https://github.com/sudheerj/reactjs-interview-questions
List of top 500 ReactJS Interview Questions & Answers....Coding exercise questions are coming soon!!
https://github.com/sudheerj/reactjs-interview-questions
interview-preparation interview-questions javascript javascript-applications javascript-framework javascript-interview-questions react react-interview-questions react-native react-router react16 reactjs redux
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List of top 500 ReactJS Interview Questions & Answers....Coding exercise questions are coming soon!!
- Host: GitHub
- URL: https://github.com/sudheerj/reactjs-interview-questions
- Owner: sudheerj
- License: mit
- Created: 2018-05-31T17:17:01.000Z (about 8 years ago)
- Default Branch: master
- Last Pushed: 2025-03-31T11:09:55.000Z (about 1 year ago)
- Last Synced: 2025-05-12T16:19:53.391Z (about 1 year ago)
- Topics: interview-preparation, interview-questions, javascript, javascript-applications, javascript-framework, javascript-interview-questions, react, react-interview-questions, react-native, react-router, react16, reactjs, redux
- Language: JavaScript
- Homepage:
- Size: 2.99 MB
- Stars: 41,960
- Watchers: 628
- Forks: 9,866
- Open Issues: 14
-
Metadata Files:
- Readme: README.md
- Funding: .github/FUNDING.yml
- License: LICENSE
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README
# React Interview Questions & Answers
> Click :star: if you like the project. Pull Requests are highly appreciated. Follow me [@SudheerJonna](https://twitter.com/SudheerJonna) for technical updates.
---
> 💡 Nail React interviews with questions and solutions from ex-interviewers! [Try GreatFrontEnd →](https://www.greatfrontend.com/questions/react?utm_source=github&utm_medium=referral&utm_campaign=sudheerj-react&gnrs=sudheerj) 💡
---
> 🚀 Ace React interview questions with solutions from FAANG+ companies! [Try FrontendLead →](https://www.frontendlead.com/coding-questions?utm_source=github&utm_medium=referral&utm_campaign=sudheerj-react) 🚀
---
- I recommend this React course if you’re serious about learning React and want to go beyond the basics.
- Want to ace your coding interview and get hired at your dream company? Take this coding interview bootcamp.
---
**Note:** This repository is specific to ReactJS. Please check [JavaScript Interview Questions](https://github.com/sudheerj/javascript-interview-questions) for core JavaScript questions and [Data Structures and Algorithms](https://github.com/sudheerj/datastructures-algorithms) for DSA-related questions or problems.
### Table of Contents
Hide/Show table of contents
| No. | Questions |
| --- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| | **Core React** |
| 1 | [What is React?](#what-is-react) |
| 2 | [What is the history behind React’s evolution?](#what-is-the-history-behind-react-evolution) |
| 3 | [What are the major features of React?](#what-are-the-major-features-of-react) |
| 4 | [What is JSX?](#what-is-jsx) |
| 5 | [What is the difference between an Element and a Component?](#what-is-the-difference-between-element-and-component) |
| 6 | [How do you create components in React?](#how-to-create-components-in-react) |
| 7 | [When should you use a Class Component over a Function Component?](#when-to-use-a-class-component-over-a-function-component) |
| 8 | [What are Pure Components?](#what-are-pure-components) |
| 9 | [What is state in React?](#what-is-state-in-react) |
| 10 | [What are props in React?](#what-are-props-in-react) |
| 11 | [What is the difference between state and props?](#what-is-the-difference-between-state-and-props) |
| 12 | [What is the difference between HTML and React event handling?](#what-is-the-difference-between-html-and-react-event-handling) |
| 13 | [What are synthetic events in React?](#what-are-synthetic-events-in-react) |
| 14 | [What are inline conditional expressions?](#what-are-inline-conditional-expressions) |
| 15 | [What is the "key" prop and what is its benefit when used in arrays of elements?](#what-is-key-prop-and-what-is-the-benefit-of-using-it-in-arrays-of-elements) |
| 16 | [What is the Virtual DOM?](#what-is-virtual-dom) |
| 17 | [How does the Virtual DOM work?](#how-virtual-dom-works) |
| 18 | [What is the difference between Shadow DOM and Virtual DOM?](#what-is-the-difference-between-shadow-dom-and-virtual-dom) |
| 19 | [What is React Fiber?](#what-is-react-fiber) |
| 20 | [What is the main goal of React Fiber?](#what-is-the-main-goal-of-react-fiber) |
| 21 | [What are controlled components?](#what-are-controlled-components) |
| 22 | [What are uncontrolled components?](#what-are-uncontrolled-components) |
| 23 | [What is the difference between createElement and cloneElement?](#what-is-the-difference-between-createelement-and-cloneelement) |
| 24 | [What is Lifting State Up in React?](#what-is-lifting-state-up-in-react) |
| 25 | [What are Higher-Order Components?](#what-are-higher-order-components) |
| 26 | [What is the children prop?](#what-is-children-prop) |
| 27 | [How do you write comments in React?](#how-to-write-comments-in-react) |
| 28 | [What is reconciliation?](#what-is-reconciliation) |
| 29 | [Does the lazy function support named exports?](#does-the-lazy-function-support-named-exports) |
| 30 | [Why does React use className instead of the class attribute?](#why-react-uses-classname-over-class-attribute) |
| 31 | [What are Fragments?](#what-are-fragments) |
| 32 | [Why are Fragments better than container divs?](#why-fragments-are-better-than-container-divs) |
| 33 | [What are portals in React?](#what-are-portals-in-react) |
| 34 | [What are stateless components?](#what-are-stateless-components) |
| 35 | [What are stateful components?](#what-are-stateful-components) |
| 36 | [How do you apply validation to props in React?](#how-to-apply-validation-on-props-in-react) |
| 37 | [What are the advantages of React?](#what-are-the-advantages-of-react) |
| 38 | [What are the limitations of React?](#what-are-the-limitations-of-react) |
| 39 | [What are the recommended ways for static type checking?](#what-are-the-recommended-ways-for-static-type-checking) |
| 40 | [What is the use of the react-dom package?](#what-is-the-use-of-react-dom-package) |
| 41 | [What is ReactDOMServer?](#what-is-reactdomserver) |
| 42 | [How do you use innerHTML in React?](#how-to-use-innerhtml-in-react) |
| 43 | [How do you apply styles in React?](#how-to-use-styles-in-react) |
| 44 | [How are events different in React?](#how-events-are-different-in-react) |
| 45 | [What is the impact of using indexes as keys?](#what-is-the-impact-of-indexes-as-keys) |
| 46 | [How do you conditionally render components?](#how-do-you-conditionally-render-components) |
| 47 | [Why do we need to be careful when spreading props on DOM elements?](#why-we-need-to-be-careful-when-spreading-props-on-dom-elements) |
| 48 | [How do you memoize a component?](#how-do-you-memoize-a-component) |
| 49 | [How do you implement Server-Side Rendering (SSR)?](#how-you-implement-server-side-rendering-or-ssr) |
| 50 | [How do you enable production mode in React?](#how-to-enable-production-mode-in-react) |
| 51 | [Do Hooks replace render props and higher-order components?](#do-hooks-replace-render-props-and-higher-order-components) |
| 52 | [What is a switching component?](#what-is-a-switching-component) |
| 53 | [What are React Mixins?](#what-are-react-mixins) |
| 54 | [What are the pointer events supported in React?](#what-are-the-pointer-events-supported-in-react) |
| 55 | [Why should component names start with a capital letter?](#why-should-component-names-start-with-capital-letter) |
| 56 | [Are custom DOM attributes supported in React v16?](#are-custom-dom-attributes-supported-in-react-v16) |
| 57 | [How do you loop inside JSX?](#how-to-loop-inside-jsx) |
| 58 | [How do you access props within attribute quotes?](#how-do-you-access-props-in-attribute-quotes) |
| 59 | [What is a React PropType array with shape?](#what-is-react-proptype-array-with-shape) |
| 60 | [How do you conditionally apply class attributes?](#how-to-conditionally-apply-class-attributes) |
| 61 | [What is the difference between React and ReactDOM?](#what-is-the-difference-between-react-and-reactdom) |
| 62 | [Why is ReactDOM separated from React?](#why-reactdom-is-separated-from-react) |
| 63 | [How do you use the React label element?](#how-to-use-react-label-element) |
| 64 | [How do you combine multiple inline style objects?](#how-to-combine-multiple-inline-style-objects) |
| 65 | [How do you re-render the view when the browser is resized?](#how-to-re-render-the-view-when-the-browser-is-resized) |
| 66 | [How do you pretty-print JSON with React?](#how-to-pretty-print-json-with-react) |
| 67 | [Why can’t you update props in React?](#why-you-cant-update-props-in-react) |
| 68 | [How do you focus an input element on page load?](#how-to-focus-an-input-element-on-page-load) |
| 69 | [How can you find the version of React at runtime in the browser?](#how-can-we-find-the-version-of-react-at-runtime-in-the-browser) |
| 70 | [How do you add Google Analytics for React Router?](#how-to-add-google-analytics-for-react-router) |
| 71 | [How do you apply vendor prefixes to inline styles in React?](#how-do-you-apply-vendor-prefixes-to-inline-styles-in-react) |
| 72 | [How do you import and export components using React and ES6?](#how-to-import-and-export-components-using-react-and-es6) |
| 73 | [What are the exceptions to React component naming?](#what-are-the-exceptions-on-react-component-naming) |
| 74 | [Is it possible to use async/await in plain React?](#is-it-possible-to-use-asyncawait-in-plain-react) |
| 75 | [What are common folder structures for React?](#what-are-the-common-folder-structures-for-react) |
| 76 | [What are popular packages for animation?](#what-are-the-popular-packages-for-animation) |
| 77 | [What are the benefits of style modules?](#what-is-the-benefit-of-styles-modules) |
| 78 | [What are popular React-specific linters?](#what-are-the-popular-react-specific-linters) |
| | **React Router** |
| 79 | [What is React Router?](#what-is-react-router) |
| 80 | [How is React Router different from the history library?](#how-react-router-is-different-from-history-library) |
| 81 | [What are the components of React Router v6?](#what-are-the-router-components-of-react-router-v6) |
| 82 | [What is the purpose of the push and replace methods of history?](#what-is-the-purpose-of-push-and-replace-methods-of-history) |
| 83 | [How do you programmatically navigate using React Router v4?](#how-do-you-programmatically-navigate-using-react-router-v4) |
| 84 | [How do you get query parameters in React Router v4?](#how-to-get-query-parameters-in-react-router-v4) |
| 85 | [Why do you get a "Router may have only one child element" warning?](#why-you-get-router-may-have-only-one-child-element-warning) |
| 86 | [How do you pass params to the history.push method in React Router v4?](#how-to-pass-params-to-historypush-method-in-react-router-v4) |
| 87 | [How do you implement a default or NotFound page?](#how-to-implement-default-or-notfound-page) |
| 88 | [How do you get history in React Router v4?](#how-to-get-history-on-react-router-v4) |
| 89 | [How do you perform an automatic redirect after login?](#how-to-perform-automatic-redirect-after-login)
| | **React Internationalization** |
| 90 | [What is React Intl?](#what-is-react-intl) |
| 91 | [What are the main features of React Intl?](#what-are-the-main-features-of-react-intl) |
| 92 | [What are the two ways of formatting in React Intl?](#what-are-the-two-ways-of-formatting-in-react-intl) |
| 93 | [How do you use FormattedMessage as a placeholder with React Intl?](#how-to-use-formattedmessage-as-placeholder-using-react-intl) |
| 94 | [How do you access the current locale with React Intl?](#how-to-access-current-locale-with-react-intl) |
| 95 | [How do you format a date using React Intl?](#how-to-format-date-using-react-intl) |
| | **React Testing** |
| 96 | [What is the Shallow Renderer in React testing?](#what-is-shallow-renderer-in-react-testing) |
| 97 | [What is the TestRenderer package in React?](#what-is-testrenderer-package-in-react) |
| 98 | [What is the purpose of the ReactTestUtils package?](#what-is-the-purpose-of-reacttestutils-package) |
| 99 | [What is Jest?](#what-is-jest) |
| 100 | [What are the advantages of Jest over Jasmine?](#what-are-the-advantages-of-jest-over-jasmine) |
| 101 | [Can you give a simple example of a Jest test case?](#give-a-simple-example-of-jest-test-case) |
| | **React Redux** |
| 102 | [What is Flux?](#what-is-flux) |
| 103 | [What is Redux?](#what-is-redux) |
| 104 | [What are the core principles of Redux?](#what-are-the-core-principles-of-redux) |
| 105 | [What are the downsides of Redux compared to Flux?](#what-are-the-downsides-of-redux-compared-to-flux) |
| 106 | [What is the difference between mapStateToProps() and mapDispatchToProps()?](#what-is-the-difference-between-mapstatetoprops-and-mapdispatchtoprops) |
| 107 | [Can you dispatch an action in a reducer?](#can-i-dispatch-an-action-in-reducer) |
| 108 | [How do you access the Redux store outside a component?](#how-to-access-redux-store-outside-a-component) |
| 109 | [What are the drawbacks of the MVW pattern?](#what-are-the-drawbacks-of-mvw-pattern) |
| 110 | [Are there any similarities between Redux and RxJS?](#are-there-any-similarities-between-redux-and-rxjs) |
| 111 | [How do you reset state in Redux?](#how-to-reset-state-in-redux) |
| 112 | [What is the difference between React Context and React Redux?](#what-is-the-difference-between-react-context-and-react-redux) |
| 113 | [Why are Redux state functions called reducers?](#why-are-redux-state-functions-called-reducers) |
| 114 | [How do you make an AJAX request in Redux?](#how-to-make-ajax-request-in-redux) |
| 115 | [Should you keep all component states in the Redux store?](#should-i-keep-all-components-state-in-redux-store) |
| 116 | [What is the proper way to access the Redux store?](#what-is-the-proper-way-to-access-redux-store) |
| 117 | [What is the difference between a component and a container in React Redux?](#what-is-the-difference-between-component-and-container-in-react-redux) |
| 118 | [What is the purpose of constants in Redux?](#what-is-the-purpose-of-the-constants-in-redux) |
| 119 | [What are the different ways to write mapDispatchToProps()?](#what-are-the-different-ways-to-write-mapdispatchtoprops) |
| 120 | [What is the use of the ownProps parameter in mapStateToProps() and mapDispatchToProps()?](#what-is-the-use-of-the-ownprops-parameter-in-mapstatetoprops-and-mapdispatchtoprops) |
| 121 | [How do you structure Redux top-level directories?](#how-to-structure-redux-top-level-directories) |
| 122 | [What is Redux Saga?](#what-is-redux-saga) |
| 123 | [What is the mental model of Redux Saga?](#what-is-the-mental-model-of-redux-saga) |
| 124 | [What are the differences between call and put in Redux Saga?](#what-are-the-differences-between-call-and-put-in-redux-saga) |
| 125 | [What is Redux Thunk?](#what-is-redux-thunk) |
| 126 | [What are the differences between Redux Saga and Redux Thunk?](#what-are-the-differences-between-redux-saga-and-redux-thunk) |
| 127 | [What is Redux DevTools?](#what-is-redux-devtools) |
| 128 | [What are the features of Redux DevTools?](#what-are-the-features-of-redux-devtools) |
| 129 | [What are Redux selectors and why should you use them?](#what-are-redux-selectors-and-why-use-them) |
| 130 | [What is Redux Form?](#what-is-redux-form) |
| 131 | [What are the main features of Redux Form?](#what-are-the-main-features-of-redux-form) |
| 132 | [How do you add multiple middlewares to Redux?](#how-to-add-multiple-middlewares-to-redux) |
| 133 | [How do you set the initial state in Redux?](#how-to-set-initial-state-in-redux) |
| 134 | [How is Relay different from Redux?](#how-relay-is-different-from-redux) |
| 135 | [What is an action in Redux?](#what-is-an-action-in-redux) |
| | **React Native** |
| 136 | [What is the difference between React Native and React?](#what-is-the-difference-between-react-native-and-react) |
| 137 | [How do you test React Native apps?](#how-to-test-react-native-apps) |
| 138 | [How do you log in React Native?](#how-to-do-logging-in-react-native) |
| 139 | [How do you debug React Native apps?](#how-to-debug-your-react-native) |
| | **React Supported Libraries and Integration** |
| 140 | [What is Reselect and how does it work?](#what-is-reselect-and-how-it-works) |
| 141 | [What is Flow?](#what-is-flow) |
| 142 | [What is the difference between Flow and PropTypes?](#what-is-the-difference-between-flow-and-proptypes) |
| 143 | [How do you use Font Awesome icons in React?](#how-to-use-font-awesome-icons-in-react) |
| 144 | [What is React DevTools?](#what-is-react-dev-tools) |
| 145 | [Why does DevTools not load in Chrome for local files?](#why-is-devtools-not-loading-in-chrome-for-local-files) |
| 146 | [How do you use Polymer in React?](#how-to-use-polymer-in-react) |
| 147 | [What are the advantages of React over Vue.js?](#what-are-the-advantages-of-react-over-vuejs) |
| 148 | [What is the difference between React and Angular?](#what-is-the-difference-between-react-and-angular) |
| 149 | [Why is the React tab not showing up in DevTools?](#why-react-tab-is-not-showing-up-in-devtools) |
| 150 | [What are styled-components?](#what-are-styled-components) |
| 151 | [Can you give an example of styled-components?](#give-an-example-of-styled-components) |
| 152 | [What is Relay?](#what-is-relay) |
| | **Miscellaneous** |
| 153 | [What are the main features of the Reselect library?](#what-are-the-main-features-of-reselect-library) |
| 154 | [Can you give an example of Reselect usage?](#give-an-example-of-reselect-usage) |
| 155 | [Can Redux only be used with React?](#can-redux-only-be-used-with-react) |
| 156 | [Do you need a specific build tool to use Redux?](#do-you-need-to-have-a-particular-build-tool-to-use-redux) |
| 157 | [How do Redux Form initial values get updated from state?](#how-redux-form-initialvalues-get-updated-from-state) |
| 158 | [How do React PropTypes allow different types for one prop?](#how-react-proptypes-allow-different-types-for-one-prop) |
| 159 | [Can you import an SVG file as a React component?](#can-i-import-an-svg-file-as-react-component) |
| 160 | [What is render hijacking in React?](#what-is-render-hijacking-in-react) |
| 161 | [How do you pass numbers to a React component?](#how-to-pass-numbers-to-react-component) |
| 162 | [Do you need to keep all state in Redux? Should you ever use React’s internal state?](#do-i-need-to-keep-all-my-state-into-redux-should-i-ever-use-react-internal-state) |
| 163 | [What is the purpose of registerServiceWorker in React?](#what-is-the-purpose-of-registerserviceworker-in-react) |
| 164 | [What is the React.memo function?](#what-is-react-memo-function) |
| 165 | [What is the React.lazy function?](#what-is-react-lazy-function) |
| 166 | [How do you prevent unnecessary updates using setState?](#how-to-prevent-unnecessary-updates-using-setstate) |
| 167 | [How do you render arrays, strings, and numbers in React v16?](#how-do-you-render-array-strings-and-numbers-in-react-16-version) |
| 168 | [What are Hooks?](#what-are-hooks) |
| 169 | [What rules must be followed for Hooks?](#what-rules-need-to-be-followed-for-hooks) |
| 170 | [How do you ensure Hooks follow the rules in your project?](#how-to-ensure-hooks-followed-the-rules-in-your-project) |
| 171 | [What are the differences between Flux and Redux?](#what-are-the-differences-between-flux-and-redux) |
| 172 | [What are the benefits of React Router v4?](#what-are-the-benefits-of-react-router-v4) |
| 173 | [Can you describe the componentDidCatch lifecycle method signature?](#can-you-describe-about-componentdidcatch-lifecycle-method-signature) |
| 174 | [In which scenarios do error boundaries not catch errors?](#in-which-scenarios-do-error-boundaries-not-catch-errors) |
| 175 | [What is the behavior of uncaught errors in React v16?](#what-is-the-behavior-of-uncaught-errors-in-react-16) |
| 176 | [What is the proper placement for error boundaries?](#what-is-the-proper-placement-for-error-boundaries) |
| 177 | [What is the benefit of a component stack trace from an error boundary?](#what-is-the-benefit-of-component-stack-trace-from-error-boundary) |
| 178 | [What are default props?](#what-are-default-props) |
| 179 | [What is the purpose of the displayName class property?](#what-is-the-purpose-of-displayname-class-property) |
| 180 | [What is the browser support for React applications?](#what-is-the-browser-support-for-react-applications) |
| 181 | [What is code-splitting?](#what-is-code-splitting) |
| 182 | [What are keyed Fragments?](#what-are-keyed-fragments) |
| 183 | [Does React support all HTML attributes?](#does-react-support-all-html-attributes) |
| 184 | [When do component props default to true?](#when-component-props-defaults-to-true) |
| 185 | [What is Next.js and what are its major features?](#what-is-nextjs-and-major-features-of-it) |
| 186 | [How do you pass an event handler to a component?](#how-do-you-pass-an-event-handler-to-a-component) |
| 187 | [How do you prevent a function from being called multiple times?](#how-to-prevent-a-function-from-being-called-multiple-times) |
| 188 | [How does JSX prevent injection attacks?](#how-jsx-prevents-injection-attacks) |
| 189 | [How do you update rendered elements?](#how-do-you-update-rendered-elements) |
| 190 | [How do you indicate that props are read-only?](#how-do-you-say-that-props-are-read-only) |
| 191 | [What are the conditions for safely using an index as a key?](#what-are-the-conditions-to-safely-use-the-index-as-a-key) |
| 192 | [Do keys need to be globally unique?](#is-it-keys-should-be-globally-unique) |
| 193 | [What is the popular choice for form handling?](#what-is-the-popular-choice-for-form-handling) |
| 194 | [What are the advantages of Formik over the Redux Form library?](#what-are-the-advantages-of-formik-over-redux-form-library) |
| 195 | [Why are you not required to use inheritance?](#why-do-you-not-required-to-use-inheritance) |
| 196 | [Can you use web components in a React application?](#can-i-use-web-components-in-react-application) |
| 197 | [What is a dynamic import?](#what-is-dynamic-import) |
| 198 | [What are loadable components?](#what-are-loadable-components) |
| 199 | [What is a Suspense component?](#what-is-suspense-component) |
| 200 | [What is route-based code splitting?](#what-is-route-based-code-splitting) |
| 201 | [What is the purpose of the default value in Context?](#what-is-the-purpose-of-default-value-in-context) |
| 202 | [What is the diffing algorithm?](#what-is-diffing-algorithm) |
| 203 | [What rules are covered by the diffing algorithm?](#what-are-the-rules-covered-by-diffing-algorithm) |
| 204 | [When do you need to use refs?](#when-do-you-need-to-use-refs) |
| 205 | [Must a prop be named "render" for render props?](#is-it-prop-must-be-named-as-render-for-render-props) |
| 206 | [What are the problems with using render props with Pure Components?](#what-are-the-problems-of-using-render-props-with-pure-components) |
| 207 | [What is the windowing technique?](#what-is-windowing-technique) |
| 208 | [How do you print falsy values in JSX?](#how-do-you-print-falsy-values-in-jsx) |
| 209 | [What is the typical use case for portals?](#what-is-the-typical-use-case-of-portals) |
| 210 | [How do you set a default value for an uncontrolled component?](#how-do-you-set-default-value-for-uncontrolled-component) |
| 211 | [What is your favorite React stack?](#what-is-your-favorite-react-stack) |
| 212 | [What is the difference between the real DOM and the Virtual DOM?](#what-is-the-difference-between-real-dom-and-virtual-dom) |
| 213 | [How do you add Bootstrap to a React application?](#how-to-add-bootstrap-to-a-react-application) |
| 214 | [Can you list the top websites or applications using React as a front-end framework?](#can-you-list-down-top-websites-or-applications-using-react-as-front-end-framework) |
| 215 | [Is it recommended to use the CSS-in-JS technique in React?](#is-it-recommended-to-use-css-in-js-technique-in-react) |
| 216 | [Do you need to rewrite all class components with Hooks?](#do-i-need-to-rewrite-all-my-class-components-with-hooks) |
| 217 | [How do you fetch data with React Hooks?](#how-to-fetch-data-with-react-hooks) |
| 218 | [Do Hooks cover all use cases for classes?](#is-hooks-cover-all-use-cases-for-classes) |
| 219 | [What is the stable release for Hooks support?](#what-is-the-stable-release-for-hooks-support) |
| 220 | [Why do we use array destructuring (square bracket notation) in useState?](#why-do-we-use-array-destructuring-square-brackets-notation-in-usestate) |
| 221 | [What sources were used for introducing Hooks?](#what-are-the-sources-used-for-introducing-hooks) |
| 222 | [How do you access the imperative API of web components?](#how-do-you-access-imperative-api-of-web-components) |
| 223 | [What is Formik?](#what-is-formik) |
| 224 | [What are typical middleware choices for handling asynchronous calls in Redux?](#what-are-typical-middleware-choices-for-handling-asynchronous-calls-in-redux) |
| 225 | [Do browsers understand JSX code?](#do-browsers-understand-jsx-code) |
| 226 | [Can you describe data flow in React?](#describe-about-data-flow-in-react) |
| 227 | [What is MobX?](#what-is-mobx) |
| 228 | [What are the differences between Redux and MobX?](#what-are-the-differences-between-redux-and-mobx) |
| 229 | [Should you learn ES6 before learning ReactJS?](#should-i-learn-es6-before-learning-reactjs) |
| 230 | [What is concurrent rendering?](#what-is-concurrent-rendering) |
| 231 | [What is the difference between async mode and concurrent mode?](#what-is-the-difference-between-async-mode-and-concurrent-mode) |
| 232 | [Can you use JavaScript URLs in React v16.9?](#can-i-use-javascript-urls-in-react169) |
| 233 | [What is the purpose of the ESLint plugin for Hooks?](#what-is-the-purpose-of-eslint-plugin-for-hooks) |
| 234 | [What is the difference between imperative and declarative programming in React?](#what-is-the-difference-between-imperative-and-declarative-in-react) |
| 235 | [What are the benefits of using TypeScript with ReactJS?](#what-are-the-benefits-of-using-typescript-with-reactjs) |
| 236 | [How do you ensure a 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) |
| 237 | [What are the benefits of the new JSX transform?](#what-are-the-benefits-of-new-jsx-transform) |
| 238 | [How is the new JSX transform different from the old transform?](#how-is-the-new-jsx-transform-different-from-old-transform) |
| 239 | [What are React Server Components?](#what-are-react-server-components) |
| 240 | [What is prop drilling?](#what-is-prop-drilling) |
| 241 | [What is the difference between the useState and useRef Hooks?](#what-is-the-difference-between-usestate-and-useref-hook) |
| 242 | [What is a wrapper component?](#what-is-a-wrapper-component) |
| 243 | [What are the differences between the useEffect and useLayoutEffect Hooks?](#what-are-the-differences-between-useeffect-and-uselayouteffect-hooks) |
| 244 | [What are the differences between functional and class components?](#what-are-the-differences-between-functional-and-class-components) |
| 245 | [What is Strict Mode in React?](#what-is-strict-mode-in-react) |
| 246 | [What is the benefit of Strict Mode?](#what-is-the-benefit-of-strict-mode) |
| 247 | [Why does Strict Mode render twice in React?](#why-does-strict-mode-render-twice-in-react) |
| 248 | [What are the rules of JSX?](#what-are-the-rules-of-jsx) |
| 249 | [What is the reason multiple JSX tags must be wrapped?](#what-is-the-reason-behind-multiple-jsx-tags-to-be-wrapped) |
| 250 | [How do you prevent mutating array variables?](#how-do-you-prevent-mutating-array-variables) |
| 251 | [What are capture phase events?](#what-are-capture-phase-events) |
| 252 | [How does React update the screen in an application?](#how-does-react-updates-screen-in-an-application) |
| 253 | [How does React batch multiple state updates?](#how-does-react-batch-multiple-state-updates) |
| 254 | [Is it possible to prevent automatic batching?](#is-it-possible-to-prevent-automatic-batching) |
| 255 | [What is React hydration?](#what-is-react-hydration) |
| 256 | [How do you update objects inside state?](#how-do-you-update-objects-inside-state) |
| 257 | [How do you update nested objects inside state?](#How-do-you-update-nested-objects-inside-state) |
| 258 | [How do you update arrays inside state?](#how-do-you-update-arrays-inside-state) |
| 259 | [How do you use the Immer library for state updates?](#how-do-you-use-immer-library-for-state-updates) |
| 260 | [What are the benefits of preventing direct state mutations?](#what-are-the-benefits-of-preventing-the-direct-state-mutations) |
| 261 | [What are the preferred and non-preferred array operations for updating state?](#what-are-the-preferred-and-non-preferred-array-operations-for-updating-the-state) |
| 262 | [What will happen when defining nested function components?](#what-will-happen-by-defining-nested-function-components) |
| 263 | [Can I use keys for non-list items?](#can-i-use-keys-for-non-list-items) |
| 264 | [What are the guidelines to follow for writing reducers?](#what-are-the-guidelines-to-be-followed-for-writing-reducers) |
| | **Hooks** |
| 265 | [What is useReducer hook? Can you describe its usage?](#what-is-use-reducer-hook-can-you-describe-its-usage) |
| 266 | [How do you compare useState and useReducer?](#how-do-you-compare-use-state-and-use-reducer) |
| 267 | [How does Context work with the useContext hook?](#how-does-context-works-using-usecontext-hook) |
| 268 | [What are the use cases of the useContext hook?](#what-are-the-use-cases-of-usecontext-hook) |
| 269 | [When should you use client and server components?](#when-to-use-client-and-server-components) |
| 270 | [What are the differences between the Page Router and App Router in Next.js?](#what-are-the-differences-between-page-router-and-app-router-in-nextjs) | | |
| |
### Table of Contents
Hide/Show table of contents
| No. | Questions |
| --- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| | **Old Q&A** |
| 1 | [Why should we not update the state directly?](#why-should-we-not-update-the-state-directly) |
| 2 | [What is the purpose of callback function as an argument of setState()?](#what-is-the-purpose-of-callback-function-as-an-argument-of-setstate) |
| 3 | [How to bind methods or event handlers in JSX callbacks?](#how-to-bind-methods-or-event-handlers-in-jsx-callbacks) |
| 4 | [How to pass a parameter to an event handler or callback?](#how-to-pass-a-parameter-to-an-event-handler-or-callback) |
| 5 | [What is the use of refs?](#what-is-the-use-of-refs) |
| 6 | [How to create refs?](#how-to-create-refs) |
| 7 | [What are forward refs?](#what-are-forward-refs) |
| 8 | [Which is preferred option with in callback refs and findDOMNode()?](#which-is-preferred-option-with-in-callback-refs-and-finddomnode) |
| 9 | [Why are String Refs legacy?](#why-are-string-refs-legacy) |
| 10 | [What are the different phases of component lifecycle?](#what-are-the-different-phases-of-component-lifecycle) |
| 11 | [What are the lifecycle methods of React?](#what-are-the-lifecycle-methods-of-react) |
| 12 | [How to create props proxy for HOC component?](#how-to-create-props-proxy-for-hoc-component) |
| 13 | [What is context?](#what-is-context) |
| 14 | [What is the purpose of using super constructor with props argument?](#what-is-the-purpose-of-using-super-constructor-with-props-argument) |
| 15 | [How to set state with a dynamic key name?](#how-to-set-state-with-a-dynamic-key-name) |
| 16 | [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) |
| 17 | [What are error boundaries in React v16](#what-are-error-boundaries-in-react-v16) |
| 18 | [How are error boundaries handled in React v15?](#how-are-error-boundaries-handled-in-react-v15) |
| 19 | [What is the purpose of render method of react-dom?](#what-is-the-purpose-of-render-method-of-react-dom) |
| 20 | [What will happen if you use setState in constructor?](#what-will-happen-if-you-use-setstate-in-constructor) |
| 21 | [Is it good to use setState() in componentWillMount() method?](#is-it-good-to-use-setstate-in-componentwillmount-method) |
| 22 | [What will happen if you use props in initial state?](#what-will-happen-if-you-use-props-in-initial-state) |
| 23 | [How you use decorators in React?](#how-you-use-decorators-in-react) |
| 24 | [What is CRA and its benefits?](#what-is-cra-and-its-benefits) |
| 25 | [What is the lifecycle methods order in mounting?](#what-is-the-lifecycle-methods-order-in-mounting) |
| 26 | [What are the lifecycle methods going to be deprecated in React v16?](#what-are-the-lifecycle-methods-going-to-be-deprecated-in-react-v16) |
| 27 | [What is the purpose of getDerivedStateFromProps() lifecycle method?](#what-is-the-purpose-of-getderivedstatefromprops-lifecycle-method) |
| 28 | [What is the purpose of getSnapshotBeforeUpdate() lifecycle method?](#what-is-the-purpose-of-getsnapshotbeforeupdate-lifecycle-method) |
| 29 | [What is the recommended way for naming components?](#what-is-the-recommended-way-for-naming-components) |
| 30 | [What is the recommended ordering of methods in component class?](#what-is-the-recommended-ordering-of-methods-in-component-class) |
| 31 | [Why we need to pass a function to setState()?](#why-we-need-to-pass-a-function-to-setstate) |
| 32 | [Why is isMounted() an anti-pattern and what is the proper solution?](#why-is-ismounted-an-anti-pattern-and-what-is-the-proper-solution) |
| 33 | [What is the difference between constructor and getInitialState?](#what-is-the-difference-between-constructor-and-getinitialstate) |
| 34 | [Can you force a component to re-render without calling setState?](#can-you-force-a-component-to-re-render-without-calling-setstate) |
| 35 | [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) |
| 36 | [What is the difference between setState and replaceState methods?](#what-is-the-difference-between-setstate-and-replacestate-methods) |
| 37 | [How to listen to state changes?](#how-to-listen-to-state-changes) |
| 38 | [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) |
| 39 | [Is it possible to use React without rendering HTML?](#is-it-possible-to-use-react-without-rendering-html) |
| 40 | [What are the possible ways of updating objects in state?](#what-are-the-possible-ways-of-updating-objects-in-state) |
| 41 | [What are the approaches to include polyfills in your create-react-app?](#what-are-the-approaches-to-include-polyfills-in-your-create-react-app) |
| 42 | [How to use https instead of http in create-react-app?](#how-to-use-https-instead-of-http-in-create-react-app) |
| 43 | [How to avoid using relative path imports in create-react-app?](#how-to-avoid-using-relative-path-imports-in-create-react-app) |
| 44 | [How to update a component every second?](#how-to-update-a-component-every-second) |
| 45 | [Why is a component constructor called only once?](#why-is-a-component-constructor-called-only-once) |
| 46 | [How to define constants in React?](#how-to-define-constants-in-react) |
| 47 | [How to programmatically trigger click event in React?](#how-to-programmatically-trigger-click-event-in-react) |
| 48 | [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) |
| 49 | [What are render props?](#what-are-render-props) |
| 50 | [How to dispatch an action on load?](#how-to-dispatch-an-action-on-load) |
| 51 | [How to use connect from React Redux?](#how-to-use-connect-from-react-redux) |
| 52 | [Whats the purpose of at symbol in the redux connect decorator?](#whats-the-purpose-of-at-symbol-in-the-redux-connect-decorator) |
| 53 | [How to use TypeScript in create-react-app application?](#how-to-use-typescript-in-create-react-app-application) |
| 54 | [Does the statics object work with ES6 classes in React?](#does-the-statics-object-work-with-es6-classes-in-react) |
| 55 | [Why are inline ref callbacks or functions not recommended?](#why-are-inline-ref-callbacks-or-functions-not-recommended) |
| 56 | [What are HOC factory implementations?](#what-are-hoc-factory-implementations) |
| 57 | [How to use class field declarations syntax in React classes?](#how-to-use-class-field-declarations-syntax-in-react-classes) |
| 58 | [Why do you not need error boundaries for event handlers?](#why-do-you-not-need-error-boundaries-for-event-handlers) |
| 59 | [What is the difference between try catch block and error boundaries?](#what-is-the-difference-between-try-catch-block-and-error-boundaries) |
| 60 | [What is the required method to be defined for a class component?](#what-is-the-required-method-to-be-defined-for-a-class-component) |
| 61 | [What are the possible return types of render method?](#what-are-the-possible-return-types-of-render-method) |
| 62 | [What is the main purpose of constructor?](#what-is-the-main-purpose-of-constructor) |
| 63 | [Is it mandatory to define constructor for React component?](#is-it-mandatory-to-define-constructor-for-react-component) |
| 64 | [Why should not call setState in componentWillUnmount?](#why-should-not-call-setstate-in-componentwillunmount) |
| 65 | [What is the purpose of getDerivedStateFromError?](#what-is-the-purpose-of-getderivedstatefromerror) |
| 66 | [What is the methods order when component re-rendered?](#what-is-the-methods-order-when-component-re-rendered) |
| 67 | [What are the methods invoked during error handling?](#what-are-the-methods-invoked-during-error-handling) |
| 68 | [What is the purpose of unmountComponentAtNode method?](#what-is-the-purpose-of-unmountcomponentatnode-method) |
| 69 | [What are the limitations with HOCs?](#what-are-the-limitations-with-hocs) |
| 70 | [How to debug forwardRefs in DevTools?](#how-to-debug-forwardrefs-in-devtools) |
| 71 | [Is it good to use arrow functions in render methods?](#is-it-good-to-use-arrow-functions-in-render-methods) |
| 72 | [How do you say that state updates are merged?](#how-do-you-say-that-state-updates-are-merged) |
| 73 | [How do you pass arguments to an event handler?](#how-do-you-pass-arguments-to-an-event-handler) |
| 74 | [How to prevent component from rendering?](#how-to-prevent-component-from-rendering) |
| 75 | [Give an example on How to use context?](#give-an-example-on-how-to-use-context) |
| 76 | [How do you use contextType?](#how-do-you-use-contexttype) |
| 77 | [What is a consumer?](#what-is-a-consumer) |
| 78 | [How do you solve performance corner cases while using context?](#how-do-you-solve-performance-corner-cases-while-using-context) |
| 79 | [What is the purpose of forward ref in HOCs?](#what-is-the-purpose-of-forward-ref-in-hocs) |
| 80 | [Is it ref argument available for all functions or class components?](#is-it-ref-argument-available-for-all-functions-or-class-components) |
| 81 | [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) |
| 82 | [How to create react class components without ES6?](#how-to-create-react-class-components-without-es6) |
| 83 | [Is it possible to use react without JSX?](#is-it-possible-to-use-react-without-jsx) |
| 84 | [How do you create HOC using render props?](#how-do-you-create-hoc-using-render-props) |
| 85 | [What is react scripts?](#what-is-react-scripts) |
| 86 | [What are the features of create react app?](#what-are-the-features-of-create-react-app) |
| 87 | [What is the purpose of renderToNodeStream method?](#what-is-the-purpose-of-rendertonodestream-method) |
| 88 | [How do you get redux scaffolding using create-react-app?](#how-do-you-get-redux-scaffolding-using-create-react-app) |
| 89 | [What is state mutation and how to prevent it?](#what-is-state-mutation-and-how-to-prevent-it) |
## 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)**
3. ### 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)**
4. ### 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)**
5. ### 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:
```javascript
{
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)**
6. ### 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 one and only one 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)**
7. ### 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.
So the summary to this question is as follows:
**Use Function Components:**
- If you don't need state or lifecycle methods, and your component is purely presentational.
- For simplicity, readability, and modern code practices, especially with the use of React Hooks for state and side effects.
**Use Class Components:**
- If you need to manage state or use lifecycle methods.
- In scenarios where backward compatibility or integration with older code is necessary.
**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.
> **_Note when using react-error-boundary:_** ErrorBoundary is a client component. You can only pass props to it that are serializable or use it in files that have a `"use client";` directive.
```jsx
"use client";
import { ErrorBoundary } from "react-error-boundary";
Something went wrong}>
;
```
**[⬆ Back to Top](#table-of-contents)**
8. ### 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)**
9. ### 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 { useState } from "react";
function User() {
const [message, setMessage] = useState("Welcome to React world");
return (
{message}
);
}
```
Whenever React calls your component or access `useState` hook, it gives you a snapshot of the state for that particular render.
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)**
10. ### 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}
{props.gender}
);
};
const ParentComponent