. JavaScript has no enums but typescript provides built-in enum support.
```javascript
enum Color {
RED, GREEN, BLUE
}
```
**[⬆ Back to Top](#table-of-contents)**
280. ### How do you list all properties of an object
You can use the `Object.getOwnPropertyNames()` method which returns an array of all properties found directly in a given object. Let's the usage of it in an example,
```javascript
const newObject = {
a: 1,
b: 2,
c: 3,
};
console.log(Object.getOwnPropertyNames(newObject));
["a", "b", "c"];
```
**[⬆ Back to Top](#table-of-contents)**
281. ### How do you get property descriptors of an object
You can use the `Object.getOwnPropertyDescriptors()` method which returns all own property descriptors of a given object. The example usage of this method is below,
```javascript
const newObject = {
a: 1,
b: 2,
c: 3,
};
const descriptorsObject = Object.getOwnPropertyDescriptors(newObject);
console.log(descriptorsObject.a.writable); //true
console.log(descriptorsObject.a.configurable); //true
console.log(descriptorsObject.a.enumerable); //true
console.log(descriptorsObject.a.value); // 1
```
**[⬆ Back to Top](#table-of-contents)**
282. ### What are the attributes provided by a property descriptor
A property descriptor is a record which has the following attributes
1. value: The value associated with the property
2. writable: Determines whether the value associated with the property can be changed or not
3. configurable: Returns true if the type of this property descriptor can be changed and if the property can be deleted from the corresponding object.
4. enumerable: Determines whether the property appears during enumeration of the properties on the corresponding object or not.
5. set: A function which serves as a setter for the property
6. get: A function which serves as a getter for the property
**[⬆ Back to Top](#table-of-contents)**
283. ### How do you extend classes
The `extends` keyword is used in class declarations/expressions to create a class which is a child of another class. It can be used to subclass custom classes as well as built-in objects. The syntax would be as below,
```javascript
class ChildClass extends ParentClass { ... }
```
Let's take an example of Square subclass from Polygon parent class,
```javascript
class Square extends Rectangle {
constructor(length) {
super(length, length);
this.name = "Square";
}
get area() {
return this.width * this.height;
}
set area(value) {
this.area = value;
}
}
```
**[⬆ Back to Top](#table-of-contents)**
284. ### How do I modify the url without reloading the page
The `window.location.href` property will be helpful to modify the url but it reloads the page. HTML5 introduced the `history.pushState()` and `history.replaceState()` methods, which allow you to add and modify history entries, respectively. For example, you can use pushState as below,
```javascript
window.history.pushState("page2", "Title", "/page2.html");
```
**[⬆ Back to Top](#table-of-contents)**
285. ### How do you check whether an array includes a particular value or not
The `Array#includes()` method is used to determine whether an array includes a particular value among its entries by returning either true or false. Let's see an example to find an element(numeric and string) within an array.
```javascript
var numericArray = [1, 2, 3, 4];
console.log(numericArray.includes(3)); // true
var stringArray = ["green", "yellow", "blue"];
console.log(stringArray.includes("blue")); //true
```
**[⬆ Back to Top](#table-of-contents)**
286. ### How do you compare scalar arrays
You can use length and every method of arrays to compare two scalar(compared directly using ===) arrays. The combination of these expressions can give the expected result,
```javascript
const arrayFirst = [1, 2, 3, 4, 5];
const arraySecond = [1, 2, 3, 4, 5];
console.log(
arrayFirst.length === arraySecond.length &&
arrayFirst.every((value, index) => value === arraySecond[index])
); // true
```
If you would like to compare arrays irrespective of order then you should sort them before,
```javascript
const arrayFirst = [2, 3, 1, 4, 5];
const arraySecond = [1, 2, 3, 4, 5];
console.log(
arrayFirst.length === arraySecond.length &&
arrayFirst.sort().every((value, index) => value === arraySecond[index])
); //true
```
**[⬆ Back to Top](#table-of-contents)**
287. ### How to get the value from get parameters
The `new URL()` object accepts the url string and `searchParams` property of this object can be used to access the get parameters. Remember that you may need to use polyfill or `window.location` to access the URL in older browsers(including IE).
```javascript
let urlString = "http://www.some-domain.com/about.html?x=1&y=2&z=3"; //window.location.href
let url = new URL(urlString);
let parameterZ = url.searchParams.get("z");
console.log(parameterZ); // 3
```
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288. ### How do you print numbers with commas as thousand separators
You can use the `Number.prototype.toLocaleString()` method which returns a string with a language-sensitive representation such as thousand separator,currency etc of this number.
```javascript
function convertToThousandFormat(x) {
return x.toLocaleString(); // 12,345.679
}
console.log(convertToThousandFormat(12345.6789));
```
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289. ### What is the difference between java and javascript
Both are totally unrelated programming languages and no relation between them. Java is statically typed, compiled, runs on its own VM. Whereas Javascript is dynamically typed, interpreted, and runs in a browser and nodejs environments. Let's see the major differences in a tabular format,
| Feature | Java | JavaScript |
|---- | ---- | -----
| Typed | It's a strongly typed language | It's a dynamic typed language |
| Paradigm | Object oriented programming | Prototype based programming |
| Scoping | Block scoped | Function-scoped |
| Concurrency | Thread based | event based |
| Memory | Uses more memory | Uses less memory. Hence it will be used for web pages |
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290. ### Does JavaScript supports namespace
JavaScript doesn’t support namespace by default. So if you create any element(function, method, object, variable) then it becomes global and pollutes the global namespace. Let's take an example of defining two functions without any namespace,
```javascript
function func1() {
console.log("This is a first definition");
}
function func1() {
console.log("This is a second definition");
}
func1(); // This is a second definition
```
It always calls the second function definition. In this case, namespace will solve the name collision problem.
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291. ### How do you declare namespace
Even though JavaScript lacks namespaces, we can use Objects , IIFE to create namespaces.
1. **Using Object Literal Notation:** Let's wrap variables and functions inside an Object literal which acts as a namespace. After that you can access them using object notation
```javascript
var namespaceOne = {
function func1() {
console.log("This is a first definition");
}
}
var namespaceTwo = {
function func1() {
console.log("This is a second definition");
}
}
namespaceOne.func1(); // This is a first definition
namespaceTwo.func1(); // This is a second definition
```
1. **Using IIFE (Immediately invoked function expression):** The outer pair of parentheses of IIFE creates a local scope for all the code inside of it and makes the anonymous function a function expression. Due to that, you can create the same function in two different function expressions to act as a namespace.
```javascript
(function () {
function fun1() {
console.log("This is a first definition");
}
fun1();
})();
(function () {
function fun1() {
console.log("This is a second definition");
}
fun1();
})();
```
1. **Using a block and a let/const declaration:** In ECMAScript 6, you can simply use a block and a let declaration to restrict the scope of a variable to a block.
```javascript
{
let myFunction = function fun1() {
console.log("This is a first definition");
};
myFunction();
}
//myFunction(): ReferenceError: myFunction is not defined.
{
let myFunction = function fun1() {
console.log("This is a second definition");
};
myFunction();
}
//myFunction(): ReferenceError: myFunction is not defined.
```
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292. ### How do you invoke javascript code in an iframe from parent page
Initially iFrame needs to be accessed using either `document.getElementBy` or `window.frames`. After that `contentWindow` property of iFrame gives the access for targetFunction
```javascript
document.getElementById("targetFrame").contentWindow.targetFunction();
window.frames[0].frameElement.contentWindow.targetFunction(); // Accessing iframe this way may not work in latest versions chrome and firefox
```
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293. ### How do get the timezone offset from date
You can use the `getTimezoneOffset` method of the date object. This method returns the time zone difference, in minutes, from current locale (host system settings) to UTC
```javascript
var offset = new Date().getTimezoneOffset();
console.log(offset); // -480
```
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294. ### How do you load CSS and JS files dynamically
You can create both link and script elements in the DOM and append them as child to head tag. Let's create a function to add script and style resources as below,
```javascript
function loadAssets(filename, filetype) {
if (filetype == "css") {
// External CSS file
var fileReference = document.createElement("link");
fileReference.setAttribute("rel", "stylesheet");
fileReference.setAttribute("type", "text/css");
fileReference.setAttribute("href", filename);
} else if (filetype == "js") {
// External JavaScript file
var fileReference = document.createElement("script");
fileReference.setAttribute("type", "text/javascript");
fileReference.setAttribute("src", filename);
}
if (typeof fileReference != "undefined")
document.getElementsByTagName("head")[0].appendChild(fileReference);
}
```
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295. ### What are the different methods to find HTML elements in DOM
If you want to access any element in an HTML page, you need to start with accessing the document object. Later you can use any of the below methods to find the HTML element,
1. document.getElementById(id): It finds an element by Id
2. document.getElementsByTagName(name): It finds an element by tag name
3. document.getElementsByClassName(name): It finds an element by class name
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296. ### What is jQuery
jQuery is a popular cross-browser JavaScript library that provides Document Object Model (DOM) traversal, event handling, animations and AJAX interactions by minimizing the discrepancies across browsers. It is widely famous with its philosophy of “Write less, do more”. For example, you can display welcome message on the page load using jQuery as below,
```javascript
$(document).ready(function () {
// It selects the document and apply the function on page load
alert("Welcome to jQuery world");
});
```
**Note:** You can download it from jquery's official site or install it from CDNs, like google.
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297. ### What is V8 JavaScript engine
V8 is an open source high-performance JavaScript engine used by the Google Chrome browser, written in C++. It is also being used in the node.js project. It implements ECMAScript and WebAssembly, and runs on Windows 7 or later, macOS 10.12+, and Linux systems that use x64, IA-32, ARM, or MIPS processors.
**Note:** It can run standalone, or can be embedded into any C++ application.
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298. ### Why do we call javascript as dynamic language
JavaScript is a loosely typed or a dynamic language because variables in JavaScript are not directly associated with any particular value type, and any variable can be assigned/reassigned with values of all types.
```javascript
let age = 50; // age is a number now
age = "old"; // age is a string now
age = true; // age is a boolean
```
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299. ### What is a void operator
The `void` operator evaluates the given expression and then returns undefined(i.e, without returning value). The syntax would be as below,
```javascript
void expression;
void expression;
```
Let's display a message without any redirection or reload
```javascript
Click here to see a message
```
**Note:** This operator is often used to obtain the undefined primitive value, using "void(0)".
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300. ### How to set the cursor to wait
The cursor can be set to wait in JavaScript by using the property "cursor". Let's perform this behavior on page load using the below function.
```javascript
function myFunction() {
window.document.body.style.cursor = "wait";
}
```
and this function invoked on page load
```html
```
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301. ### How do you create an infinite loop
You can create infinite loops using for and while loops without using any expressions. The for loop construct or syntax is better approach in terms of ESLint and code optimizer tools,
```javascript
for (;;) {}
while (true) {}
```
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302. ### Why do you need to avoid with statement
JavaScript's with statement was intended to provide a shorthand for writing recurring accesses to objects. So it can help reduce file size by reducing the need to repeat a lengthy object reference without performance penalty. Let's take an example where it is used to avoid redundancy when accessing an object several times.
```javascript
a.b.c.greeting = "welcome";
a.b.c.age = 32;
```
Using `with` it turns this into:
```javascript
with (a.b.c) {
greeting = "welcome";
age = 32;
}
```
But this `with` statement creates performance problems since one cannot predict whether an argument will refer to a real variable or to a property inside the with argument.
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303. ### What is the output of below for loops
```javascript
for (var i = 0; i < 4; i++) {
// global scope
setTimeout(() => console.log(i));
}
for (let i = 0; i < 4; i++) {
// block scope
setTimeout(() => console.log(i));
}
```
The output of the above for loops is 4 4 4 4 and 0 1 2 3
**Explanation:** Due to the event queue/loop of javascript, the `setTimeout` callback function is called after the loop has been executed. Since the variable i is declared with the `var` keyword it became a global variable and the value was equal to 4 using iteration when the time `setTimeout` function is invoked. Hence, the output of the first loop is `4 4 4 4`.
Whereas in the second loop, the variable i is declared as the `let` keyword it becomes a block scoped variable and it holds a new value(0, 1 ,2 3) for each iteration. Hence, the output of the first loop is `0 1 2 3`.
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304. ### List down some of the features of ES6
Below are the list of some new features of ES6,
1. Support for constants or immutable variables
2. Block-scope support for variables, constants and functions
3. Arrow functions
4. Default parameters
5. Rest and Spread Parameters
6. Template Literals
7. Multi-line Strings
8. Destructuring Assignment
9. Enhanced Object Literals
10. Promises
11. Classes
12. Modules
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305. ### What is ES6
ES6 is the sixth edition of the javascript language and it was released in June 2015. It was initially known as ECMAScript 6 (ES6) and later renamed to ECMAScript 2015. Almost all the modern browsers support ES6 but for the old browsers there are many transpilers, like Babel.js etc.
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306. ### Can I redeclare let and const variables
No, you cannot redeclare let and const variables. If you do, it throws below error
```bash
Uncaught SyntaxError: Identifier 'someVariable' has already been declared
```
**Explanation:** The variable declaration with `var` keyword refers to a function scope and the variable is treated as if it were declared at the top of the enclosing scope due to hoisting feature. So all the multiple declarations contributing to the same hoisted variable without any error. Let's take an example of re-declaring variables in the same scope for both var and let/const variables.
```javascript
var name = "John";
function myFunc() {
var name = "Nick";
var name = "Abraham"; // Re-assigned in the same function block
alert(name); // Abraham
}
myFunc();
alert(name); // John
```
The block-scoped multi-declaration throws syntax error,
```javascript
let name = "John";
function myFunc() {
let name = "Nick";
let name = "Abraham"; // Uncaught SyntaxError: Identifier 'name' has already been declared
alert(name);
}
myFunc();
alert(name);
```
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307. ### Is const variable makes the value immutable
No, the const variable doesn't make the value immutable. But it disallows subsequent assignments(i.e, You can declare with assignment but can't assign another value later)
```javascript
const userList = [];
userList.push("John"); // Can mutate even though it can't re-assign
console.log(userList); // ['John']
```
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308. ### What are default parameters
In ES5, we need to depend on logical OR operators to handle default values of function parameters. Whereas in ES6, Default function parameters feature allows parameters to be initialized with default values if no value or undefined is passed. Let's compare the behavior with an examples,
```javascript
//ES5
var calculateArea = function (height, width) {
height = height || 50;
width = width || 60;
return width * height;
};
console.log(calculateArea()); //300
```
The default parameters makes the initialization more simpler,
```javascript
//ES6
var calculateArea = function (height = 50, width = 60) {
return width * height;
};
console.log(calculateArea()); //300
```
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309. ### What are template literals
Template literals or template strings are string literals allowing embedded expressions. These are enclosed by the back-tick (`) character instead of double or single quotes.
In ES6, this feature enables using dynamic expressions as below,
```javascript
var greeting = `Welcome to JS World, Mr. ${firstName} ${lastName}.`;
```
In ES5, you need break string like below,
```javascript
var greeting = 'Welcome to JS World, Mr. ' + firstName + ' ' + lastName.`
```
**Note:** You can use multi-line strings and string interpolation features with template literals.
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310. ### How do you write multi-line strings in template literals
In ES5, you would have to use newline escape characters('\\n') and concatenation symbols(+) in order to get multi-line strings.
```javascript
console.log("This is string sentence 1\n" + "This is string sentence 2");
```
Whereas in ES6, You don't need to mention any newline sequence character,
```javascript
console.log(`This is string sentence
'This is string sentence 2`);
```
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311. ### What are nesting templates
The nesting template is a feature supported within template literals syntax to allow inner backticks inside a placeholder ${ } within the template. For example, the below nesting template is used to display the icons based on user permissions whereas outer template checks for platform type,
```javascript
const iconStyles = `icon ${
isMobilePlatform()
? ""
: `icon-${user.isAuthorized ? "submit" : "disabled"}`
}`;
```
You can write the above use case without nesting template features as well. However, the nesting template feature is more compact and readable.
```javascript
//Without nesting templates
const iconStyles = `icon ${
isMobilePlatform()
? ""
: user.isAuthorized
? "icon-submit"
: "icon-disabled"
}`;
```
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312. ### What are tagged templates
Tagged templates are the advanced form of templates in which tags allow you to parse template literals with a function. The tag function accepts the first parameter as an array of strings and remaining parameters as expressions. This function can also return manipulated strings based on parameters. Let's see the usage of this tagged template behavior of an IT professional skill set in an organization,
```javascript
var user1 = "John";
var skill1 = "JavaScript";
var experience1 = 15;
var user2 = "Kane";
var skill2 = "JavaScript";
var experience2 = 5;
function myInfoTag(strings, userExp, experienceExp, skillExp) {
var str0 = strings[0]; // "Mr/Ms. "
var str1 = strings[1]; // " is a/an "
var str2 = strings[2]; // "in"
var expertiseStr;
if (experienceExp > 10) {
expertiseStr = "expert developer";
} else if (skillExp > 5 && skillExp <= 10) {
expertiseStr = "senior developer";
} else {
expertiseStr = "junior developer";
}
return `${str0}${userExp}${str1}${expertiseStr}${str2}${skillExp}`;
}
var output1 = myInfoTag`Mr/Ms. ${user1} is a/an ${experience1} in ${skill1}`;
var output2 = myInfoTag`Mr/Ms. ${user2} is a/an ${experience2} in ${skill2}`;
console.log(output1); // Mr/Ms. John is a/an expert developer in JavaScript
console.log(output2); // Mr/Ms. Kane is a/an junior developer in JavaScript
```
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313. ### What are raw strings
ES6 provides a raw strings feature using the `String.raw()` method which is used to get the raw string form of template strings. This feature allows you to access the raw strings as they were entered, without processing escape sequences. For example, the usage would be as below,
```javascript
var calculationString = String.raw`The sum of numbers is \n${
1 + 2 + 3 + 4
}!`;
console.log(calculationString); // The sum of numbers is \n10!
```
If you don't use raw strings, the newline character sequence will be processed by displaying the output in multiple lines
```javascript
var calculationString = `The sum of numbers is \n${1 + 2 + 3 + 4}!`;
console.log(calculationString);
// The sum of numbers is
// 10!
```
Also, the raw property is available on the first argument to the tag function
```javascript
function tag(strings) {
console.log(strings.raw[0]);
}
```
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314. ### What is destructuring assignment
The destructuring assignment is a JavaScript expression that makes it possible to unpack values from arrays or properties from objects into distinct variables.
Let's get the month values from an array using destructuring assignment
```javascript
var [one, two, three] = ["JAN", "FEB", "MARCH"];
console.log(one); // "JAN"
console.log(two); // "FEB"
console.log(three); // "MARCH"
```
and you can get user properties of an object using destructuring assignment,
```javascript
var { name, age } = { name: "John", age: 32 };
console.log(name); // John
console.log(age); // 32
```
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315. ### What are default values in destructuring assignment
A variable can be assigned a default value when the value unpacked from the array or object is undefined during destructuring assignment. It helps to avoid setting default values separately for each assignment. Let's take an example for both arrays and object use cases,
**Arrays destructuring:**
```javascript
var x, y, z;
[x = 2, y = 4, z = 6] = [10];
console.log(x); // 10
console.log(y); // 4
console.log(z); // 6
```
**Objects destructuring:**
```javascript
var { x = 2, y = 4, z = 6 } = { x: 10 };
console.log(x); // 10
console.log(y); // 4
console.log(z); // 6
```
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316. ### How do you swap variables in destructuring assignment
If you don't use destructuring assignment, swapping two values requires a temporary variable. Whereas using a destructuring feature, two variable values can be swapped in one destructuring expression. Let's swap two number variables in array destructuring assignment,
```javascript
var x = 10,
y = 20;
[x, y] = [y, x];
console.log(x); // 20
console.log(y); // 10
```
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317. ### What are enhanced object literals
Object literals make it easy to quickly create objects with properties inside the curly braces. For example, it provides shorter syntax for common object property definition as below.
```javascript
//ES6
var x = 10,
y = 20;
obj = { x, y };
console.log(obj); // {x: 10, y:20}
//ES5
var x = 10,
y = 20;
obj = { x: x, y: y };
console.log(obj); // {x: 10, y:20}
```
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318. ### What are dynamic imports
The dynamic imports using `import()` function syntax allows us to load modules on demand by using promises or the async/await syntax. Currently this feature is in [stage4 proposal](https://github.com/tc39/proposal-dynamic-import). The main advantage of dynamic imports is reduction of our bundle's sizes, the size/payload response of our requests and overall improvements in the user experience.
The syntax of dynamic imports would be as below,
```javascript
import("./Module").then((Module) => Module.method());
```
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319. ### What are the use cases for dynamic imports
Below are some of the use cases of using dynamic imports over static imports,
1. Import a module on-demand or conditionally. For example, if you want to load a polyfill on legacy browser
```javascript
if (isLegacyBrowser()) {
import(···)
.then(···);
}
```
1. Compute the module specifier at runtime. For example, you can use it for internationalization.
```javascript
import(`messages_${getLocale()}.js`).then(···);
```
1. Import a module from within a regular script instead a module.
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320. ### What are typed arrays
Typed arrays are array-like objects from ECMAScript 6 API for handling binary data. JavaScript provides 8 Typed array types,
1. Int8Array: An array of 8-bit signed integers
2. Int16Array: An array of 16-bit signed integers
3. Int32Array: An array of 32-bit signed integers
4. Uint8Array: An array of 8-bit unsigned integers
5. Uint16Array: An array of 16-bit unsigned integers
6. Uint32Array: An array of 32-bit unsigned integers
7. Float32Array: An array of 32-bit floating point numbers
8. Float64Array: An array of 64-bit floating point numbers
For example, you can create an array of 8-bit signed integers as below
```javascript
const a = new Int8Array();
// You can pre-allocate n bytes
const bytes = 1024;
const a = new Int8Array(bytes);
```
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321. ### What are the advantages of module loaders
The module loaders provides the below features,
1. Dynamic loading
2. State isolation
3. Global namespace isolation
4. Compilation hooks
5. Nested virtualization
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322. ### What is collation
Collation is used for sorting a set of strings and searching within a set of strings. It is parameterized by locale and aware of Unicode. Let's take comparison and sorting features,
1. **Comparison:**
```javascript
var list = ["ä", "a", "z"]; // In German, "ä" sorts with "a" Whereas in Swedish, "ä" sorts after "z"
var l10nDE = new Intl.Collator("de");
var l10nSV = new Intl.Collator("sv");
console.log(l10nDE.compare("ä", "z") === -1); // true
console.log(l10nSV.compare("ä", "z") === +1); // true
```
1. **Sorting:**
```javascript
var list = ["ä", "a", "z"]; // In German, "ä" sorts with "a" Whereas in Swedish, "ä" sorts after "z"
var l10nDE = new Intl.Collator("de");
var l10nSV = new Intl.Collator("sv");
console.log(list.sort(l10nDE.compare)); // [ "a", "ä", "z" ]
console.log(list.sort(l10nSV.compare)); // [ "a", "z", "ä" ]
```
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323. ### What is for...of statement
The for...of statement creates a loop iterating over iterable objects or elements such as built-in String, Array, Array-like objects (like arguments or NodeList), TypedArray, Map, Set, and user-defined iterables. The basic usage of for...of statement on arrays would be as below,
```javascript
let arrayIterable = [10, 20, 30, 40, 50];
for (let value of arrayIterable) {
value++;
console.log(value); // 11 21 31 41 51
}
```
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324. ### What is the output of below spread operator array
```javascript
[..."John Resig"];
```
The output of the array is ['J', 'o', 'h', 'n', '', 'R', 'e', 's', 'i', 'g']
**Explanation:** The string is an iterable type and the spread operator within an array maps every character of an iterable to one element. Hence, each character of a string becomes an element within an Array.
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325. ### Is PostMessage secure
Yes, postMessages can be considered very secure as long as the programmer/developer is careful about checking the origin and source of an arriving message. But if you try to send/receive a message without verifying its source will create cross-site scripting attacks.
**[⬆ Back to Top](#table-of-contents)**
326. ### What are the problems with postmessage target origin as wildcard
The second argument of postMessage method specifies which origin is allowed to receive the message. If you use the wildcard “\*” as an argument then any origin is allowed to receive the message. In this case, there is no way for the sender window to know if the target window is at the target origin when sending the message. If the target window has been navigated to another origin, the other origin would receive the data. Hence, this may lead to XSS vulnerabilities.
```javascript
targetWindow.postMessage(message, "*");
```
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327. ### How do you avoid receiving postMessages from attackers
Since the listener listens for any message, an attacker can trick the application by sending a message from the attacker’s origin, which gives an impression that the receiver received the message from the actual sender’s window. You can avoid this issue by validating the origin of the message on the receiver's end using the “message.origin” attribute. For examples, let's check the sender's origin [http://www.some-sender.com](http://www.some-sender.com) on receiver side [www.some-receiver.com](www.some-receiver.com),
```javascript
//Listener on http://www.some-receiver.com/
window.addEventListener("message", function(message){
if(/^http://www\.some-sender\.com$/.test(message.origin)){
console.log('You received the data from valid sender', message.data);
}
});
```
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328. ### Can I avoid using postMessages completely
You cannot avoid using postMessages completely(or 100%). Even though your application doesn’t use postMessage considering the risks, a lot of third party scripts use postMessage to communicate with the third party service. So your application might be using postMessage without your knowledge.
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329. ### Is postMessages synchronous
The postMessages are synchronous in IE8 browser but they are asynchronous in IE9 and all other modern browsers (i.e, IE9+, Firefox, Chrome, Safari).Due to this asynchronous behaviour, we use a callback mechanism when the postMessage is returned.
**[⬆ Back to Top](#table-of-contents)**
330. ### What paradigm is Javascript
JavaScript is a multi-paradigm language, supporting imperative/procedural programming, Object-Oriented Programming and functional programming. JavaScript supports Object-Oriented Programming with prototypical inheritance.
**[⬆ Back to Top](#table-of-contents)**
331. ### What is the difference between internal and external javascript
**Internal JavaScript:** It is the source code within the script tag.
**External JavaScript:** The source code is stored in an external file(stored with .js extension) and referred with in the tag.
**[⬆ Back to Top](#table-of-contents)**
332. ### Is JavaScript faster than server side script
Yes, JavaScript is faster than server side scripts. Because JavaScript is a client-side script it does not require any web server’s help for its computation or calculation. So JavaScript is always faster than any server-side script like ASP, PHP, etc.
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333. ### How do you get the status of a checkbox
You can apply the `checked` property on the selected checkbox in the DOM. If the value is `true` it means the checkbox is checked, otherwise it is unchecked. For example, the below HTML checkbox element can be access using javascript as below:
```html
Agree the
conditions
```
```javascript
console.log(document.getElementById(‘checkboxname’).checked); // true or false
```
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334. ### What is the purpose of double tilde operator
The double tilde operator(~~) is known as double NOT bitwise operator. This operator is a slightly quicker substitute for Math.floor().
**[⬆ Back to Top](#table-of-contents)**
335. ### How do you convert character to ASCII code
You can use the `String.prototype.charCodeAt()` method to convert string characters to ASCII numbers. For example, let's find ASCII code for the first letter of 'ABC' string,
```javascript
"ABC".charCodeAt(0); // returns 65
```
Whereas `String.fromCharCode()` method converts numbers to equal ASCII characters.
```javascript
String.fromCharCode(65, 66, 67); // returns 'ABC'
```
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336. ### What is ArrayBuffer
An ArrayBuffer object is used to represent a generic, fixed-length raw binary data buffer. You can create it as below,
```javascript
let buffer = new ArrayBuffer(16); // create a buffer of length 16
alert(buffer.byteLength); // 16
```
To manipulate an ArrayBuffer, we need to use a “view” object.
```javascript
//Create a DataView referring to the buffer
let view = new DataView(buffer);
```
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337. ### What is the output of below string expression
```javascript
console.log("Welcome to JS world"[0]);
```
The output of the above expression is "W".
**Explanation:** The bracket notation with specific index on a string returns the character at a specific location. Hence, it returns the character "W" of the string. Since this is not supported in IE7 and below versions, you may need to use the .charAt() method to get the desired result.
**[⬆ Back to Top](#table-of-contents)**
338. ### What is the purpose of Error object
The Error constructor creates an error object and the instances of error objects are thrown when runtime errors occur. The Error object can also be used as a base object for user-defined exceptions. The syntax of error object would be as below,
```javascript
new Error([message[, fileName[, lineNumber]]])
```
You can throw user defined exceptions or errors using Error object in try...catch block as below,
```javascript
try {
if (withdraw > balance)
throw new Error("Oops! You don't have enough balance");
} catch (e) {
console.log(e.name + ": " + e.message);
}
```
**[⬆ Back to Top](#table-of-contents)**
339. ### What is the purpose of EvalError object
The EvalError object indicates an error regarding the global `eval()` function. Even though this exception is not thrown by JavaScript anymore, the EvalError object remains for compatibility. The syntax of this expression would be as below,
```javascript
new EvalError([message[, fileName[, lineNumber]]])
```
You can throw EvalError with in try...catch block as below,
```javascript
try {
throw new EvalError('Eval function error', 'someFile.js', 100);
} catch (e) {
console.log(e.message, e.name, e.fileName); // "Eval function error", "EvalError", "someFile.js"
```
**[⬆ Back to Top](#table-of-contents)**
340. ### What are the list of cases error thrown from non-strict mode to strict mode
When you apply 'use strict'; syntax, some of the below cases will throw a SyntaxError before executing the script
1. When you use Octal syntax
```javascript
var n = 022;
```
1. Using `with` statement
2. When you use delete operator on a variable name
3. Using eval or arguments as variable or function argument name
4. When you use newly reserved keywords
5. When you declare a function in a block
```javascript
if (someCondition) {
function f() {}
}
```
Hence, the errors from above cases are helpful to avoid errors in development/production environments.
**[⬆ Back to Top](#table-of-contents)**
341. ### Do all objects have prototypes
No. All objects have prototypes except for the base object which is created by the user, or an object that is created using the new keyword.
**[⬆ Back to Top](#table-of-contents)**
342. ### What is the difference between a parameter and an argument
Parameter is the variable name of a function definition whereas an argument represents the value given to a function when it is invoked. Let's explain this with a simple function
```javascript
function myFunction(parameter1, parameter2, parameter3) {
console.log(arguments[0]); // "argument1"
console.log(arguments[1]); // "argument2"
console.log(arguments[2]); // "argument3"
}
myFunction("argument1", "argument2", "argument3");
```
**[⬆ Back to Top](#table-of-contents)**
343. ### What is the purpose of some method in arrays
The some() method is used to test whether at least one element in the array passes the test implemented by the provided function. The method returns a boolean value. Let's take an example to test for any odd elements,
```javascript
var array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
var odd = (element) => element % 2 !== 0;
console.log(array.some(odd)); // true (the odd element exists)
```
**[⬆ Back to Top](#table-of-contents)**
344. ### How do you combine two or more arrays
The concat() method is used to join two or more arrays by returning a new array containing all the elements. The syntax would be as below,
```javascript
array1.concat(array2, array3, ..., arrayX)
```
Let's take an example of array's concatenation with veggies and fruits arrays,
```javascript
var veggies = ["Tomato", "Carrot", "Cabbage"];
var fruits = ["Apple", "Orange", "Pears"];
var veggiesAndFruits = veggies.concat(fruits);
console.log(veggiesAndFruits); // Tomato, Carrot, Cabbage, Apple, Orange, Pears
```
**[⬆ Back to Top](#table-of-contents)**
345. ### What is the difference between Shallow and Deep copy
There are two ways to copy an object,
**Shallow Copy:**
Shallow copy is a bitwise copy of an object. A new object is created that has an exact copy of the values in the original object. If any of the fields of the object are references to other objects, just the reference addresses are copied i.e., only the memory address is copied.
**Example**
```javascript
var empDetails = {
name: "John",
age: 25,
expertise: "Software Developer",
};
```
to create a duplicate
```javascript
var empDetailsShallowCopy = empDetails; //Shallow copying!
```
if we change some property value in the duplicate one like this:
```javascript
empDetailsShallowCopy.name = "Johnson";
```
The above statement will also change the name of `empDetails`, since we have a shallow copy. That means we're losing the original data as well.
**Deep copy:**
A deep copy copies all fields, and makes copies of dynamically allocated memory pointed to by the fields. A deep copy occurs when an object is copied along with the objects to which it refers.
**Example**
```javascript
var empDetails = {
name: "John",
age: 25,
expertise: "Software Developer",
};
```
Create a deep copy by using the properties from the original object into new variable
```javascript
var empDetailsDeepCopy = {
name: empDetails.name,
age: empDetails.age,
expertise: empDetails.expertise,
};
```
Now if you change `empDetailsDeepCopy.name`, it will only affect `empDetailsDeepCopy` & not `empDetails`
**[⬆ Back to Top](#table-of-contents)**
346. ### How do you create specific number of copies of a string
The `repeat()` method is used to construct and return a new string which contains the specified number of copies of the string on which it was called, concatenated together. Remember that this method has been added to the ECMAScript 2015 specification.
Let's take an example of Hello string to repeat it 4 times,
```javascript
"Hello".repeat(4); // 'HelloHelloHelloHello'
```
347. ### How do you return all matching strings against a regular expression
The `matchAll()` method can be used to return an iterator of all results matching a string against a regular expression. For example, the below example returns an array of matching string results against a regular expression,
```javascript
let regexp = /Hello(\d?))/g;
let greeting = "Hello1Hello2Hello3";
let greetingList = [...greeting.matchAll(regexp)];
console.log(greetingList[0]); //Hello1
console.log(greetingList[1]); //Hello2
console.log(greetingList[2]); //Hello3
```
**[⬆ Back to Top](#table-of-contents)**
348. ### How do you trim a string at the beginning or ending
The `trim` method of string prototype is used to trim on both sides of a string. But if you want to trim especially at the beginning or ending of the string then you can use `trimStart/trimLeft` and `trimEnd/trimRight` methods. Let's see an example of these methods on a greeting message,
```javascript
var greeting = " Hello, Goodmorning! ";
console.log(greeting); // " Hello, Goodmorning! "
console.log(greeting.trimStart()); // "Hello, Goodmorning! "
console.log(greeting.trimLeft()); // "Hello, Goodmorning! "
console.log(greeting.trimEnd()); // " Hello, Goodmorning!"
console.log(greeting.trimRight()); // " Hello, Goodmorning!"
```
**[⬆ Back to Top](#table-of-contents)**
349. ### What is the output of below console statement with unary operator
Let's take console statement with unary operator as given below,
```javascript
console.log(+"Hello");
```
The output of the above console log statement returns NaN. Because the element is prefixed by the unary operator and the JavaScript interpreter will try to convert that element into a number type. Since the conversion fails, the value of the statement results in NaN value.
**[⬆ Back to Top](#table-of-contents)**
350. ### Does javascript uses mixins
Mixin is a generic object-oriented programming term - is a class containing methods that can be used by other classes without a need to inherit from it. In JavaScript we can only inherit from a single object. ie. There can be only one `[[prototype]]` for an object.
But sometimes we require to extend more than one, to overcome this we can use Mixin which helps to copy methods to the prototype of another class.
Say for instance, we've two classes `User` and `CleanRoom`. Suppose we need to add `CleanRoom` functionality to `User`, so that user can clean the room at demand. Here's where concept called mixins comes into picture.
```javascript
// mixin
let cleanRoomMixin = {
cleanRoom() {
alert(`Hello ${this.name}, your room is clean now`);
},
sayBye() {
alert(`Bye ${this.name}`);
},
};
// usage:
class User {
constructor(name) {
this.name = name;
}
}
// copy the methods
Object.assign(User.prototype, cleanRoomMixin);
// now User can clean the room
new User("Dude").cleanRoom(); // Hello Dude, your room is clean now!
```
**[⬆ Back to Top](#table-of-contents)**
351. ### What is a thunk function
A thunk is just a function which delays the evaluation of the value. It doesn’t take any arguments but gives the value whenever you invoke the thunk. i.e, It is used not to execute now but it will be sometime in the future. Let's take a synchronous example,
```javascript
const add = (x, y) => x + y;
const thunk = () => add(2, 3);
thunk(); // 5
```
**[⬆ Back to Top](#table-of-contents)**
352. ### What are asynchronous thunks
The asynchronous thunks are useful to make network requests. Let's see an example of network requests,
```javascript
function fetchData(fn) {
fetch("https://jsonplaceholder.typicode.com/todos/1")
.then((response) => response.json())
.then((json) => fn(json));
}
const asyncThunk = function () {
return fetchData(function getData(data) {
console.log(data);
});
};
asyncThunk();
```
The `getData` function won't be called immediately but it will be invoked only when the data is available from API endpoint. The setTimeout function is also used to make our code asynchronous. The best real time example is redux state management library which uses the asynchronous thunks to delay the actions to dispatch.
**[⬆ Back to Top](#table-of-contents)**
353. ### What is the output of below function calls
**Code snippet:**
```javascript
const circle = {
radius: 20,
diameter() {
return this.radius * 2;
},
perimeter: () => 2 * Math.PI * this.radius,
};
```
```javascript
console.log(circle.diameter());
console.log(circle.perimeter());
```
**Output:**
The output is 40 and NaN. Remember that diameter is a regular function, whereas the value of perimeter is an arrow function. The `this` keyword of a regular function(i.e, diameter) refers to the surrounding scope which is a class(i.e, Shape object). Whereas this keyword of perimeter function refers to the surrounding scope which is a window object. Since there is no radius property on window objects it returns an undefined value and the multiple of number value returns NaN value.
**[⬆ Back to Top](#table-of-contents)**
354. ### How to remove all line breaks from a string
The easiest approach is using regular expressions to detect and replace newlines in the string. In this case, we use replace function along with string to replace with, which in our case is an empty string.
```javascript
function remove_linebreaks( var message ) {
return message.replace( /[\r\n]+/gm, "" );
}
```
In the above expression, g and m are for global and multiline flags.
**[⬆ Back to Top](#table-of-contents)**
355. ### What is the difference between reflow and repaint
A _repaint_ occurs when changes are made which affect the visibility of an element, but not its layout. Examples of this include outline, visibility, or background color. A _reflow_ involves changes that affect the layout of a portion of the page (or the whole page). Resizing the browser window, changing the font, content changing (such as user typing text), using JavaScript methods involving computed styles, adding or removing elements from the DOM, and changing an element's classes are a few of the things that can trigger reflow. Reflow of an element causes the subsequent reflow of all child and ancestor elements as well as any elements following it in the DOM.
**[⬆ Back to Top](#table-of-contents)**
356. ### What happens with negating an array
Negating an array with `!` character will coerce the array into a boolean. Since Arrays are considered to be truthy So negating it will return `false`.
```javascript
console.log(![]); // false
```
**[⬆ Back to Top](#table-of-contents)**
357. ### What happens if we add two arrays
If you add two arrays together, it will convert them both to strings and concatenate them. For example, the result of adding arrays would be as below,
```javascript
console.log(["a"] + ["b"]); // "ab"
console.log([] + []); // ""
console.log(![] + []); // "false", because ![] returns false.
```
**[⬆ Back to Top](#table-of-contents)**
358. ### What is the output of prepend additive operator on falsy values
If you prepend the additive(+) operator on falsy values(null, undefined, NaN, false, ""), the falsy value converts to a number value zero. Let's display them on browser console as below,
```javascript
console.log(+null); // 0
console.log(+undefined); // NaN
console.log(+false); // 0
console.log(+NaN); // NaN
console.log(+""); // 0
```
**[⬆ Back to Top](#table-of-contents)**
359. ### How do you create self string using special characters
The self string can be formed with the combination of `[]()!+` characters. You need to remember the below conventions to achieve this pattern.
1. Since Arrays are truthful values, negating the arrays will produce false: ![] === false
2. As per JavaScript coercion rules, the addition of arrays together will toString them: [] + [] === ""
3. Prepend an array with + operator will convert an array to false, the negation will make it true and finally converting the result will produce value '1': +(!(+[])) === 1
By applying the above rules, we can derive below conditions
```javascript
(![] + [] === "false" + !+[]) === 1;
```
Now the character pattern would be created as below,
```javascript
s e l f
^^^^^^^^^^^^^ ^^^^^^^^^^^^^ ^^^^^^^^^^^^^ ^^^^^^^^^^^^^
(![] + [])[3] + (![] + [])[4] + (![] + [])[2] + (![] + [])[0]
^^^^^^^^^^^^^ ^^^^^^^^^^^^^ ^^^^^^^^^^^^^ ^^^^^^^^^^^^^
(![] + [])[+!+[]+!+[]+!+[]] +
(![] + [])[+!+[]+!+[]+!+[]+!+[]] +
(![] + [])[+!+[]+!+[]] +
(![] + [])[+[]]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(![]+[])[+!+[]+!+[]+!+[]]+(![]+[])[+!+[]+!+[]+!+[]+!+[]]+(![]+[])[+!+[]+!+[]]+(![]+[])[+[]]
```
**[⬆ Back to Top](#table-of-contents)**
360. ### How do you remove falsy values from an array
You can apply the filter method on the array by passing Boolean as a parameter. This way it removes all falsy values(0, undefined, null, false and "") from the array.
```javascript
const myArray = [false, null, 1, 5, undefined];
myArray.filter(Boolean); // [1, 5] // is same as myArray.filter(x => x);
```
**[⬆ Back to Top](#table-of-contents)**
361. ### How do you get unique values of an array
You can get unique values of an array with the combination of `Set` and rest expression/spread(...) syntax.
```javascript
console.log([...new Set([1, 2, 4, 4, 3])]); // [1, 2, 4, 3]
```
**[⬆ Back to Top](#table-of-contents)**
362. ### What is destructuring aliases
Sometimes you would like to have a destructured variable with a different name than the property name. In that case, you'll use a `: newName` to specify a name for the variable. This process is called destructuring aliases.
```javascript
const obj = { x: 1 };
// Grabs obj.x as as { otherName }
const { x: otherName } = obj;
```
**[⬆ Back to Top](#table-of-contents)**
363. ### How do you map the array values without using map method
You can map the array values without using the `map` method by just using the `from` method of Array. Let's map city names from Countries array,
```javascript
const countries = [
{ name: "India", capital: "Delhi" },
{ name: "US", capital: "Washington" },
{ name: "Russia", capital: "Moscow" },
{ name: "Singapore", capital: "Singapore" },
{ name: "China", capital: "Beijing" },
{ name: "France", capital: "Paris" },
];
const cityNames = Array.from(countries, ({ capital }) => capital);
console.log(cityNames); // ['Delhi, 'Washington', 'Moscow', 'Singapore', 'Beijing', 'Paris']
```
**[⬆ Back to Top](#table-of-contents)**
364. ### How do you empty an array
You can empty an array quickly by setting the array length to zero.
```javascript
let cities = ["Singapore", "Delhi", "London"];
cities.length = 0; // cities becomes []
```
**[⬆ Back to Top](#table-of-contents)**
365. ### How do you rounding numbers to certain decimals
You can round numbers to a certain number of decimals using `toFixed` method from native javascript.
```javascript
let pie = 3.141592653;
pie = pie.toFixed(3); // 3.142
```
**[⬆ Back to Top](#table-of-contents)**
366. ### What is the easiest way to convert an array to an object
You can convert an array to an object with the same data using spread(...) operator.
```javascript
var fruits = ["banana", "apple", "orange", "watermelon"];
var fruitsObject = { ...fruits };
console.log(fruitsObject); // {0: "banana", 1: "apple", 2: "orange", 3: "watermelon"}
```
**[⬆ Back to Top](#table-of-contents)**
367. ### How do you create an array with some data
You can create an array with some data or an array with the same values using `fill` method.
```javascript
var newArray = new Array(5).fill("0");
console.log(newArray); // ["0", "0", "0", "0", "0"]
```
**[⬆ Back to Top](#table-of-contents)**
368. ### What are the placeholders from console object
Below are the list of placeholders available from console object,
1. %o — It takes an object,
2. %s — It takes a string,
3. %d — It is used for a decimal or integer
These placeholders can be represented in the console.log as below
```javascript
const user = { name: "John", id: 1, city: "Delhi" };
console.log(
"Hello %s, your details %o are available in the object form",
"John",
user
); // Hello John, your details {name: "John", id: 1, city: "Delhi"} are available in object
```
**[⬆ Back to Top](#table-of-contents)**
369. ### Is it possible to add CSS to console messages
Yes, you can apply CSS styles to console messages similar to html text on the web page.
```javascript
console.log(
"%c The text has blue color, with large font and red background",
"color: blue; font-size: x-large; background: red"
);
```
The text will be displayed as below,
**Note:** All CSS styles can be applied to console messages.
**[⬆ Back to Top](#table-of-contents)**
370. ### What is the purpose of dir method of console object
The `console.dir()` is used to display an interactive list of the properties of the specified JavaScript object as JSON.
```javascript
const user = { name: "John", id: 1, city: "Delhi" };
console.dir(user);
```
The user object displayed in JSON representation
**[⬆ Back to Top](#table-of-contents)**
371. ### Is it possible to debug HTML elements in console
Yes, it is possible to get and debug HTML elements in the console just like inspecting elements.
```javascript
const element = document.getElementsByTagName("body")[0];
console.log(element);
```
It prints the HTML element in the console,
**[⬆ Back to Top](#table-of-contents)**
372. ### How do you display data in a tabular format using console object
The `console.table()` is used to display data in the console in a tabular format to visualize complex arrays or objects.
```js
const users = [
{ name: "John", id: 1, city: "Delhi" },
{ name: "Max", id: 2, city: "London" },
{ name: "Rod", id: 3, city: "Paris" },
];
console.table(users);
```
The data visualized in a table format,
**Not:** Remember that `console.table()` is not supported in IE.
**[⬆ Back to Top](#table-of-contents)**
373. ### How do you verify that an argument is a Number or not
The combination of IsNaN and isFinite methods are used to confirm whether an argument is a number or not.
```javascript
function isNumber(n) {
return !isNaN(parseFloat(n)) && isFinite(n);
}
```
**[⬆ Back to Top](#table-of-contents)**
374. ### How do you create copy to clipboard button
You need to select the content(using .select() method) of the input element and execute the copy command with execCommand (i.e, execCommand('copy')). You can also execute other system commands like cut and paste.
```javascript
document.querySelector("#copy-button").onclick = function () {
// Select the content
document.querySelector("#copy-input").select();
// Copy to the clipboard
document.execCommand("copy");
};
```
**[⬆ Back to Top](#table-of-contents)**
375. ### What is the shortcut to get timestamp
You can use `new Date().getTime()` to get the current timestamp. There is an alternative shortcut to get the value.
```javascript
console.log(+new Date());
console.log(Date.now());
```
**[⬆ Back to Top](#table-of-contents)**
376. ### How do you flattening multi dimensional arrays
Flattening bi-dimensional arrays is trivial with Spread operator.
```javascript
const biDimensionalArr = [11, [22, 33], [44, 55], [66, 77], 88, 99];
const flattenArr = [].concat(...biDimensionalArr); // [11, 22, 33, 44, 55, 66, 77, 88, 99]
```
But you can make it work with multi-dimensional arrays by recursive calls,
```javascript
function flattenMultiArray(arr) {
const flattened = [].concat(...arr);
return flattened.some((item) => Array.isArray(item))
? flattenMultiArray(flattened)
: flattened;
}
const multiDimensionalArr = [11, [22, 33], [44, [55, 66, [77, [88]], 99]]];
const flatArr = flattenMultiArray(multiDimensionalArr); // [11, 22, 33, 44, 55, 66, 77, 88, 99]
```
Also you can use the `flat` method of Array.
```javascript
const arr = [1, [2, 3], 4, 5, [6, 7]];
const fllattenArr = arr.flat(); // [1, 2, 3, 4, 5, 6, 7]
// And for multiDemensional arrays
const multiDimensionalArr = [11, [22, 33], [44, [55, 66, [77, [88]], 99]]];
const oneStepFlat = multiDimensionalArr.flat(1); // [11, 22, 33, 44, [55, 66, [77, [88]], 99]]
const towStep = multiDimensionalArr.flat(2); // [11, 22, 33, 44, 55, 66, [77, [88]], 99]
const fullyFlatArray = multiDimensionalArr.flat(Infinity); // [11, 22, 33, 44, 55, 66, 77, 88, 99]
```
**[⬆ Back to Top](#table-of-contents)**
377. ### What is the easiest multi condition checking
You can use `indexOf` to compare input with multiple values instead of checking each value as one condition.
```javascript
// Verbose approach
if (
input === "first" ||
input === 1 ||
input === "second" ||
input === 2
) {
someFunction();
}
// Shortcut
if (["first", 1, "second", 2].indexOf(input) !== -1) {
someFunction();
}
```
**[⬆ Back to Top](#table-of-contents)**
378. ### How do you capture browser back button
The `window.onbeforeunload` method is used to capture browser back button events. This is helpful to warn users about losing the current data.
```javascript
window.onbeforeunload = function () {
alert("You work will be lost");
};
```
**[⬆ Back to Top](#table-of-contents)**
379. ### How do you disable right click in the web page
The right click on the page can be disabled by returning false from the `oncontextmenu` attribute on the body element.
```html
```
**[⬆ Back to Top](#table-of-contents)**
380. ### What are wrapper objects
Primitive Values like string,number and boolean don't have properties and methods but they are temporarily converted or coerced to an object(Wrapper object) when you try to perform actions on them. For example, if you apply toUpperCase() method on a primitive string value, it does not throw an error but returns uppercase of the string.
```javascript
let name = "john";
console.log(name.toUpperCase()); // Behind the scenes treated as console.log(new String(name).toUpperCase());
```
i.e, Every primitive except null and undefined have Wrapper Objects and the list of wrapper objects are String,Number,Boolean,Symbol and BigInt.
**[⬆ Back to Top](#table-of-contents)**
381. ### What is AJAX
AJAX stands for Asynchronous JavaScript and XML and it is a group of related technologies(HTML, CSS, JavaScript, XMLHttpRequest API etc) used to display data asynchronously. i.e. We can send data to the server and get data from the server without reloading the web page.
**[⬆ Back to Top](#table-of-contents)**
382. ### What are the different ways to deal with Asynchronous Code
Below are the list of different ways to deal with Asynchronous code.
1. Callbacks
2. Promises
3. Async/await
4. Third-party libraries such as async.js,bluebird etc
**[⬆ Back to Top](#table-of-contents)**
383. ### How to cancel a fetch request
Until a few days back, One shortcoming of native promises is no direct way to cancel a fetch request. But the new `AbortController` from js specification allows you to use a signal to abort one or multiple fetch calls.
The basic flow of cancelling a fetch request would be as below,
1. Create an `AbortController` instance
2. Get the signal property of an instance and pass the signal as a fetch option for signal
3. Call the AbortController's abort property to cancel all fetches that use that signal
For example, let's pass the same signal to multiple fetch calls will cancel all requests with that signal,
```javascript
const controller = new AbortController();
const { signal } = controller;
fetch("http://localhost:8000", { signal })
.then((response) => {
console.log(`Request 1 is complete!`);
})
.catch((e) => {
if (e.name === "AbortError") {
// We know it's been canceled!
}
});
fetch("http://localhost:8000", { signal })
.then((response) => {
console.log(`Request 2 is complete!`);
})
.catch((e) => {
if (e.name === "AbortError") {
// We know it's been canceled!
}
});
// Wait 2 seconds to abort both requests
setTimeout(() => controller.abort(), 2000);
```
**[⬆ Back to Top](#table-of-contents)**
384. ### What is web speech API
Web speech API is used to enable modern browsers recognize and synthesize speech(i.e, voice data into web apps). This API has been introduced by W3C Community in the year 2012. It has two main parts,
1. **SpeechRecognition (Asynchronous Speech Recognition or Speech-to-Text):** It provides the ability to recognize voice context from an audio input and respond accordingly. This is accessed by the `SpeechRecognition` interface.
The below example shows on how to use this API to get text from speech,
```javascript
window.SpeechRecognition =
window.webkitSpeechRecognition || window.SpeechRecognition; // webkitSpeechRecognition for Chrome and SpeechRecognition for FF
const recognition = new window.SpeechRecognition();
recognition.onresult = (event) => {
// SpeechRecognitionEvent type
const speechToText = event.results[0][0].transcript;
console.log(speechToText);
};
recognition.start();
```
In this API, browser is going to ask you for permission to use your microphone
1. **SpeechSynthesis (Text-to-Speech):** It provides the ability to recognize voice context from an audio input and respond. This is accessed by the `SpeechSynthesis` interface.
For example, the below code is used to get voice/speech from text,
```javascript
if ("speechSynthesis" in window) {
var speech = new SpeechSynthesisUtterance("Hello World!");
speech.lang = "en-US";
window.speechSynthesis.speak(speech);
}
```
The above examples can be tested on chrome(33+) browser's developer console.
**Note:** This API is still a working draft and only available in Chrome and Firefox browsers(ofcourse Chrome only implemented the specification)
**[⬆ Back to Top](#table-of-contents)**
385. ### What is minimum timeout throttling
Both browser and NodeJS javascript environments throttles with a minimum delay that is greater than 0ms. That means even though setting a delay of 0ms will not happen instantaneously.
**Browsers:** They have a minimum delay of 4ms. This throttle occurs when successive calls are triggered due to callback nesting(certain depth) or after a certain number of successive intervals.
Note: The older browsers have a minimum delay of 10ms.
**Nodejs:** They have a minimum delay of 1ms. This throttle happens when the delay is larger than 2147483647 or less than 1.
The best example to explain this timeout throttling behavior is the order of below code snippet.
```javascript
function runMeFirst() {
console.log("My script is initialized");
}
setTimeout(runMeFirst, 0);
console.log("Script loaded");
```
and the output would be in
```cmd
Script loaded
My script is initialized
```
If you don't use `setTimeout`, the order of logs will be sequential.
```javascript
function runMeFirst() {
console.log("My script is initialized");
}
runMeFirst();
console.log("Script loaded");
```
and the output is,
```cmd
My script is initialized
Script loaded
```
**[⬆ Back to Top](#table-of-contents)**
386. ### How do you implement zero timeout in modern browsers
You can't use setTimeout(fn, 0) to execute the code immediately due to minimum delay of greater than 0ms. But you can use window.postMessage() to achieve this behavior.
**[⬆ Back to Top](#table-of-contents)**
387. ### What are tasks in event loop
A task is any javascript code/program which is scheduled to be run by the standard mechanisms such as initially starting to run a program, run an event callback, or an interval or timeout being fired. All these tasks are scheduled on a task queue.
Below are the list of use cases to add tasks to the task queue,
1. When a new javascript program is executed directly from console or running by the `` element, the task will be added to the task queue.
2. When an event fires, the event callback added to task queue
3. When a setTimeout or setInterval is reached, the corresponding callback added to task queue
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388. ### What is microtask
Microtask is the javascript code which needs to be executed immediately after the currently executing task/microtask is completed. They are kind of blocking in nature. i.e, The main thread will be blocked until the microtask queue is empty.
The main sources of microtasks are Promise.resolve, Promise.reject, MutationObservers, IntersectionObservers etc
**Note:** All of these microtasks are processed in the same turn of the event loop.
**[⬆ Back to Top](#table-of-contents)**
389. ### What are different event loops
In JavaScript, there are multiple event loops that can be used depending on the context of your application. The most common event loops are:
390. The Browser Event Loop
391. The Node.js Event Loop
- Browser Event Loop: The Browser Event Loop is used in client-side JavaScript applications and is responsible for handling events that occur within the browser environment, such as user interactions (clicks, keypresses, etc.), HTTP requests, and other asynchronous actions.
- The Node.js Event Loop is used in server-side JavaScript applications and is responsible for handling events that occur within the Node.js runtime environment, such as file I/O, network I/O, and other asynchronous actions.
**[⬆ Back to Top](#table-of-contents)**
390. ### What is the purpose of queueMicrotask
The `queueMicrotask` function is used to schedule a microtask, which is a function that will be executed asynchronously in the microtask queue. The purpose of `queueMicrotask` is to ensure that a function is executed after the current task has finished, but before the browser performs any rendering or handles user events.
Example:
```javascript
console.log("Start"); //1
queueMicrotask(() => {
console.log("Inside microtask"); // 3
});
console.log("End"); //2
```
By using queueMicrotask, you can ensure that certain tasks or callbacks are executed at the earliest opportunity during the JavaScript event loop, making it useful for performing work that needs to be done asynchronously but with higher priority than regular `setTimeout` or `setInterval` callbacks.
**[⬆ Back to Top](#table-of-contents)**
391. ### How do you use javascript libraries in typescript file
It is known that not all JavaScript libraries or frameworks have TypeScript declaration files. But if you still want to use libraries or frameworks in our TypeScript files without getting compilation errors, the only solution is `declare` keyword along with a variable declaration. For example, let's imagine you have a library called `customLibrary` that doesn’t have a TypeScript declaration and have a namespace called `customLibrary` in the global namespace. You can use this library in typescript code as below,
```javascript
declare var customLibrary;
```
In the runtime, typescript will provide the type to the `customLibrary` variable as `any` type. The another alternative without using declare keyword is below
```javascript
var customLibrary: any;
```
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392. ### What are the differences between promises and observables
Some of the major difference in a tabular form
| Promises | Observables |
| ------------------------------------------------------------------ | ---------------------------------------------------------------------------------------- |
| Emits only a single value at a time | Emits multiple values over a period of time(stream of values ranging from 0 to multiple) |
| Eager in nature; they are going to be called immediately | Lazy in nature; they require subscription to be invoked |
| Promise is always asynchronous even though it resolved immediately | Observable can be either synchronous or asynchronous |
| Doesn't provide any operators | Provides operators such as map, forEach, filter, reduce, retry, and retryWhen etc |
| Cannot be canceled | Canceled by using unsubscribe() method |
**[⬆ Back to Top](#table-of-contents)**
393. ### What is heap
Heap(Or memory heap) is the memory location where objects are stored when we define variables. i.e, This is the place where all the memory allocations and de-allocation take place. Both heap and call-stack are two containers of JS runtime.
Whenever runtime comes across variables and function declarations in the code it stores them in the Heap.
**[⬆ Back to Top](#table-of-contents)**
394. ### What is an event table
Event Table is a data structure that stores and keeps track of all the events which will be executed asynchronously like after some time interval or after the resolution of some API requests. i.e Whenever you call a setTimeout function or invoke async operation, it is added to the Event Table.
It doesn't not execute functions on it’s own. The main purpose of the event table is to keep track of events and send them to the Event Queue as shown in the below diagram.
**[⬆ Back to Top](#table-of-contents)**
395. ### What is a microTask queue
Microtask Queue is the new queue where all the tasks initiated by promise objects get processed before the callback queue.
The microtasks queue are processed before the next rendering and painting jobs. But if these microtasks are running for a long time then it leads to visual degradation.
**[⬆ Back to Top](#table-of-contents)**
396. ### What is the difference between shim and polyfill
A shim is a library that brings a new API to an older environment, using only the means of that environment. It isn't necessarily restricted to a web application. For example, es5-shim.js is used to emulate ES5 features on older browsers (mainly pre IE9).
Whereas polyfill is a piece of code (or plugin) that provides the technology that you, the developer, expect the browser to provide natively.
In a simple sentence, A polyfill is a shim for a browser API.
**[⬆ Back to Top](#table-of-contents)**
397. ### How do you detect primitive or non primitive value type
In JavaScript, primitive types include boolean, string, number, BigInt, null, Symbol and undefined. Whereas non-primitive types include the Objects. But you can easily identify them with the below function,
```javascript
var myPrimitive = 30;
var myNonPrimitive = {};
function isPrimitive(val) {
return Object(val) !== val;
}
isPrimitive(myPrimitive);
isPrimitive(myNonPrimitive);
```
If the value is a primitive data type, the Object constructor creates a new wrapper object for the value. But If the value is a non-primitive data type (an object), the Object constructor will give the same object.
**[⬆ Back to Top](#table-of-contents)**
398. ### What is babel
Babel is a JavaScript transpiler to convert ECMAScript 2015+ code into a backwards compatible version of JavaScript in current and older browsers or environments. Some of the main features are listed below,
1. Transform syntax
2. Polyfill features that are missing in your target environment (using @babel/polyfill)
3. Source code transformations (or codemods)
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399. ### Is Node.js completely single threaded
Node is a single thread, but some of the functions included in the Node.js standard library(e.g, fs module functions) are not single threaded. i.e, Their logic runs outside of the Node.js single thread to improve the speed and performance of a program.
**[⬆ Back to Top](#table-of-contents)**
400. ### What are the common use cases of observables
Some of the most common use cases of observables are web sockets with push notifications, user input changes, repeating intervals, etc
**[⬆ Back to Top](#table-of-contents)**
401. ### What is RxJS
RxJS (Reactive Extensions for JavaScript) is a library for implementing reactive programming using observables that makes it easier to compose asynchronous or callback-based code. It also provides utility functions for creating and working with observables.
**[⬆ Back to Top](#table-of-contents)**
402. ### What is the difference between Function constructor and function declaration
The functions which are created with `Function constructor` do not create closures to their creation contexts but they are always created in the global scope. i.e, the function can access its own local variables and global scope variables only. Whereas function declarations can access outer function variables(closures) too.
Let's see this difference with an example,
**Function Constructor:**
```javascript
var a = 100;
function createFunction() {
var a = 200;
return new Function("return a;");
}
console.log(createFunction()()); // 100
```
**Function declaration:**
```javascript
var a = 100;
function createFunction() {
var a = 200;
return function func() {
return a;
};
}
console.log(createFunction()()); // 200
```
**[⬆ Back to Top](#table-of-contents)**
403. ### What is a Short circuit condition
Short circuit conditions are meant for condensed way of writing simple if statements. Let's demonstrate the scenario using an example. If you would like to login to a portal with an authentication condition, the expression would be as below,
```javascript
if (authenticate) {
loginToPorta();
}
```
Since the javascript logical operators evaluated from left to right, the above expression can be simplified using && logical operator
```javascript
authenticate && loginToPorta();
```
**[⬆ Back to Top](#table-of-contents)**
404. ### What is the easiest way to resize an array
The length property of an array is useful to resize or empty an array quickly. Let's apply length property on number array to resize the number of elements from 5 to 2,
```javascript
var array = [1, 2, 3, 4, 5];
console.log(array.length); // 5
array.length = 2;
console.log(array.length); // 2
console.log(array); // [1,2]
```
and the array can be emptied too
```javascript
var array = [1, 2, 3, 4, 5];
array.length = 0;
console.log(array.length); // 0
console.log(array); // []
```
**[⬆ Back to Top](#table-of-contents)**
405. ### What is an observable
An Observable is basically a function that can return a stream of values either synchronously or asynchronously to an observer over time. The consumer can get the value by calling `subscribe()` method.
Let's look at a simple example of an Observable
```javascript
import { Observable } from "rxjs";
const observable = new Observable((observer) => {
setTimeout(() => {
observer.next("Message from a Observable!");
}, 3000);
});
observable.subscribe((value) => console.log(value));
```
**Note:** Observables are not part of the JavaScript language yet but they are being proposed to be added to the language
**[⬆ Back to Top](#table-of-contents)**
406. ### What is the difference between function and class declarations
The main difference between function declarations and class declarations is `hoisting`. The function declarations are hoisted but not class declarations.
**Classes:**
```javascript
const user = new User(); // ReferenceError
class User {}
```
**Constructor Function:**
```javascript
const user = new User(); // No error
function User() {}
```
**[⬆ Back to Top](#table-of-contents)**
407. ### What is an async function
An async function is a function declared with the `async` keyword which enables asynchronous, promise-based behavior to be written in a cleaner style by avoiding promise chains. These functions can contain zero or more `await` expressions.
Let's take a below async function example,
```javascript
async function logger() {
let data = await fetch("http://someapi.com/users"); // pause until fetch returns
console.log(data);
}
logger();
```
It is basically syntax sugar over ES2015 promises and generators.
**[⬆ Back to Top](#table-of-contents)**
408. ### How do you prevent promises swallowing errors
While using asynchronous code, JavaScript’s ES6 promises can make your life a lot easier without having callback pyramids and error handling on every second line. But Promises have some pitfalls and the biggest one is swallowing errors by default.
Let's say you expect to print an error to the console for all the below cases,
```javascript
Promise.resolve("promised value").then(function () {
throw new Error("error");
});
Promise.reject("error value").catch(function () {
throw new Error("error");
});
new Promise(function (resolve, reject) {
throw new Error("error");
});
```
But there are many modern JavaScript environments that won't print any errors. You can fix this problem in different ways,
1. **Add catch block at the end of each chain:** You can add catch block to the end of each of your promise chains
```javascript
Promise.resolve("promised value")
.then(function () {
throw new Error("error");
})
.catch(function (error) {
console.error(error.stack);
});
```
But it is quite difficult to type for each promise chain and verbose too.
2. **Add done method:** You can replace first solution's then and catch blocks with done method
```javascript
Promise.resolve("promised value").done(function () {
throw new Error("error");
});
```
Let's say you want to fetch data using HTTP and later perform processing on the resulting data asynchronously. You can write `done` block as below,
```javascript
getDataFromHttp()
.then(function (result) {
return processDataAsync(result);
})
.done(function (processed) {
displayData(processed);
});
```
In future, if the processing library API changed to synchronous then you can remove `done` block as below,
```javascript
getDataFromHttp().then(function (result) {
return displayData(processDataAsync(result));
});
```
and then you forgot to add `done` block to `then` block leads to silent errors.
3. **Extend ES6 Promises by Bluebird:**
Bluebird extends the ES6 Promises API to avoid the issue in the second solution. This library has a “default” onRejection handler which will print all errors from rejected Promises to stderr. After installation, you can process unhandled rejections
```javascript
Promise.onPossiblyUnhandledRejection(function (error) {
throw error;
});
```
and discard a rejection, just handle it with an empty catch
```javascript
Promise.reject("error value").catch(function () {});
```
**[⬆ Back to Top](#table-of-contents)**
409. ### What is deno
Deno is a simple, modern and secure runtime for JavaScript and TypeScript that uses V8 JavaScript engine and the Rust programming language.
**[⬆ Back to Top](#table-of-contents)**
410. ### How do you make an object iterable in javascript
By default, plain objects are not iterable. But you can make the object iterable by defining a `Symbol.iterator` property on it.
Let's demonstrate this with an example,
```javascript
const collection = {
one: 1,
two: 2,
three: 3,
[Symbol.iterator]() {
const values = Object.keys(this);
let i = 0;
return {
next: () => {
return {
value: this[values[i++]],
done: i > values.length,
};
},
};
},
};
const iterator = collection[Symbol.iterator]();
console.log(iterator.next()); // → {value: 1, done: false}
console.log(iterator.next()); // → {value: 2, done: false}
console.log(iterator.next()); // → {value: 3, done: false}
console.log(iterator.next()); // → {value: undefined, done: true}
```
The above process can be simplified using a generator function,
```javascript
const collection = {
one: 1,
two: 2,
three: 3,
[Symbol.iterator]: function* () {
for (let key in this) {
yield this[key];
}
},
};
const iterator = collection[Symbol.iterator]();
console.log(iterator.next()); // {value: 1, done: false}
console.log(iterator.next()); // {value: 2, done: false}
console.log(iterator.next()); // {value: 3, done: false}
console.log(iterator.next()); // {value: undefined, done: true}
```
**[⬆ Back to Top](#table-of-contents)**
411. ### What is a Proper Tail Call
First, we should know about tail call before talking about "Proper Tail Call". A tail call is a subroutine or function call performed as the final action of a calling function. Whereas **Proper tail call(PTC)** is a technique where the program or code will not create additional stack frames for a recursion when the function call is a tail call.
For example, the below classic or head recursion of factorial function relies on stack for each step. Each step need to be processed upto `n * factorial(n - 1)`
```javascript
function factorial(n) {
if (n === 0) {
return 1;
}
return n * factorial(n - 1);
}
console.log(factorial(5)); //120
```
But if you use Tail recursion functions, they keep passing all the necessary data it needs down the recursion without relying on the stack.
```javascript
function factorial(n, acc = 1) {
if (n === 0) {
return acc;
}
return factorial(n - 1, n * acc);
}
console.log(factorial(5)); //120
```
The above pattern returns the same output as the first one. But the accumulator keeps track of total as an argument without using stack memory on recursive calls.
**[⬆ Back to Top](#table-of-contents)**
412. ### How do you check an object is a promise or not
If you don't know if a value is a promise or not, wrapping the value as `Promise.resolve(value)` which returns a promise
```javascript
function isPromise(object) {
if (Promise && Promise.resolve) {
return Promise.resolve(object) == object;
} else {
throw "Promise not supported in your environment";
}
}
var i = 1;
var promise = new Promise(function (resolve, reject) {
resolve();
});
console.log(isPromise(i)); // false
console.log(isPromise(promise)); // true
```
Another way is to check for `.then()` handler type
```javascript
function isPromise(value) {
return Boolean(value && typeof value.then === "function");
}
var i = 1;
var promise = new Promise(function (resolve, reject) {
resolve();
});
console.log(isPromise(i)); // false
console.log(isPromise(promise)); // true
```
**[⬆ Back to Top](#table-of-contents)**
413. ### How to detect if a function is called as constructor
You can use `new.target` pseudo-property to detect whether a function was called as a constructor(using the new operator) or as a regular function call.
1. If a constructor or function invoked using the new operator, new.target returns a reference to the constructor or function.
2. For function calls, new.target is undefined.
```javascript
function Myfunc() {
if (new.target) {
console.log("called with new");
} else {
console.log("not called with new");
}
}
new Myfunc(); // called with new
Myfunc(); // not called with new
Myfunc.call({}); // not called with new
```
**[⬆ Back to Top](#table-of-contents)**
414. ### What are the differences between arguments object and rest parameter
There are three main differences between arguments object and rest parameters
1. The arguments object is an array-like but not an array. Whereas the rest parameters are array instances.
2. The arguments object does not support methods such as sort, map, forEach, or pop. Whereas these methods can be used in rest parameters.
3. The rest parameters are only the ones that haven’t been given a separate name, while the arguments object contains all arguments passed to the function
**[⬆ Back to Top](#table-of-contents)**
415. ### What are the differences between spread operator and rest parameter
Rest parameter collects all remaining elements into an array. Whereas Spread operator allows iterables( arrays / objects / strings ) to be expanded into single arguments/elements. i.e, Rest parameter is opposite to the spread operator.
**[⬆ Back to Top](#table-of-contents)**
416. ### What are the different kinds of generators
There are five kinds of generators,
1. **Generator function declaration:**
```javascript
function* myGenFunc() {
yield 1;
yield 2;
yield 3;
}
const genObj = myGenFunc();
```
2. **Generator function expressions:**
```javascript
const myGenFunc = function* () {
yield 1;
yield 2;
yield 3;
};
const genObj = myGenFunc();
```
3. **Generator method definitions in object literals:**
```javascript
const myObj = {
*myGeneratorMethod() {
yield 1;
yield 2;
yield 3;
},
};
const genObj = myObj.myGeneratorMethod();
```
4. **Generator method definitions in class:**
```javascript
class MyClass {
*myGeneratorMethod() {
yield 1;
yield 2;
yield 3;
}
}
const myObject = new MyClass();
const genObj = myObject.myGeneratorMethod();
```
5. **Generator as a computed property:**
```javascript
const SomeObj = {
*[Symbol.iterator]() {
yield 1;
yield 2;
yield 3;
},
};
console.log(Array.from(SomeObj)); // [ 1, 2, 3 ]
```
**[⬆ Back to Top](#table-of-contents)**
417. ### What are the built-in iterables
Below are the list of built-in iterables in javascript,
1. Arrays and TypedArrays
2. Strings: Iterate over each character or Unicode code-points
3. Maps: iterate over its key-value pairs
4. Sets: iterates over their elements
5. arguments: An array-like special variable in functions
6. DOM collection such as NodeList
**[⬆ Back to Top](#table-of-contents)**
418. ### What are the differences between for...of and for...in statements
Both for...in and for...of statements iterate over js data structures. The only difference is over what they iterate:
1. for..in iterates over all enumerable property keys of an object
2. for..of iterates over the values of an iterable object.
Let's explain this difference with an example,
```javascript
let arr = ["a", "b", "c"];
arr.newProp = "newVlue";
// key are the property keys
for (let key in arr) {
console.log(key); // 0, 1, 2 & newValue
}
// value are the property values
for (let value of arr) {
console.log(value); // a, b, c
}
```
Since for..in loop iterates over the keys of the object, the first loop logs 0, 1, 2 and newProp while iterating over the array object. The for..of loop iterates over the values of a arr data structure and logs a, b, c in the console.
**[⬆ Back to Top](#table-of-contents)**
419. ### How do you define instance and non-instance properties
The Instance properties must be defined inside of class methods. For example, name and age properties defined inside constructor as below,
```javascript
class Person {
constructor(name, age) {
this.name = name;
this.age = age;
}
}
```
But Static(class) and prototype data properties must be defined outside of the ClassBody declaration. Let's assign the age value for Person class as below,
```javascript
Person.staticAge = 30;
Person.prototype.prototypeAge = 40;
```
**[⬆ Back to Top](#table-of-contents)**
420. ### What is the difference between isNaN and Number.isNaN?
1. **isNaN**: The global function `isNaN` converts the argument to a Number and returns true if the resulting value is NaN.
2. **Number.isNaN**: This method does not convert the argument. But it returns true when the type is a Number and value is NaN.
Let's see the difference with an example,
```javascript
isNaN(‘hello’); // true
Number.isNaN('hello'); // false
```
**[⬆ Back to Top](#table-of-contents)**
421. ### How to invoke an IIFE without any extra brackets?
Immediately Invoked Function Expressions(IIFE) requires a pair of parenthesis to wrap the function which contains set of statements.
```js
(function (dt) {
console.log(dt.toLocaleTimeString());
})(new Date());
```
Since both IIFE and void operator discard the result of an expression, you can avoid the extra brackets using `void operator` for IIFE as below,
```js
void (function (dt) {
console.log(dt.toLocaleTimeString());
})(new Date());
```
**[⬆ Back to Top](#table-of-contents)**
422. ### Is that possible to use expressions in switch cases?
You might have seen expressions used in switch condition but it is also possible to use for switch cases by assigning true value for the switch condition. Let's see the weather condition based on temparature as an example,
```js
const weather = (function getWeather(temp) {
switch (true) {
case temp < 0:
return "freezing";
case temp < 10:
return "cold";
case temp < 24:
return "cool";
default:
return "unknown";
}
})(10);
```
**[⬆ Back to Top](#table-of-contents)**
423. ### What is the easiest way to ignore promise errors?
The easiest and safest way to ignore promise errors is void that error. This approach is ESLint friendly too.
```js
await promise.catch((e) => void e);
```
**[⬆ Back to Top](#table-of-contents)**
424. ### How do style the console output using CSS?
You can add CSS styling to the console output using the CSS format content specifier %c. The console string message can be appended after the specifier and CSS style in another argument. Let's print the red the color text using console.log and CSS specifier as below,
```js
console.log("%cThis is a red text", "color:red");
```
It is also possible to add more styles for the content. For example, the font-size can be modified for the above text
```js
console.log(
"%cThis is a red text with bigger font",
"color:red; font-size:20px"
);
```
**[⬆ Back to Top](#table-of-contents)**
425. ### What is nullish coalescing operator (??)?
It is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined, and otherwise returns its left-hand side operand. This can be contrasted with the logical OR (||) operator, which returns the right-hand side operand if the left operand is any falsy value, not only null or undefined.
```js
console.log(null ?? true); // true
console.log(false ?? true); // false
console.log(undefined ?? true); // true
```
**[⬆ Back to Top](#table-of-contents)**
426. ### How do you group and nest console output?
The `console.group()` can be used to group related log messages to be able to easily read the logs and use console.groupEnd()to close the group. Along with this, you can also nest groups which allows to output message in hierarchical manner.
For example, if you’re logging a user’s details:
```js
console.group("User Details");
console.log("name: Sudheer Jonna");
console.log("job: Software Developer");
// Nested Group
console.group("Address");
console.log("Street: Commonwealth");
console.log("City: Los Angeles");
console.log("State: California");
// Close nested group
console.groupEnd();
// Close outer group
console.groupEnd();
```
You can also use `console.groupCollapsed()` instead of `console.group()` if you want the groups to be collapsed by default.
**[⬆ Back to Top](#table-of-contents)**
427. ### What is the difference between dense and sparse arrays?
An array contains items at each index starting from first(0) to last(array.length - 1) is called as Dense array. Whereas if at least one item is missing at any index, the array is called as sparse.
Let's see the below two kind of arrays,
```js
const avengers = ["Ironman", "Hulk", "CaptainAmerica"];
console.log(avengers[0]); // 'Ironman'
console.log(avengers[1]); // 'Hulk'
console.log(avengers[2]); // 'CaptainAmerica'
console.log(avengers.length); // 3
const justiceLeague = ["Superman", "Aquaman", , "Batman"];
console.log(justiceLeague[0]); // 'Superman'
console.log(justiceLeague[1]); // 'Aquaman'
console.log(justiceLeague[2]); // undefined
console.log(justiceLeague[3]); // 'Batman'
console.log(justiceLeague.length); // 4
```
**[⬆ Back to Top](#table-of-contents)**
428. ### What are the different ways to create sparse arrays?
There are 4 different ways to create sparse arrays in JavaScript
1. **Array literal:** Omit a value when using the array literal
```js
const justiceLeague = ["Superman", "Aquaman", , "Batman"];
console.log(justiceLeague); // ['Superman', 'Aquaman', empty ,'Batman']
```
2. **Array() constructor:** Invoking Array(length) or new Array(length)
```js
const array = Array(3);
console.log(array); // [empty, empty ,empty]
```
3. **Delete operator:** Using delete array[index] operator on the array
```js
const justiceLeague = ["Superman", "Aquaman", "Batman"];
delete justiceLeague[1];
console.log(justiceLeague); // ['Superman', empty, ,'Batman']
```
4. **Increase length property:** Increasing length property of an array
```js
const justiceLeague = ["Superman", "Aquaman", "Batman"];
justiceLeague.length = 5;
console.log(justiceLeague); // ['Superman', 'Aquaman', 'Batman', empty, empty]
```
**[⬆ Back to Top](#table-of-contents)**
429. ### What is the difference between setTimeout, setImmediate and process.nextTick?
1. **Set Timeout:** setTimeout() is to schedule execution of a one-time callback after delay milliseconds.
2. **Set Immediate:** The setImmediate function is used to execute a function right after the current event loop finishes.
3. **Process NextTick:** If process.nextTick() is called in a given phase, all the callbacks passed to process.nextTick() will be resolved before the event loop continues. This will block the event loop and create I/O Starvation if process.nextTick() is called recursively.
**[⬆ Back to Top](#table-of-contents)**
430. ### How do you reverse an array without modifying original array?
The `reverse()` method reverses the order of the elements in an array but it mutates the original array. Let's take a simple example to demonistrate this case,
```javascript
const originalArray = [1, 2, 3, 4, 5];
const newArray = originalArray.reverse();
console.log(newArray); // [ 5, 4, 3, 2, 1]
console.log(originalArray); // [ 5, 4, 3, 2, 1]
```
There are few solutions that won't mutate the original array. Let's take a look.
1. **Using slice and reverse methods:**
In this case, just invoke the `slice()` method on the array to create a shallow copy followed by `reverse()` method call on the copy.
```javascript
const originalArray = [1, 2, 3, 4, 5];
const newArray = originalArray.slice().reverse(); //Slice an array gives a new copy
console.log(originalArray); // [1, 2, 3, 4, 5]
console.log(newArray); // [ 5, 4, 3, 2, 1]
```
2. **Using spread and reverse methods:**
In this case, let's use the spread syntax (...) to create a copy of the array followed by `reverse()` method call on the copy.
```javascript
const originalArray = [1, 2, 3, 4, 5];
const newArray = [...originalArray].reverse();
console.log(originalArray); // [1, 2, 3, 4, 5]
console.log(newArray); // [ 5, 4, 3, 2, 1]
```
3. **Using reduce and spread methods:**
Here execute a reducer function on an array elements and append the accumulated array on right side using spread syntax
```javascript
const originalArray = [1, 2, 3, 4, 5];
const newArray = originalArray.reduce((accumulator, value) => {
return [value, ...accumulator];
}, []);
console.log(originalArray); // [1, 2, 3, 4, 5]
console.log(newArray); // [ 5, 4, 3, 2, 1]
```
4. **Using reduceRight and spread methods:**
Here execute a right reducer function(i.e. opposite direction of reduce method) on an array elements and append the accumulated array on left side using spread syntax
```javascript
const originalArray = [1, 2, 3, 4, 5];
const newArray = originalArray.reduceRight((accumulator, value) => {
return [...accumulator, value];
}, []);
console.log(originalArray); // [1, 2, 3, 4, 5]
console.log(newArray); // [ 5, 4, 3, 2, 1]
```
5. **Using reduceRight and push methods:**
Here execute a right reducer function(i.e. opposite direction of reduce method) on an array elements and push the iterated value to the accumulator
```javascript
const originalArray = [1, 2, 3, 4, 5];
const newArray = originalArray.reduceRight((accumulator, value) => {
accumulator.push(value);
return accumulator;
}, []);
console.log(originalArray); // [1, 2, 3, 4, 5]
console.log(newArray); // [ 5, 4, 3, 2, 1]
```
**[⬆ Back to Top](#table-of-contents)**
431. ### How do you create custom HTML element?
The creation of custom HTML elements involves two main steps,
1. **Define your custom HTML element:** First you need to define some custom class by extending HTMLElement class.
After that define your component properties (styles,text etc) using `connectedCallback` method.
**Note:** The browser exposes a function called `customElements.define` inorder to reuse the element.
```javascript
class CustomElement extends HTMLElement {
connectedCallback() {
this.innerHTML = "This is a custom element";
}
}
customElements.define("custom-element", CustomElement);
```
2. **Use custome element just like other HTML element:** Declare your custom element as a HTML tag.
```javascript
<body>
<custom-element>
</body>
```
**[⬆ Back to Top](#table-of-contents)**
432. ### What is global execution context?
The global execution context is the default or first execution context that is created by the JavaScript engine before any code is executed(i.e, when the file first loads in the browser). All the global code that is not inside a function or object will be executed inside this global execution context. Since JS engine is single threaded there will be only one global environment and there will be only one global execution context.
For example, the below code other than code inside any function or object is executed inside the global execution context.
```javascript
var x = 10;
function A() {
console.log("Start function A");
function B() {
console.log("In function B");
}
B();
}
A();
console.log("GlobalContext");
```
**[⬆ Back to Top](#table-of-contents)**
433. ### What is function execution context?
Whenever a function is invoked, the JavaScript engine creates a different type of Execution Context known as a Function Execution Context (FEC) within the Global Execution Context (GEC) to evaluate and execute the code within that function.
**[⬆ Back to Top](#table-of-contents)**
434. ### What is debouncing?
Debouncing is a programming pattern that allows delaying execution of some piece of code until a specified time to avoid unnecessary _CPU cycles, API calls and improve performance_. The debounce function make sure that your code is only triggered once per user input. The common usecases are Search box suggestions, text-field auto-saves, and eliminating double-button clicks.
Let's say you want to show suggestions for a search query, but only after a visitor has finished typing it. So here you write a debounce function where the user keeps writing the characters with in 500ms then previous timer cleared out using `clearTimeout` and reschedule API call/DB query for a new time—300 ms in the future.
```js
function debounce(func, timeout = 500) {
let timer;
return (...args) => {
clearTimeout(timer);
timer = setTimeout(() => {
func.apply(this, args);
}, timeout);
};
}
function fetchResults() {
console.log("Fetching input suggestions");
}
const processChange = debounce(() => fetchResults());
```
The _debounce()_ function can be used on input, button and window events
**Input:**
```html
<input type="text" onkeyup="processChange()" />
```
**Button:**
```html
<button onclick="processChange()">Click me</button>
```
**Windows event:**
```html
window.addEventListener("scroll", processChange);
```
**[⬆ Back to Top](#table-of-contents)**
435. ### What is throttling?
Throttling is a technique used to limit the execution of an event handler function, even when this event triggers continuously due to user actions. The common use cases are browser resizing, window scrolling etc.
The below example creates a throttle function to reduce the number of events for each pixel change and trigger scroll event for each 100ms except for the first event.
```js
const throttle = (func, limit) => {
let inThrottle;
return (...args) => {
if (!inThrottle) {
func.apply(this, args);
inThrottle = true;
setTimeout(() => (inThrottle = false), limit);
}
};
};
window.addEventListener("scroll", () => {
throttle(handleScrollAnimation, 100);
});
```
**[⬆ Back to Top](#table-of-contents)**
436. ### What is optional chaining?
According to MDN official docs, the optional chaining operator (?.) permits reading the value of a property located deep within a chain of connected objects without having to expressly validate that each reference in the chain is valid.
The ?. operator is like the . chaining operator, except that instead of causing an error if a reference is nullish (null or undefined), the expression short-circuits with a return value of undefined. When used with function calls, it returns undefined if the given function does not exist.
```js
const adventurer = {
name: "Alice",
cat: {
name: "Dinah",
},
};
const dogName = adventurer.dog?.name;
console.log(dogName);
// expected output: undefined
console.log(adventurer.someNonExistentMethod?.());
// expected output: undefined
```
**[⬆ Back to Top](#table-of-contents)**
437. ### What is an environment record?
According to ECMAScript specification 262 (9.1):
> [Environment Record](https://262.ecma-international.org/12.0/#sec-environment-records) is a specification type used to define the association of Identifiers to specific variables and functions, based upon the lexical nesting structure of ECMAScript code.
Usually an Environment Record is associated with some specific syntactic structure of ECMAScript code such as a FunctionDeclaration, a BlockStatement, or a Catch clause of a TryStatement.
Each time such code is evaluated, a new Environment Record is created to record the identifier bindings that are created by that code.
**[⬆ Back to Top](#table-of-contents)**
438. ### How to verify if a variable is an array?
It is possible to check if a variable is an array instance using 3 different ways,
1. Array.isArray() method:
The `Array.isArray(value)` utility function is used to determine whether value is an array or not. This function returns a true boolean value if the variable is an array and a false value if it is not.
```javascript
const numbers = [1, 2, 3];
const user = { name: "John" };
Array.isArray(numbers); // true
Array.isArray(user); //false
```
2. instanceof operator:
The instanceof operator is used to check the type of an array at run time. It returns true if the type of a variable is an Array other false for other type.
```javascript
const numbers = [1, 2, 3];
const user = { name: "John" };
console.log(numbers instanceof Array); // true
console.log(user instanceof Array); // false
```
3. Checking constructor type:
The constructor property of the variable is used to determine whether the variable Array type or not.
```javascript
const numbers = [1, 2, 3];
const user = { name: "John" };
console.log(numbers.constructor === Array); // true
console.log(user.constructor === Array); // false
```
**[⬆ Back to Top](#table-of-contents)**
439. ### What is pass by value and pass by reference?
Pass-by-value creates a new space in memory and makes a copy of a value. Primitives such as string, number, boolean etc will actually create a new copy. Hence, updating one value doesn't impact the other value. i.e, The values are independent of each other.
```javascript
let a = 5;
let b = a;
b++;
console.log(a, b); //5, 6
```
In the above code snippet, the value of `a` is assigned to `b` and the variable `b` has been incremented. Since there is a new space created for variable `b`, any update on this variable doesn't impact the variable `a`.
Pass by reference doesn't create a new space in memory but the new variable adopts a memory address of an initial variable. Non-primitives such as objects, arrays and functions gets the reference of the initiable variable. i.e, updating one value will impact the other variable.
```javascript
let user1 = {
name: "John",
age: 27,
};
let user2 = user1;
user2.age = 30;
console.log(user1.age, user2.age); // 30, 30
```
In the above code snippet, updating the `age` property of one object will impact the other property due to the same reference.
**[⬆ Back to Top](#table-of-contents)**
440. ### What are the differences between primitives and non-primitives?
JavaScript language has both primitives and non-primitives but there are few differences between them as below,
| Primitives | Non-primitives |
| -------------------------- | -------------------- |
| These types are predefined | Created by developer |
| These are immutable | Mutable |
| Compare by value | Compare by reference |
| Stored in Stack | Stored in heap |
| Contain certain value | Can contain NULL too |
**[⬆ Back to Top](#table-of-contents)**
441. ### How do you create your own bind method using either call or apply method?
The custom bind function needs to be created on Function prototype inorder to use it as other builtin functions. This custom function should return a function similar to original bind method and the implementation of inner function needs to use apply method call.
The function which is going to bind using custom `myOwnBind` method act as the attached function(`boundTargetFunction`) and argument as the object for `apply` method call.
```js
Function.prototype.myOwnBind = function (whoIsCallingMe) {
if (typeof this !== "function") {
throw new Error(this + "cannot be bound as it's not callable");
}
const boundTargetFunction = this;
return function () {
boundTargetFunction.apply(whoIsCallingMe, arguments);
};
};
```
**[⬆ Back to Top](#table-of-contents)**
442. ### What are the differences between pure and impure functions?
Some of the major differences between pure and impure function are as below,
| Pure function | Impure function |
| ----------------------------------- | --------------------------------------------------------------------- |
| It has no side effects | It causes side effects |
| It is always return the same result | It returns different result on each call |
| Easy to read and debug | Difficult to read and debug because they are affected by extenal code |
**[⬆ Back to Top](#table-of-contents)**
445. ### What is referential transparency?
An expression in javascript that can be replaced by its value without affecting the behaviour of the program is called referential transparency. Pure functions are referentially transparent.
```javascript
const add = (x, y) => x + y;
const multiplyBy2 = (x) => x * 2;
//Now add (2, 3) can be replaced by 5.
multiplyBy2(add(2, 3));
```
**[⬆ Back to Top](#table-of-contents)**
446. ### What are the possible side-effects in javascript?
A side effect is the modification of state through the invocation of a function or expression. These side effects makes our function impure by default. Below are some side effects which makes function impure,
1. Making an HTTP request. Asynchronous functions such as fetch and promise are impure.
1. DOM manipulations
1. Mutating the input data
1. Printing to a screen or console: For example, console.log() and alert()
1. Fetching the current time
1. Math.random() calls: Modifies the internal state of Math object
**[⬆ Back to Top](#table-of-contents)**
447. ### What are compose and pipe functions?
The "compose" and "pipe" are two techniques commonly used in functional programming to simplify complex operations and make code more readable. They are not native to JavaScript and higher-order functions. the `compose()` applies right to left any number of functions to the output of the previous function.
**[⬆ Back to Top](#table-of-contents)**
448. ### What is module pattern?
Module pattern is a designed pattern used to wrap a set of variables and functions together in a single scope returned as an object. JavaScript doesn't have access specifiers similar to other languages(Java, Python, etc) to provide private scope. It uses IIFE (Immediately invoked function expression) to allow for private scopes. i.e., a closure that protect variables and methods.
The module pattern looks like below,
```javascript
(function () {
// Private variables or functions goes here.
return {
// Return public variables or functions here.
};
})();
```
Let's see an example of a module pattern for an employee with private and public access,
```javascript
const createEmployee = (function () {
// Private
const name = "John";
const department = "Sales";
const getEmployeeName = () => name;
const getDepartmentName = () => department;
// Public
return {
name,
department,
getName: () => getEmployeeName(),
getDepartment: () => getDepartmentName(),
};
})();
console.log(createEmployee.name);
console.log(createEmployee.department);
console.log(createEmployee.getName());
console.log(createEmployee.getDepartment());
```
**Note:** It mimic the concepts of classes with private variables and methods.
**[⬆ Back to Top](#table-of-contents)**
449. ### What is Function Composition?
It is an approach where the result of one function is passed on to the next function, which is passed to another until the final function is executed for the final result.
```javascript
//example
const double = (x) => x * 2;
const square = (x) => x * x;
var output1 = double(2);
var output2 = square(output1);
console.log(output2);
var output_final = square(double(2));
console.log(output_final);
```
**[⬆ Back to Top](#table-of-contents)**
450. ### How to use await outside of async function prior to ES2022?
Prior to ES2022, if you attempted to use an await outside of an async function resulted in a SyntaxError.
```javascript
await Promise.resolve(console.log("Hello await")); // SyntaxError: await is only valid in async function
```
But you can fix this issue with an alternative IIFE (Immediately Invoked Function Expression) to get access to the feature.
```javascript
(async function () {
await Promise.resolve(console.log("Hello await")); // Hello await
})();
```
In ES2022, you can write top-level await without writing any hacks.
```javascript
await Promise.resolve(console.log("Hello await")); //Hello await
```
**[⬆ Back to Top](#table-of-contents)**
### Coding Exercise
#### 1. What is the output of below code
```javascript
var car = new Vehicle("Honda", "white", "2010", "UK");
console.log(car);
function Vehicle(model, color, year, country) {
this.model = model;
this.color = color;
this.year = year;
this.country = country;
}
```
- 1: Undefined
- 2: ReferenceError
- 3: null
- 4: {model: "Honda", color: "white", year: "2010", country: "UK"}
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
The function declarations are hoisted similar to any variables. So the placement for `Vehicle` function declaration doesn't make any difference.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 2. What is the output of below code
```javascript
function foo() {
let x = (y = 0);
x++;
y++;
return x;
}
console.log(foo(), typeof x, typeof y);
```
- 1: 1, undefined and undefined
- 2: ReferenceError: X is not defined
- 3: 1, undefined and number
- 4: 1, number and number
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
Of course the return value of `foo()` is 1 due to the increment operator. But the statement `let x = y = 0` declares a local variable x. Whereas y declared as a global variable accidentally. This statement is equivalent to,
```javascript
let x;
window.y = 0;
x = window.y;
```
Since the block scoped variable x is undefined outside of the function, the type will be undefined too. Whereas the global variable `y` is available outside the function, the value is 0 and type is number.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 3. What is the output of below code
```javascript
function main() {
console.log("A");
setTimeout(function print() {
console.log("B");
}, 0);
console.log("C");
}
main();
```
- 1: A, B and C
- 2: B, A and C
- 3: A and C
- 4: A, C and B
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
The statements order is based on the event loop mechanism. The order of statements follows the below order,
1. At first, the main function is pushed to the stack.
2. Then the browser pushes the first statement of the main function( i.e, A's console.log) to the stack, executing and popping out immediately.
3. But `setTimeout` statement moved to Browser API to apply the delay for callback.
4. In the meantime, C's console.log added to stack, executed and popped out.
5. The callback of `setTimeout` moved from Browser API to message queue.
6. The `main` function popped out from stack because there are no statements to execute
7. The callback moved from message queue to the stack since the stack is empty.
8. The console.log for B is added to the stack and display on the console.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 4. What is the output of below equality check
```javascript
console.log(0.1 + 0.2 === 0.3);
```
- 1: false
- 2: true
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
This is due to the float point math problem. Since the floating point numbers are encoded in binary format, the addition operations on them lead to rounding errors. Hence, the comparison of floating points doesn't give expected results.
You can find more details about the explanation here [0.30000000000000004.com/](https://0.30000000000000004.com/)
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 5. What is the output of below code
```javascript
var y = 1;
if (function f() {}) {
y += typeof f;
}
console.log(y);
```
- 1: 1function
- 2: 1object
- 3: ReferenceError
- 4: 1undefined
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
The main points in the above code snippets are,
1. You can see function expression instead function declaration inside if statement. So it always returns true.
2. Since it is not declared(or assigned) anywhere, f is undefined and typeof f is undefined too.
In other words, it is same as
```javascript
var y = 1;
if ("foo") {
y += typeof f;
}
console.log(y);
```
**Note:** It returns 1object for MS Edge browser
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 6. What is the output of below code
```javascript
function foo() {
return;
{
message: "Hello World";
}
}
console.log(foo());
```
- 1: Hello World
- 2: Object {message: "Hello World"}
- 3: Undefined
- 4: SyntaxError
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
This is a semicolon issue. Normally semicolons are optional in JavaScript. So if there are any statements(in this case, return) missing semicolon, it is automatically inserted immediately. Hence, the function returned as undefined.
Whereas if the opening curly brace is along with the return keyword then the function is going to be returned as expected.
```javascript
function foo() {
return {
message: "Hello World",
};
}
console.log(foo()); // {message: "Hello World"}
```
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 7. What is the output of below code
```javascript
var myChars = ["a", "b", "c", "d"];
delete myChars[0];
console.log(myChars);
console.log(myChars[0]);
console.log(myChars.length);
```
- 1: [empty, 'b', 'c', 'd'], empty, 3
- 2: [null, 'b', 'c', 'd'], empty, 3
- 3: [empty, 'b', 'c', 'd'], undefined, 4
- 4: [null, 'b', 'c', 'd'], undefined, 4
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
The `delete` operator will delete the object property but it will not reindex the array or change its length. So the number or elements or length of the array won't be changed.
If you try to print myChars then you can observe that it doesn't set an undefined value, rather the property is removed from the array. The newer versions of Chrome use `empty` instead of `undefined` to make the difference a bit clearer.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 8. What is the output of below code in latest Chrome
```javascript
var array1 = new Array(3);
console.log(array1);
var array2 = [];
array2[2] = 100;
console.log(array2);
var array3 = [, , ,];
console.log(array3);
```
- 1: [undefined × 3], [undefined × 2, 100], [undefined × 3]
- 2: [empty × 3], [empty × 2, 100], [empty × 3]
- 3: [null × 3], [null × 2, 100], [null × 3]
- 4: [], [100], []
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
The latest chrome versions display `sparse array`(they are filled with holes) using this empty x n notation. Whereas the older versions have undefined x n notation.
**Note:** The latest version of FF displays `n empty slots` notation.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 9. What is the output of below code
```javascript
const obj = {
prop1: function () {
return 0;
},
prop2() {
return 1;
},
["prop" + 3]() {
return 2;
},
};
console.log(obj.prop1());
console.log(obj.prop2());
console.log(obj.prop3());
```
- 1: 0, 1, 2
- 2: 0, { return 1 }, 2
- 3: 0, { return 1 }, { return 2 }
- 4: 0, 1, undefined
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
ES6 provides method definitions and property shorthands for objects. So both prop2 and prop3 are treated as regular function values.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 10. What is the output of below code
```javascript
console.log(1 < 2 < 3);
console.log(3 > 2 > 1);
```
- 1: true, true
- 2: true, false
- 3: SyntaxError, SyntaxError,
- 4: false, false
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
The important point is that if the statement contains the same operators(e.g, < or >) then it can be evaluated from left to right.
The first statement follows the below order,
1. console.log(1 < 2 < 3);
2. console.log(true < 3);
3. console.log(1 < 3); // True converted as `1` during comparison
4. True
Whereas the second statement follows the below order,
1. console.log(3 > 2 > 1);
2. console.log(true > 1);
3. console.log(1 > 1); // False converted as `0` during comparison
4. False
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 11. What is the output of below code in non-strict mode
```javascript
function printNumbers(first, second, first) {
console.log(first, second, first);
}
printNumbers(1, 2, 3);
```
- 1: 1, 2, 3
- 2: 3, 2, 3
- 3: SyntaxError: Duplicate parameter name not allowed in this context
- 4: 1, 2, 1
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
In non-strict mode, the regular JavaScript functions allow duplicate named parameters. The above code snippet has duplicate parameters on 1st and 3rd parameters.
The value of the first parameter is mapped to the third argument which is passed to the function. Hence, the 3rd argument overrides the first parameter.
**Note:** In strict mode, duplicate parameters will throw a Syntax Error.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 12. What is the output of below code
```javascript
const printNumbersArrow = (first, second, first) => {
console.log(first, second, first);
};
printNumbersArrow(1, 2, 3);
```
- 1: 1, 2, 3
- 2: 3, 2, 3
- 3: SyntaxError: Duplicate parameter name not allowed in this context
- 4: 1, 2, 1
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
Unlike regular functions, the arrow functions doesn't not allow duplicate parameters in either strict or non-strict mode. So you can see `SyntaxError` in the console.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 13. What is the output of below code
```javascript
const arrowFunc = () => arguments.length;
console.log(arrowFunc(1, 2, 3));
```
- 1: ReferenceError: arguments is not defined
- 2: 3
- 3: undefined
- 4: null
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
Arrow functions do not have an `arguments, super, this, or new.target` bindings. So any reference to `arguments` variable tries to resolve to a binding in a lexically enclosing environment. In this case, the arguments variable is not defined outside of the arrow function. Hence, you will receive a reference error.
Where as the normal function provides the number of arguments passed to the function
```javascript
const func = function () {
return arguments.length;
};
console.log(func(1, 2, 3));
```
But If you still want to use an arrow function then rest operator on arguments provides the expected arguments
```javascript
const arrowFunc = (...args) => args.length;
console.log(arrowFunc(1, 2, 3));
```
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 14. What is the output of below code
```javascript
console.log(String.prototype.trimLeft.name === "trimLeft");
console.log(String.prototype.trimLeft.name === "trimStart");
```
- 1: True, False
- 2: False, True
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
In order to be consistent with functions like `String.prototype.padStart`, the standard method name for trimming the whitespaces is considered as `trimStart`. Due to web web compatibility reasons, the old method name 'trimLeft' still acts as an alias for 'trimStart'. Hence, the prototype for 'trimLeft' is always 'trimStart'
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 15. What is the output of below code
```javascript
console.log(Math.max());
```
- 1: undefined
- 2: Infinity
- 3: 0
- 4: -Infinity
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
-Infinity is the initial comparant because almost every other value is bigger. So when no arguments are provided, -Infinity is going to be returned.
**Note:** Zero number of arguments is a valid case.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 16. What is the output of below code
```javascript
console.log(10 == [10]);
console.log(10 == [[[[[[[10]]]]]]]);
```
- 1: True, True
- 2: True, False
- 3: False, False
- 4: False, True
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
As per the comparison algorithm in the ECMAScript specification(ECMA-262), the above expression converted into JS as below
```javascript
10 === Number([10].valueOf().toString()); // 10
```
So it doesn't matter about number brackets([]) around the number, it is always converted to a number in the expression.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 17. What is the output of below code
```javascript
console.log(10 + "10");
console.log(10 - "10");
```
- 1: 20, 0
- 2: 1010, 0
- 3: 1010, 10-10
- 4: NaN, NaN
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
The concatenation operator(+) is applicable for both number and string types. So if any operand is string type then both operands concatenated as strings. Whereas subtract(-) operator tries to convert the operands as number type.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 18. What is the output of below code
```javascript
console.log([0] == false);
if ([0]) {
console.log("I'm True");
} else {
console.log("I'm False");
}
```
- 1: True, I'm True
- 2: True, I'm False
- 3: False, I'm True
- 4: False, I'm False
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
In comparison operators, the expression `[0]` converted to Number([0].valueOf().toString()) which is resolved to false. Whereas `[0]` just becomes a truthy value without any conversion because there is no comparison operator.
</p>
</details>
#### 19. What is the output of below code
```javascript
console.log([1, 2] + [3, 4]);
```
- 1: [1,2,3,4]
- 2: [1,2][3,4]
- 3: SyntaxError
- 4: 1,23,4
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
The + operator is not meant or defined for arrays. So it converts arrays into strings and concatenates them.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 20. What is the output of below code
```javascript
const numbers = new Set([1, 1, 2, 3, 4]);
console.log(numbers);
const browser = new Set("Firefox");
console.log(browser);
```
- 1: {1, 2, 3, 4}, {"F", "i", "r", "e", "f", "o", "x"}
- 2: {1, 2, 3, 4}, {"F", "i", "r", "e", "o", "x"}
- 3: [1, 2, 3, 4], ["F", "i", "r", "e", "o", "x"]
- 4: {1, 1, 2, 3, 4}, {"F", "i", "r", "e", "f", "o", "x"}
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
Since `Set` object is a collection of unique values, it won't allow duplicate values in the collection. At the same time, it is case sensitive data structure.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 21. What is the output of below code
```javascript
console.log(NaN === NaN);
```
- 1: True
- 2: False
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
JavaScript follows IEEE 754 spec standards. As per this spec, NaNs are never equal for floating-point numbers.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 22. What is the output of below code
```javascript
let numbers = [1, 2, 3, 4, NaN];
console.log(numbers.indexOf(NaN));
```
- 1: 4
- 2: NaN
- 3: SyntaxError
- 4: -1
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
The `indexOf` uses strict equality operator(===) internally and `NaN === NaN` evaluates to false. Since indexOf won't be able to find NaN inside an array, it returns -1 always.
But you can use `Array.prototype.findIndex` method to find out the index of NaN in an array or You can use `Array.prototype.includes` to check if NaN is present in an array or not.
```javascript
let numbers = [1, 2, 3, 4, NaN];
console.log(numbers.findIndex(Number.isNaN)); // 4
console.log(numbers.includes(NaN)); // true
```
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 23. What is the output of below code
```javascript
let [a, ...b] = [1, 2, 3, 4, 5];
console.log(a, b);
```
- 1: 1, [2, 3, 4, 5]
- 2: 1, {2, 3, 4, 5}
- 3: SyntaxError
- 4: 1, [2, 3, 4]
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
When using rest parameters, trailing commas are not allowed and will throw a SyntaxError.
If you remove the trailing comma then it displays 1st answer
```javascript
let [a, ...b] = [1, 2, 3, 4, 5];
console.log(a, b); // 1, [2, 3, 4, 5]
```
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 25. What is the output of below code
```javascript
async function func() {
return 10;
}
console.log(func());
```
- 1: Promise {\<fulfilled\>: 10}
- 2: 10
- 3: SyntaxError
- 4: Promise {\<rejected\>: 10}
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
Async functions always return a promise. But even if the return value of an async function is not explicitly a promise, it will be implicitly wrapped in a promise. The above async function is equivalent to below expression,
```javascript
function func() {
return Promise.resolve(10);
}
```
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 26. What is the output of below code
```javascript
async function func() {
await 10;
}
console.log(func());
```
- 1: Promise {\<fulfilled\>: 10}
- 2: 10
- 3: SyntaxError
- 4: Promise {\<resolved\>: undefined}
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
The await expression returns value 10 with promise resolution and the code after each await expression can be treated as existing in a `.then` callback. In this case, there is no return expression at the end of the function. Hence, the default return value of `undefined` is returned as the resolution of the promise. The above async function is equivalent to below expression,
```javascript
function func() {
return Promise.resolve(10).then(() => undefined);
}
```
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 27. What is the output of below code
```javascript
function delay() {
return new Promise(resolve => setTimeout(resolve, 2000));
}
async function delayedLog(item) {
await delay();
console.log(item);
}
async function processArray(array) {
array.forEach(item => {
await delayedLog(item);
})
}
processArray([1, 2, 3, 4]);
```
- 1: SyntaxError
- 2: 1, 2, 3, 4
- 3: 4, 4, 4, 4
- 4: 4, 3, 2, 1
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
Even though “processArray” is an async function, the anonymous function that we use for `forEach` is synchronous. If you use await inside a synchronous function then it throws a syntax error.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 28. What is the output of below code
```javascript
function delay() {
return new Promise((resolve) => setTimeout(resolve, 2000));
}
async function delayedLog(item) {
await delay();
console.log(item);
}
async function process(array) {
array.forEach(async (item) => {
await delayedLog(item);
});
console.log("Process completed!");
}
process([1, 2, 3, 5]);
```
- 1: 1 2 3 5 and Process completed!
- 2: 5 5 5 5 and Process completed!
- 3: Process completed! and 5 5 5 5
- 4: Process completed! and 1 2 3 5
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
The forEach method will not wait until all items are finished but it just runs the tasks and goes next. Hence, the last statement is displayed first followed by a sequence of promise resolutions.
But you control the array sequence using for..of loop,
```javascript
async function processArray(array) {
for (const item of array) {
await delayedLog(item);
}
console.log("Process completed!");
}
```
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 29. What is the output of below code
```javascript
var set = new Set();
set.add("+0").add("-0").add(NaN).add(undefined).add(NaN);
console.log(set);
```
- 1: Set(4) {"+0", "-0", NaN, undefined}
- 2: Set(3) {"+0", NaN, undefined}
- 3: Set(5) {"+0", "-0", NaN, undefined, NaN}
- 4: Set(4) {"+0", NaN, undefined, NaN}
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
Set has few exceptions from equality check,
1. All NaN values are equal
2. Both +0 and -0 considered as different values
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 30. What is the output of below code
```javascript
const sym1 = Symbol("one");
const sym2 = Symbol("one");
const sym3 = Symbol.for("two");
const sym4 = Symbol.for("two");
console.log(sym1 === sym2, sym3 === sym4);
```
- 1: true, true
- 2: true, false
- 3: false, true
- 4: false, false
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
Symbol follows below conventions,
1. Every symbol value returned from Symbol() is unique irrespective of the optional string.
2. `Symbol.for()` function creates a symbol in a global symbol registry list. But it doesn't necessarily create a new symbol on every call, it checks first if a symbol with the given key is already present in the registry and returns the symbol if it is found. Otherwise a new symbol created in the registry.
**Note:** The symbol description is just useful for debugging purposes.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 31. What is the output of below code
```javascript
const sym1 = new Symbol("one");
console.log(sym1);
```
- 1: SyntaxError
- 2: one
- 3: Symbol('one')
- 4: Symbol
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
`Symbol` is a just a standard function and not an object constructor(unlike other primitives new Boolean, new String and new Number). So if you try to call it with the new operator will result in a TypeError
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 32. What is the output of below code
```javascript
let myNumber = 100;
let myString = "100";
if (!typeof myNumber === "string") {
console.log("It is not a string!");
} else {
console.log("It is a string!");
}
if (!typeof myString === "number") {
console.log("It is not a number!");
} else {
console.log("It is a number!");
}
```
- 1: SyntaxError
- 2: It is not a string!, It is not a number!
- 3: It is not a string!, It is a number!
- 4: It is a string!, It is a number!
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
The return value of `typeof myNumber` or `typeof myString` is always a truthy value (either "number" or "string"). The ! operator operates on either `typeof myNumber` or `typeof myString`, converting them to boolean values. Since the value of both `!typeof myNumber` and `!typeof myString` is false, the if condition fails, and control goes to else block.
To make the ! operator operate on the equality expression, one needs to add parentheses:
```
if (!(typeof myNumber === "string"))
```
Or simply use the inequality operator:
```
if (typeof myNumber !== "string")
```
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 33. What is the output of below code
```javascript
console.log(
JSON.stringify({ myArray: ["one", undefined, function () {}, Symbol("")] })
);
console.log(
JSON.stringify({ [Symbol.for("one")]: "one" }, [Symbol.for("one")])
);
```
- 1: {"myArray":['one', undefined, {}, Symbol]}, {}
- 2: {"myArray":['one', null,null,null]}, {}
- 3: {"myArray":['one', null,null,null]}, "{ [Symbol.for('one')]: 'one' }, [Symbol.for('one')]"
- 4: {"myArray":['one', undefined, function(){}, Symbol('')]}, {}
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
The symbols has below constraints,
1. The undefined, Functions, and Symbols are not valid JSON values. So those values are either omitted (in an object) or changed to null (in an array). Hence, it returns null values for the value array.
2. All Symbol-keyed properties will be completely ignored. Hence it returns an empty object({}).
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 34. What is the output of below code
```javascript
class A {
constructor() {
console.log(new.target.name);
}
}
class B extends A {
constructor() {
super();
}
}
new A();
new B();
```
- 1: A, A
- 2: A, B
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
Using constructors, `new.target` refers to the constructor (points to the class definition of class which is initialized) that was directly invoked by new. This also applies to the case if the constructor is in a parent class and was delegated from a child constructor.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 35. What is the output of below code
```javascript
const [x, ...y, z] = [1, 2, 3, 4];
console.log(x, y, z);
```
- 1: 1, [2, 3], 4
- 2: 1, [2, 3, 4], undefined
- 3: 1, [2], 3
- 4: SyntaxError
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
It throws a syntax error because the rest element should not have a trailing comma. You should always consider using a rest operator as the last element.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 36. What is the output of below code
```javascript
const { a: x = 10, b: y = 20 } = { a: 30 };
console.log(x);
console.log(y);
```
- 1: 30, 20
- 2: 10, 20
- 3: 10, undefined
- 4: 30, undefined
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
The object property follows below rules,
1. The object properties can be retrieved and assigned to a variable with a different name
2. The property assigned a default value when the retrieved value is `undefined`
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 37. What is the output of below code
```javascript
function area({ length = 10, width = 20 }) {
console.log(length * width);
}
area();
```
- 1: 200
- 2: Error
- 3: undefined
- 4: 0
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
If you leave out the right-hand side assignment for the destructuring object, the function will look for at least one argument to be supplied when invoked. Otherwise you will receive an error `Error: Cannot read property 'length' of undefined` as mentioned above.
You can avoid the error with either of the below changes,
1. **Pass at least an empty object:**
```javascript
function area({ length = 10, width = 20 }) {
console.log(length * width);
}
area({});
```
2. **Assign default empty object:**
```javascript
function area({ length = 10, width = 20 } = {}) {
console.log(length * width);
}
area();
```
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 38. What is the output of below code
```javascript
const props = [
{ id: 1, name: "John" },
{ id: 2, name: "Jack" },
{ id: 3, name: "Tom" },
];
const [, , { name }] = props;
console.log(name);
```
- 1: Tom
- 2: Error
- 3: undefined
- 4: John
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
It is possible to combine Array and Object destructuring. In this case, the third element in the array props accessed first followed by name property in the object.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 39. What is the output of below code
```javascript
function checkType(num = 1) {
console.log(typeof num);
}
checkType();
checkType(undefined);
checkType("");
checkType(null);
```
- 1: number, undefined, string, object
- 2: undefined, undefined, string, object
- 3: number, number, string, object
- 4: number, number, number, number
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
If the function argument is set implicitly(not passing argument) or explicitly to undefined, the value of the argument is the default parameter. Whereas for other falsy values('' or null), the value of the argument is passed as a parameter.
Hence, the result of function calls categorized as below,
1. The first two function calls logs number type since the type of default value is number
2. The type of '' and null values are string and object type respectively.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 40. What is the output of below code
```javascript
function add(item, items = []) {
items.push(item);
return items;
}
console.log(add("Orange"));
console.log(add("Apple"));
```
- 1: ['Orange'], ['Orange', 'Apple']
- 2: ['Orange'], ['Apple']
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
Since the default argument is evaluated at call time, a new object is created each time the function is called. So in this case, the new array is created and an element pushed to the default empty array.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 41. What is the output of below code
```javascript
function greet(greeting, name, message = greeting + " " + name) {
console.log([greeting, name, message]);
}
greet("Hello", "John");
greet("Hello", "John", "Good morning!");
```
- 1: SyntaxError
- 2: ['Hello', 'John', 'Hello John'], ['Hello', 'John', 'Good morning!']
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
Since parameters defined earlier are available to later default parameters, this code snippet doesn't throw any error.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 42. What is the output of below code
```javascript
function outer(f = inner()) {
function inner() {
return "Inner";
}
}
outer();
```
- 1: ReferenceError
- 2: Inner
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
The functions and variables declared in the function body cannot be referred from default value parameter initializers. If you still try to access, it throws a run-time ReferenceError(i.e, `inner` is not defined).
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 43. What is the output of below code
```javascript
function myFun(x, y, ...manyMoreArgs) {
console.log(manyMoreArgs);
}
myFun(1, 2, 3, 4, 5);
myFun(1, 2);
```
- 1: [3, 4, 5], undefined
- 2: SyntaxError
- 3: [3, 4, 5], []
- 4: [3, 4, 5], [undefined]
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
The rest parameter is used to hold the remaining parameters of a function and it becomes an empty array if the argument is not provided.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 44. What is the output of below code
```javascript
const obj = { key: "value" };
const array = [...obj];
console.log(array);
```
- 1: ['key', 'value']
- 2: TypeError
- 3: []
- 4: ['key']
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
Spread syntax can be applied only to iterable objects. By default, Objects are not iterable, but they become iterable when used in an Array, or with iterating functions such as `map(), reduce(), and assign()`. If you still try to do it, it still throws `TypeError: obj is not iterable`.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 45. What is the output of below code
```javascript
function* myGenFunc() {
yield 1;
yield 2;
yield 3;
}
var myGenObj = new myGenFunc();
console.log(myGenObj.next().value);
```
- 1: 1
- 2: undefined
- 3: SyntaxError
- 4: TypeError
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
Generators are not constructible type. But if you still proceed to do, there will be an error saying "TypeError: myGenFunc is not a constructor"
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 46. What is the output of below code
```javascript
function* yieldAndReturn() {
yield 1;
return 2;
yield 3;
}
var myGenObj = yieldAndReturn();
console.log(myGenObj.next());
console.log(myGenObj.next());
console.log(myGenObj.next());
```
- 1: { value: 1, done: false }, { value: 2, done: true }, { value: undefined, done: true }
- 2: { value: 1, done: false }, { value: 2, done: false }, { value: undefined, done: true }
- 3: { value: 1, done: false }, { value: 2, done: true }, { value: 3, done: true }
- 4: { value: 1, done: false }, { value: 2, done: false }, { value: 3, done: true }
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
A return statement in a generator function will make the generator finish. If a value is returned, it will be set as the value property of the object and done property to true. When a generator is finished, subsequent next() calls return an object of this form: `{value: undefined, done: true}`.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 47. What is the output of below code
```javascript
const myGenerator = (function* () {
yield 1;
yield 2;
yield 3;
})();
for (const value of myGenerator) {
console.log(value);
break;
}
for (const value of myGenerator) {
console.log(value);
}
```
- 1: 1,2,3 and 1,2,3
- 2: 1,2,3 and 4,5,6
- 3: 1 and 1
- 4: 1
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
The generator should not be re-used once the iterator is closed. i.e, Upon exiting a loop(on completion or using break & return), the generator is closed and trying to iterate over it again does not yield any more results. Hence, the second loop doesn't print any value.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 48. What is the output of below code
```javascript
const num = 0o38;
console.log(num);
```
- 1: SyntaxError
- 2: 38
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
If you use an invalid number(outside of 0-7 range) in the octal literal, JavaScript will throw a SyntaxError. In ES5, it treats the octal literal as a decimal number.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 49. What is the output of below code
```javascript
const squareObj = new Square(10);
console.log(squareObj.area);
class Square {
constructor(length) {
this.length = length;
}
get area() {
return this.length * this.length;
}
set area(value) {
this.area = value;
}
}
```
- 1: 100
- 2: ReferenceError
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
Unlike function declarations, class declarations are not hoisted. i.e, First You need to declare your class and then access it, otherwise it will throw a ReferenceError "Uncaught ReferenceError: Square is not defined".
**Note:** Class expressions also applies to the same hoisting restrictions of class declarations.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 50. What is the output of below code
```javascript
function Person() {}
Person.prototype.walk = function () {
return this;
};
Person.run = function () {
return this;
};
let user = new Person();
let walk = user.walk;
console.log(walk());
let run = Person.run;
console.log(run());
```
- 1: undefined, undefined
- 2: Person, Person
- 3: SyntaxError
- 4: Window, Window
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
When a regular or prototype method is called without a value for **this**, the methods return an initial this value if the value is not undefined. Otherwise global window object will be returned. In our case, the initial `this` value is undefined so both methods return window objects.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 51. What is the output of below code
```javascript
class Vehicle {
constructor(name) {
this.name = name;
}
start() {
console.log(`${this.name} vehicle started`);
}
}
class Car extends Vehicle {
start() {
console.log(`${this.name} car started`);
super.start();
}
}
const car = new Car("BMW");
console.log(car.start());
```
- 1: SyntaxError
- 2: BMW vehicle started, BMW car started
- 3: BMW car started, BMW vehicle started
- 4: BMW car started, BMW car started
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
The super keyword is used to call methods of a superclass. Unlike other languages the super invocation doesn't need to be a first statement. i.e, The statements will be executed in the same order of code.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 52. What is the output of below code
```javascript
const USER = { age: 30 };
USER.age = 25;
console.log(USER.age);
```
- 1: 30
- 2: 25
- 3: Uncaught TypeError
- 4: SyntaxError
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
Even though we used constant variables, the content of it is an object and the object's contents (e.g properties) can be altered. Hence, the change is going to be valid in this case.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 53. What is the output of below code
```javascript
console.log("🙂" === "🙂");
```
- 1: false
- 2: true
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
Emojis are unicodes and the unicode for smile symbol is "U+1F642". The unicode comparision of same emojies is equivalent to string comparison. Hence, the output is always true.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 54. What is the output of below code?
```javascript
console.log(typeof typeof typeof true);
```
- 1: string
- 2: boolean
- 3: NaN
- 4: number
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
The typeof operator on any primitive returns a string value. So even if you apply the chain of typeof operators on the return value, it is always string.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 55. What is the output of below code?
```javascript
let zero = new Number(0);
if (zero) {
console.log("If");
} else {
console.log("Else");
}
```
- 1: If
- 2: Else
- 3: NaN
- 4: SyntaxError
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
1. The type of operator on new Number always returns object. i.e, typeof new Number(0) --> object.
2. Objects are always truthy in if block
Hence the above code block always goes to if section.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 55. What is the output of below code in non strict mode?
```javascript
let msg = "Good morning!!";
msg.name = "John";
console.log(msg.name);
```
- 1: ""
- 2: Error
- 3: John
- 4: Undefined
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
It returns undefined for non-strict mode and returns Error for strict mode. In non-strict mode, the wrapper object is going to be created and get the mentioned property. But the object get disappeared after accessing the property in next line.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 56. What is the output of below code?
```javascript
let count = 10;
(function innerFunc() {
if (count === 10) {
let count = 11;
console.log(count);
}
console.log(count);
})();
```
- 1: 11, 10
- 2: 11, 11
- 3: 10, 11
- 4: 10, 10
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
11 and 10 is logged to the console.
The innerFunc is a closure which captures the count variable from the outerscope. i.e, 10. But the conditional has another local variable `count` which overwrites the ourter `count` variable. So the first console.log displays value 11.
Whereas the second console.log logs 10 by capturing the count variable from outerscope.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 57. What is the output of below code ?
- 1: console.log(true && 'hi');
- 2: console.log(true && 'hi' && 1);
- 3: console.log(true && '' && 0);
<details><summary><b>Answer</b></summary>
- 1: hi
- 2: 1
- 3: ''
Reason : The operator returns the value of the first falsy operand encountered when evaluating from left to right, or the value of the last operand if they are all truthy.
**Note:** Below these values are consider as falsy value
- 1: 0
- 2: ''
- 3: null
- 4: undefined
- 5: NAN
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 58. What is the output of below code ?
```javascript
let arr = [1, 2, 3];
let str = "1,2,3";
console.log(arr == str);
```
- 1: false
- 2: Error
- 3: true
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
Arrays have their own implementation of `toString` method that returns a comma-separated list of elements. So the above code snippet returns true. In order to avoid conversion of array type, we should use === for comparison.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 59. What is the output of below code?
```javascript
getMessage();
var getMessage = () => {
console.log("Good morning");
};
```
- 1: Good morning
- 2: getMessage is not a function
- 3: getMessage is not defined
- 4: Undefined
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
Hoisting will move variables and functions to be the top of scope. Even though getMessage is an arrow function the above function will considered as a varible due to it's variable declaration or assignment. So the variables will have undefined value in memory phase and throws an error '`getMessage` is not a function' at the code execution phase.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 60. What is the output of below code?
```javascript
let quickPromise = Promise.resolve();
quickPromise.then(() => console.log("promise finished"));
console.log("program finished");
```
- 1: program finished
- 2: Cannot predict the order
- 3: program finished, promise finished
- 4: promise finished, program finished
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
Even though a promise is resolved immediately, it won't be executed immediately because its **.then/catch/finally** handlers or callbacks(aka task) are pushed into the queue. Whenever the JavaScript engine becomes free from the current program, it pulls a task from the queue and executes it. This is the reason why last statement is printed first before the log of promise handler.
**Note:** We call the above queue as "MicroTask Queue"
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 61. What is the output of below code?
```javascript
console
.log("First line")
[("a", "b", "c")].forEach((element) => console.log(element));
console.log("Third line");
```
- 1: `First line`, then print `a, b, c` in a new line, and finally print `Third line` as next line
- 2: `First line`, then print `a, b, c` in a first line, and print `Third line` as next line
- 3: Missing semi-colon error
- 4: Cannot read properties of undefined
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
When JavaScript encounters a line break without a semicolon, the JavaScript parser will automatically add a semicolon based on a set of rules called `Automatic Semicolon Insertion` which determines whether line break as end of statement or not to insert semicolon. But it does not assume a semicolon before square brackets [...]. So the first two lines considered as a single statement as below.
```javascript
console
.log("First line")
[("a", "b", "c")].forEach((element) => console.log(element));
```
Hence, there will be **cannot read properties of undefined** error while applying the array square bracket on log function.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 62. Write a function that returns a random HEX color
<details><summary><b>Solution 1 (Iterative generation)</b></summary>
<p>
```javascript
const HEX_ALPHABET = [
"0",
"1",
"2",
"3",
"4",
"5",
"6",
"7",
"8",
"9",
"a",
"b",
"c",
"d",
"e",
"f",
];
const HEX_PREFIX = "#";
const HEX_LENGTH = 6;
function generateRandomHex() {
let randomHex = "";
for (let i = 0; i < HEX_LENGTH; i++) {
const randomIndex = Math.floor(Math.random() * HEX_ALPHABET.length);
randomHex += HEX_ALPHABET[randomIndex];
}
return HEX_PREFIX + randomHex;
}
```
</p>
</details>
<details><summary><b>Solution 2 (One-liner)</b></summary>
<p>
```javascript
const HEX_PREFIX = "#";
const HEX_RADIX = 16;
const HEX_LENGTH = 6;
function generateRandomHex() {
return (
HEX_PREFIX +
Math.floor(Math.random() * 0xffffff)
.toString(HEX_RADIX)
.padStart(HEX_LENGTH, "0")
);
}
```
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 63. What is the output of below code?
```javascript
var of = ["of"];
for (var of of of) {
console.log(of);
}
```
- 1: of
- 2: SyntaxError: Unexpected token of
- 3: SyntaxError: Identifier 'of' has already been declared
- 4: ReferenceError: of is not defined
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
In JavaScript, `of` is not considered as a reserved keyword. So the variable declaration with `of` is accepted and prints the array value `of` using for..of loop.
But if you use reserved keyword such as `in` then there will be a syntax error saying `SyntaxError: Unexpected token in`,
```javascript
var in = ['in'];
for(var in in in) {
console.log(in[in]);
}
```
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 64. What is the output of below code?
```javascript
const numbers = [11, 25, 31, 23, 33, 18, 200];
numbers.sort();
console.log(numbers);
```
- 1: [11, 18, 23, 25, 31, 33, 200]
- 2: [11, 18, 200, 23, 25, 31, 33]
- 3: [11, 25, 31, 23, 33, 18, 200]
- 4: Cannot sort numbers
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
By default, the sort method sorts elements alphabetically. This is because elemented converted to strings and strings compared in UTF-16 code units order. Hence, you will see the above numbers not sorted as expected. In order to sort numerically just supply a comparator function which handles numeric sorts.
```javascript
const numbers = [11, 25, 31, 23, 33, 18, 200];
numbers.sort((a, b) => a - b);
console.log(numbers);
```
**Note:** Sort() method changes the original array.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 65. What is the output order of below code?
```javascript
setTimeout(() => {
console.log("1");
}, 0);
Promise.resolve("hello").then(() => console.log("2"));
console.log("3");
```
- 1: 1, 2, 3
- 2: 1, 3, 2
- 3: 3, 1, 2
- 4: 3, 2, 1
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
When the JavaScript engine parses the above code, the first two statements are asynchronous which will be executed later and third statement is synchronous statement which will be moved to callstack, executed and prints the number 3 in the console. Next, Promise is native in ES6 and it will be moved to Job queue which has high priority than callback queue in the execution order. At last, since setTimeout is part of WebAPI the callback function moved to callback queue and executed. Hence, you will see number 2 printed first followed by 1.
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 66. What is the output of below code?
```javascript
console.log(name);
console.log(message());
var name = "John";
(function message() {
console.log("Hello John: Welcome");
});
```
- 1: John, Hello John: Welcome
- 2: undefined, Hello John, Welcome
- 3: Reference error: name is not defined, Reference error: message is not defined
- 4: undefined, Reference error: message is not defined
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
IIFE(Immediately Invoked Function Expression) is just like any other function expression which won't be hoisted. Hence, there will be a reference error for message call.
The behavior would be the same with below function expression of message1,
```javascript
console.log(name);
console.log(message());
var name = 'John';
var message = function () {
console.log('Hello John: Welcome');
});
```
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 67. What is the output of below code?
```javascript
message();
function message() {
console.log("Hello");
}
function message() {
console.log("Bye");
}
```
- 1: Reference error: message is not defined
- 2: Hello
- 3: Bye
- 4: Compile time error
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
As part of hoisting, initially JavaScript Engine or compiler will store first function in heap memory but later rewrite or replaces with redefined function content.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 68. What is the output of below code?
```javascript
var currentCity = "NewYork";
var changeCurrentCity = function () {
console.log("Current City:", currentCity);
var currentCity = "Singapore";
console.log("Current City:", currentCity);
};
changeCurrentCity();
```
- 1: NewYork, Singapore
- 2: NewYork, NewYork
- 3: undefined, Singapore
- 4: Singapore, Singapore
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
Due to hositing feature, the variables declared with `var` will have `undefined` value in the creation phase so the outer variable `currentCity` will get same `undefined` value. But after few lines of code JavaScript engine found a new function call(`changeCurrentCity()`) to update the current city with `var` re-declaration. Since each function call will create a new execution context, the same variable will have `undefined` value before the declaration and new value(`Singapore`) after the declarion. Hence, the value `undefined` print first followed by new value `Singapore` in the execution phase.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 69. What is the output of below code in an order?
```javascript
function second() {
var message;
console.log(message);
}
function first() {
var message = "first";
second();
console.log(message);
}
var message = "default";
first();
console.log(message);
```
- 1: undefined, first, default
- 2: default, default, default
- 3: first, first, default
- 4: undefined, undefined, undefined
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
Each context(global or functional) has it's own variable environment and the callstack of variables in a LIFO order. So you can see the message variable value from second, first functions in an order followed by global context message variable value at the end.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 70. What is the output of below code?
```javascript
var expressionOne = function functionOne() {
console.log("functionOne");
};
functionOne();
```
- 1: functionOne is not defined
- 2: functionOne
- 3: console.log("functionOne")
- 4: undefined
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
The function call `functionOne` is not going to be part of scope chain and it has it's own execution context with the enclosed variable environment. i.e, It won't be accessed from global context. Hence, there will be an error while invoking the function as `functionOne is not defined`.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 71. What is the output of below code?
```javascript
const user = {
name: "John",
eat() {
console.log(this);
var eatFruit = function () {
console.log(this);
};
eatFruit();
},
};
user.eat();
```
- 1: {name: "John", eat: f}, {name: "John", eat: f}
- 2: Window {...}, Window {...}
- 3: {name: "John", eat: f}, undefined
- 4: {name: "John", eat: f}, Window {...}
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
`this` keyword is dynamic scoped but not lexically scoped . In other words, it doesn't matter where `this` has been written but how it has been invoked really matter. In the above code snippet, the `user` object invokes `eat` function so `this` keyword refers to `user` object but `eatFruit` has been invoked by `eat` function and `this` will have default `Window` object.
The above pit fall fixed by three ways,
1. In ES6, the arrow function will make `this` keyword as lexically scoped. Since the surrounding object of `this` object is `user` object, the `eatFruit` function will contain `user` object for `this` object.
```javascript
const user = {
name: "John",
eat() {
console.log(this);
var eatFruit = () => {
console.log(this);
};
eatFruit();
},
};
user.eat();
```
The next two solutions have been used before ES6 introduced.
2. It is possible create a reference of `this` into a separate variable and use that new variable inplace of `this` keyword inside `eatFruit` function. This is a common practice in jQuery and AngularJS before ES6 introduced.
```javascript
const user = {
name: "John",
eat() {
console.log(this);
var self = this;
var eatFruit = () => {
console.log(self);
};
eatFruit();
},
};
user.eat();
```
3. The `eatFruit` function can bind explicitly with `this` keyword where it refers `Window` object.
```javascript
const user = {
name: "John",
eat() {
console.log(this);
var eatFruit = function () {
console.log(this);
};
return eatFruit.bind(this);
},
};
user.eat()();
```
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 72. What is the output of below code?
```javascript
let message = "Hello World!";
message[0] = "J";
console.log(message);
let name = "John";
name = name + " Smith";
console.log(name);
```
- 1: Jello World!, John Smith
- 2: Jello World!, John
- 3: Hello World!, John Smith
- 4: Hello World!, John
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
In JavaScript, primitives are immutable i.e. there is no way to change a primitive value once it gets created. So when you try to update the string's first character, there is no change in the string value and prints the same initial value `Hello World!`. Whereas in the later example, the concatenated value is re-assigned to the same variable which will result into creation of new memory block with the reference pointing to `John Smith` value and the old memory block value(`John`) will be garbage collected.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 73. What is the output of below code?
```javascript
let user1 = {
name: "Jacob",
age: 28,
};
let user2 = {
name: "Jacob",
age: 28,
};
console.log(user1 === user2);
```
- 1: True
- 2: False
- 3: Compile time error
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
In JavaScript, the variables such as objects, arrays and functions comes under pass by reference. When you try to compare two objects with same content, it is going to compare memory address or reference of those variables. These variables always create separate memory blocks hence the comparison is always going to return false value.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 74. What is the output of below code?
```javascript
function greeting() {
setTimeout(function () {
console.log(message);
}, 5000);
const message = "Hello, Good morning";
}
greeting();
```
- 1: Undefined
- 2: Reference error:
- 3: Hello, Good morning
- 4: null
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
The variable `message` is still treated as closure(since it has been used in inner function) eventhough it has been declared after setTimeout function. The function with in setTimeout function will be sent to WebAPI and the variable declaration executed with in 5 seconds with the assigned value. Hence, the text declared for the variable will be displayed.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 75. What is the output of below code?
```javascript
const a = new Number(10);
const b = 10;
console.log(a === b);
```
- 1: False
- 2: True
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 1
Eventhough both variables `a` and `b` refer a number value, the first declaration is based on constructor function and the type of the variable is going to be `object` type. Whereas the second declaration is primitive assignment with a number and the type is `number` type. Hence, the equality operator `===` will output `false` value.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 76. What is the type of below function?
```javascript
function add(a, b) {
console.log("The input arguments are: ", a, b);
return a + b;
}
```
- 1: Pure function
- 2: Impure function
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
Eventhough the above function returns the same result for the same arguments(input) that are passed in the function, the `console.log()` statement causes a function to have side effects because it affects the state of an external code. i.e, the `console` object's state and depends on it to perform the job. Hence, the above function considered as impure function.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 77. What is the output of below code?
```javascript
const promiseOne = new Promise((resolve, reject) => setTimeout(resolve, 4000));
const promiseTwo = new Promise((resolve, reject) => setTimeout(reject, 4000));
Promise.all([promiseOne, promiseTwo]).then((data) => console.log(data));
```
- 1: [{status: "fullfilled", value: undefined}, {status: "rejected", reason: undefined}]
- 2: [{status: "fullfilled", value: undefined}, Uncaught(in promise)]
- 3: Uncaught (in promise)
- 4: [Uncaught(in promise), Uncaught(in promise)]
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
The above promises settled at the same time but one of them resolved and other one rejected. When you use `.all` method on these promises, the result will be short circuted by throwing an error due to rejection in second promise. But If you use `.allSettled` method then result of both the promises will be returned irrespective of resolved or rejected promise status without throwing any error.
```javascript
Promise.allSettled([promiseOne, promiseTwo]).then((data) => console.log(data));
```
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 78. What is the output of below code?
```javascript
try {
setTimeout(() => {
console.log("try block");
throw new Error(`An exception is thrown`);
}, 1000);
} catch (err) {
console.log("Error: ", err);
}
```
- 1: try block, Error: An exception is thrown
- 2: Error: An exception is thrown
- 3: try block, Uncaught Error: Exception is thrown
- 4: Uncaught Error: Exception is thrown
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 3
If you put `setTimeout` and `setInterval` methods inside the try clause and an exception is thrown, the catch clause will not catch any of them. This is because the try...catch statement works synchronously, and the function in the above code is executed asynchronously after a certain period of time. Hence, you will see runtime exception without catching the error. To resolve this issue, you have to put the try...catch block inside the function as below,
```javascript
setTimeout(() => {
try {
console.log("try block");
throw new Error(`An exception is thrown`);
} catch (err) {
console.log("Error: ", err);
}
}, 1000);
```
You can use `.catch()` function in promises to avoid these issues with asynchronous code.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
#### 79. What is the output of below code?
```javascript
let a = 10;
if (true) {
let a = 20;
console.log(a, "inside");
}
console.log(a, "outside");
```
- 1: 20, "inside" and 20, "outside"
- 2: 20, "inside" and 10, "outside"
- 3: 10, "inside" and 10, "outside"
- 4: 10, "inside" and 20, "outside"
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 2
The variable "a" declared inside "if" has block scope and does not affect the value of the outer "a" variable.
</p>
</details>
---
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#### 80. What is the output of below code?
```javascript
let arr = [1, 2, 3, 4, 5, -6, 7];
arr.length = 0;
console.log(arr);
```
- 1: 0
- 2: Undefined
- 3: null
- 4: [ ]
<details><summary><b>Answer</b></summary>
<p>
##### Answer: 4
The length of the array 'arr' has been set to 0, so the array becomes empty.
</p>
</details>
---
**[⬆ Back to Top](#table-of-contents)**
## Disclaimer
The questions provided in this repository are the summary of frequently asked questions across numerous companies. We cannot guarantee that these questions will actually be asked during your interview process, nor should you focus on memorizing all of them. The primary purpose is for you to get a sense of what some companies might ask — do not get discouraged if you don't know the answer to all of them — that is ok!
Good luck with your interview 😊
---