https://github.com/sudheerj/angular-interview-questions
List of 300 Angular Interview Questions and answers
https://github.com/sudheerj/angular-interview-questions
angular angular-interview-questions angular6 angular7 angularjs interview-questions javascript-framework javascript-interview-questions rxjs typescript
Last synced: about 1 year ago
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List of 300 Angular Interview Questions and answers
- Host: GitHub
- URL: https://github.com/sudheerj/angular-interview-questions
- Owner: sudheerj
- Created: 2018-08-08T14:39:19.000Z (almost 8 years ago)
- Default Branch: master
- Last Pushed: 2025-03-07T01:25:33.000Z (over 1 year ago)
- Last Synced: 2025-04-08T23:13:34.141Z (about 1 year ago)
- Topics: angular, angular-interview-questions, angular6, angular7, angularjs, interview-questions, javascript-framework, javascript-interview-questions, rxjs, typescript
- Homepage:
- Size: 2.08 MB
- Stars: 4,544
- Watchers: 192
- Forks: 2,102
- Open Issues: 9
-
Metadata Files:
- Readme: README.md
- Funding: .github/FUNDING.yml
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README
# Angular Interview Questions & Answers
> Click :star:if you like the project and follow [@SudheerJonna](https://twitter.com/SudheerJonna) for technical updates.
---
- Take this Angular course to go from a complete Angular beginner to confidently building enterprise-level applications from scratch
- Take this coding interview bootcamp if you’re serious about getting hired and don’t have a CS degree
---
### Table of Contents
| No. | Questions |
|---- | ---------
|1 | [What is Angular Framework?](#what-is-angular-framework)|
|2 | [What is the difference between AngularJS and Angular?](#what-is-the-difference-between-angularjs-and-angular)|
|3 | [What is TypeScript?](#what-is-typescript)|
|4 | [Write a pictorial diagram of Angular architecture?](#write-a-pictorial-diagram-of-angular-architecture)|
|5 | [What are the key components of Angular?](#what-are-the-key-components-of-angular)|
|6 | [What are directives?](#what-are-directives)|
|7 | [What are components?](#what-are-components)|
|8 | [What are the differences between Component and Directive?](#what-are-the-differences-between-component-and-directive)|
|9 | [What is a template?](#what-is-a-template)|
|10| [What is a module?](#what-is-a-module)|
|11| [What are lifecycle hooks available?](#what-are-lifecycle-hooks-available)|
|12| [What is a data binding?](#what-is-a-data-binding)|
|13| [What is metadata?](#what-is-metadata)|
|14| [What is Angular CLI?](#what-is-angular-cli)|
|15| [What is the difference between constructor and ngOnInit?](#what-is-the-difference-between-constructor-and-ngoninit)|
|16| [What is a service](#what-is-a-service)|
|17| [What is dependency injection in Angular?](#what-is-dependency-injection-in-angular)|
|18| [How is Dependency Hierarchy formed?](#how-is-dependency-hierarchy-formed)|
|19| [What is the purpose of async pipe?](#what-is-the-purpose-of-async-pipe)|
|20| [What is the option to choose between inline and external template file?](#what-is-the-option-to-choose-between-inline-and-external-template-file)|
|21| [What is the purpose of *ngFor directive?](#what-is-the-purpose-of-ngfor-directive)|
|22| [What is the purpose of ngIf directive?](#what-is-the-purpose-of-ngif-directive)|
|23| [What happens if you use script tag inside template?](#what-happens-if-you-use-script-tag-inside-template)|
|24| [What is interpolation?](#what-is-interpolation)|
|25| [What are template expressions?](#what-are-template-expressions)|
|26| [What are template statements?](#what-are-template-statements)|
|27| [How do you categorize data binding types?](#how-do-you-categorize-data-binding-types)|
|28| [What are pipes?](#what-are-pipes)|
|29| [What is a parameterized pipe?](#what-is-a-parameterized-pipe)|
|30| [How do you chain pipes?](#how-do-you-chain-pipes)|
|31| [What is a custom pipe?](#what-is-a-custom-pipe)|
|32| [Give an example of custom pipe?](#give-an-example-of-custom-pipe)|
|33| [What is the difference between pure and impure pipe?](#what-is-the-difference-between-pure-and-impure-pipe)|
|34| [What is a bootstrapping module?](#what-is-a-bootstrapping-module)|
|35| [What are observables?](#what-are-observables)|
|36| [What is HttpClient and its benefits?](#what-is-httpclient-and-its-benefits)|
|37| [Explain on how to use HttpClient with an example?](#explain-on-how-to-use-httpclient-with-an-example)|
|38| [How can you read full response?](#how-can-you-read-full-response)|
|39| [How do you perform Error handling?](#how-do-you-perform-error-handling)|
|40| [What is RxJS?](#what-is-rxjs)|
|41| [What is subscribing?](#what-is-subscribing)|
|42| [What is an observable?](#what-is-an-observable)|
|43| [What is an observer?](#what-is-an-observer)|
|44| [What is the difference between promise and observable?](#what-is-the-difference-between-promise-and-observable)|
|45| [What is multicasting?](#what-is-multicasting)|
|46| [How do you perform error handling in observables?](#how-do-you-perform-error-handling-in-observables)|
|47| [What is the shorthand notation for subscribe method?](#what-is-the-shorthand-notation-for-subscribe-method)|
|48| [What are the utility functions provided by RxJS?](#what-are-the-utility-functions-provided-by-rxjs)|
|49| [What are observable creation functions?](#what-are-observable-creation-functions)|
|50| [What will happen if you do not supply handler for the observer?](#what-will-happen-if-you-do-not-supply-handler-for-the-observer)|
|51| [What are Angular elements?](#what-are-angular-elements)|
|52| [What is the browser support of Angular Elements?](#what-is-the-browser-support-of-angular-elements)|
|53| [What are custom elements?](#what-are-custom-elements)|
|54| [Do I need to bootstrap custom elements?](#do-i-need-to-bootstrap-custom-elements)|
|55| [Explain how custom elements works internally?](#explain-how-custom-elements-works-internally)|
|56| [How to transfer components to custom elements?](#how-to-transfer-components-to-custom-elements)|
|57| [What are the mapping rules between Angular component and custom element?](#what-are-the-mapping-rules-between-angular-component-and-custom-element)|
|58| [How do you define typings for custom elements?](#how-do-you-define-typings-for-custom-elements)|
|59| [What are dynamic components?](#what-are-dynamic-components)|
|60| [What are the various kinds of directives?](#what-are-the-various-kinds-of-directives)|
|61| [How do you create directives using CLI?](#how-do-you-create-directives-using-cli)|
|62| [Give an example for attribute directives?](#give-an-example-for-attribute-directives)|
|63| [What is Angular Router?](#what-is-angular-router)|
|64| [What is the purpose of base href tag?](#what-is-the-purpose-of-base-href-tag)|
|65| [What are the router imports?](#what-are-the-router-imports)|
|66| [What is router outlet?](#what-is-router-outlet)|
|67| [What are router links?](#what-are-router-links)|
|68| [What are active router links?](#what-are-active-router-links)|
|69| [What is router state?](#what-is-router-state)|
|70| [What are router events?](#what-are-router-events)|
|71| [What is activated route?](#what-is-activated-route)|
|72| [How do you define routes?](#how-do-you-define-routes)|
|73| [What is the purpose of Wildcard route?](#what-is-the-purpose-of-wildcard-route)|
|74| [Do I need a Routing Module always?](#do-i-need-a-routing-module-always)|
|75| [What is Angular Universal?](#what-is-angular-universal)|
|76| [What are different types of compilation in Angular?](#what-are-different-types-of-compilation-in-angular)|
|77| [What is JIT?](#what-is-jit)|
|78| [What is AOT?](#what-is-aot)|
|79| [Why do we need compilation process?](#why-do-we-need-compilation-process)|
|80| [What are the advantages with AOT?](#what-are-the-advantages-with-aot)|
|81| [What are the ways to control AOT compilation?](#what-are-the-ways-to-control-aot-compilation)|
|82| [What are the restrictions of metadata?](#what-are-the-restrictions-of-metadata)|
|83| [What are the three phases of AOT?](#what-are-the-three-phases-of-aot)|
|84| [Can I use arrow functions in AOT?](#can-i-use-arrow-functions-in-aot)|
|85| [What is the purpose of metadata json files?](#what-is-the-purpose-of-metadata-json-files)|
|86| [Can I use any javascript feature for expression syntax in AOT?](#can-i-use-any-javascript-feature-for-expression-syntax-in-aot)|
|87| [What is folding?](#what-is-folding)|
|88| [What are macros?](#what-are-macros)|
|89| [Give an example of few metadata errors?](#give-an-example-of-few-metadata-errors)|
|90| [What is metadata rewriting?](#what-is-metadata-rewriting)|
|91| [How do you provide configuration inheritance?](#how-do-you-provide-configuration-inheritance)|
|92| [How do you specify angular template compiler options?](#how-do-you-specify-angular-template-compiler-options)|
|93| [How do you enable binding expression validation?](#how-do-you-enable-binding-expression-validation)|
|94| [What is the purpose of any type cast function?](#what-is-the-purpose-of-any-type-cast-function)|
|95| [What is Non null type assertion operator?](#what-is-non-null-type-assertion-operator)|
|96| [What is type narrowing?](#what-is-type-narrowing)|
|97| [How do you describe various dependencies in angular application?](#how-do-you-describe-various-dependencies-in-angular-application)|
|98| [What is zone?](#what-is-zone)|
|99| [What is the purpose of common module?](#what-is-the-purpose-of-common-module)|
|100| [What is codelyzer?](#what-is-codelyzer)|
|101| [What is angular animation?](#what-is-angular-animation)|
|102| [What are the steps to use animation module?](#what-are-the-steps-to-use-animation-module)|
|103| [What is State function?](#what-is-state-function)|
|104| [What is Style function?](#what-is-style-function)|
|105| [What is the purpose of animate function?](#what-is-the-purpose-of-animate-function)|
|106| [What is transition function?](#what-is-transition-function)|
|107| [How to inject the dynamic script in angular?](#how-to-inject-the-dynamic-script-in-angular)|
|108| [What is a service worker and its role in Angular?](#what-is-a-service-worker-and-its-role-in-angular)|
|109| [What are the design goals of service workers?](#what-are-the-design-goals-of-service-workers)|
|110| [What are the differences between AngularJS and Angular with respect to dependency injection?](#what-are-the-differences-between-angularjs-and-angular-with-respect-to-dependency-injection)|
|111| [What is Angular Ivy?](#what-is-angular-ivy)|
|112| [What are the features included in ivy preview?](#what-are-the-features-included-in-ivy-preview)|
|113| [Can I use AOT compilation with Ivy?](#can-i-use-aot-compilation-with-ivy)|
|114| [What is Angular Language Service?](#what-is-angular-language-service)|
|115| [How do you install angular language service in the project?](#how-do-you-install-angular-language-service-in-the-project)|
|116| [Is there any editor support for Angular Language Service?](#is-there-any-editor-support-for-angular-language-service)|
|117| [Explain the features provided by Angular Language Service?](#explain-the-features-provided-by-angular-language-service)|
|118| [How do you add web workers in your application?](#how-do-you-add-web-workers-in-your-application)|
|119| [What are the limitations with web workers?](#what-are-the-limitations-with-web-workers)|
|120| [What is Angular CLI Builder?](#what-is-angular-cli-builder)|
|121| [What is a builder?](#what-is-a-builder)|
|122| [How do you invoke a builder?](#how-do-you-invoke-a-builder)|
|123| [How do you create app shell in Angular?](#how-do-you-create-app-shell-in-angular)|
|124| [What are the case types in Angular?](#what-are-the-case-types-in-angular)|
|125| [What are the class decorators in Angular?](#what-are-the-class-decorators-in-angular)|
|126| [What are class field decorators?](#what-are-class-field-decorators)|
|127| [What is declarable in Angular?](#what-is-declarable-in-angular)|
|128| [What are the restrictions on declarable classes?](#what-are-the-restrictions-on-declarable-classes)|
|129| [What is a DI token?](#what-is-a-di-token)|
|130| [What is Angular DSL?](#what-is-angular-dsl)|
|131| [What is an rxjs Subject?](#what-is-an-rxjs-Subject)|
|132| [What is Bazel tool?](#what-is-bazel-tool)|
|133| [What are the advantages of Bazel tool?](#what-are-the-advantages-of-bazel-tool)|
|134| [How do you use Bazel with Angular CLI?](#how-do-you-use-bazel-with-angular-cli)|
|135| [How do you run Bazel directly?](#how-do-you-run-bazel-directly)|
|136| [What is platform in Angular?](#what-is-platform-in-angular)|
|137| [What happens if I import the same module twice?](#what-happens-if-i-import-the-same-module-twice)|
|138| [How do you select an element with in a component template?](#how-do-you-select-an-element-with-in-a-component-template)|
|139| [How do you detect route change in Angular?](#how-do-you-detect-route-change-in-angular)|
|140| [How do you pass headers for HTTP client?](#how-do-you-pass-headers-for-http-client)|
|141| [What is the purpose of differential loading in CLI?](#what-is-the-purpose-of-differential-loading-in-cli)|
|142| [Is Angular supports dynamic imports?](#is-angular-supports-dynamic-imports)|
|143| [What is lazy loading?](#what-is-lazy-loading)|
|144| [What are workspace APIs?](#what-are-workspace-apis)|
|145| [How do you upgrade angular version?](#how-do-you-upgrade-angular-version)|
|146| [What is Angular Material?](#what-is-angular-material)|
|147| [How do you upgrade location service of angularjs?](#how-do-you-upgrade-location-service-of-angularjs)|
|148| [What is NgUpgrade?](#what-is-ngupgrade)|
|149| [How do you test Angular application using CLI?](#how-do-you-test-angular-application-using-cli)|
|150| [How to use polyfills in Angular application?](#how-to-use-polyfills-in-angular-application)|
|151| [What are the ways to trigger change detection in Angular?](#what-are-the-ways-to-trigger-change-detection-in-angular)|
|152| [What are the differences of various versions of Angular?](#what-are-the-differences-of-various-versions-of-angular)|
|153| [What are the security principles in angular?](#what-are-the-security-principles-in-angular)|
|154| [What is the reason to deprecate Web Tracing Framework?](#what-is-the-reason-to-deprecate-web-tracing-framework)|
|155| [What is the reason to deprecate web worker packages?](#what-is-the-reason-to-deprecate-web-worker-packages)|
|156| [How do you find angular CLI version?](#how-do-you-find-angular-cli-version)|
|157| [What is the browser support for Angular?](#what-is-the-browser-support-for-angular)|
|158| [What is schematic](#what-is-schematic)|
|159| [What is rule in Schematics?](#what-is-rule-in-schematics)|
|160| [What is Schematics CLI?](#what-is-schematics-cli)|
|161| [What are the best practices for security in angular?](#what-are-the-best-practices-for-security-in-angular)|
|162| [What is Angular security model for preventing XSS attacks?](#what-is-angular-security-model-for-preventing-xss-attacks)|
|163| [What is the role of template compiler for prevention of XSS attacks?](#what-is-the-role-of-template-compiler-for-prevention-of-xss-attacks)|
|164| [What are the various security contexts in Angular?](#what-are-the-various-security-contexts-in-Angular)|
|165| [What is Sanitization? Is angular supports it?](#what-is-sanitization?Is-angular-supports-it)|
|166| [What is the purpose of innerHTML?](#what-is-the-purpose-of-innerhtml)|
|167| [What is the difference between interpolated content and innerHTML?](#what-is-the-difference-between-interpolated-content-and-innerhtml)|
|168| [How do you prevent automatic sanitization?](#how-do-you-prevent-automatic-sanitization)|
|169| [Is safe to use direct DOM API methods in terms of security?](#is-safe-to-use-direct-dom-api-methods-in-terms-of-security)|
|170| [What is DOM sanitizer?](#what-is-dom-sanitizer)|
|171| [How do you support server side XSS protection in Angular application?](#how-do-you-support-server-side-xss-protection-in-angular-application)
|172| [Is angular prevents http level vulnerabilities?](#is-angular-prevents-http-level-vulnerabilities)|
|173| [What are Http Interceptors?](#what-are-http-interceptors)|
|174| [What are the applications of HTTP interceptors?](#what-are-the-applications-of-http-interceptors)|
|175| [Is multiple interceptors supported in Angular?](#is-multiple-interceptors-supported-in-angular)|
|176| [How can I use interceptor for an entire application?](#how-can-i-use-interceptor-for-an-entire-application)|
|177| [How does Angular simplifies Internationalization?](#how-does-angular-simplifies-internationalization)|
|178| [How do you manually register locale data?](#how-do-you-manually-register-locale-data)|
|179| [What are the four phases of template translation?](#what-are-the-four-phases-of-template-translation)|
|180| [What is the purpose of i18n attribute?](#what-is-the-purpose-of-i18n-attribute)|
|181| [What is the purpose of custom id?](#what-is-the-purpose-of-custom-id)|
|182| [What happens if the custom id is not unique?](#what-happens-if-the-custom-id-is-not-unique)|
|183| [Can I translate text without creating an element?](#can-i-translate-text-without-creating-an-element)|
|184| [How can I translate attribute?](#how-can-i-translate-attribute)|
|185| [List down the pluralization categories?](#list-down-the-pluralization-categories)|
|186| [What is select ICU expression?](#what-is-select-icu-expression)|
|187| [How do you report missing translations?](#how-do-you-report-missing-translations)|
|188| [How do you provide build configuration for multiple locales?](#how-do-you-provide-build-configuration-for-multiple-locales)|
|189| [What is an angular library?](#what-is-an-angular-library)|
|190| [What is AOT compiler?](#what-is-aot-compiler)|
|191| [How do you select an element in component template?](#how-do-you-select-an-element-in-component-template)|
|192| [What is TestBed?](#what-is-testbed)|
|193| [What is protractor?](#what-is-protractor)|
|194| [What is collection?](#what-is-collection)|
|195| [How do you create schematics for libraries?](#how-do-you-create-schematics-for-libraries)|
|196| [How do you use jquery in Angular?](#how-do-you-use-jquery-in-angular)|
|197| [What is the reason for No provider for HTTP exception?](#what-is-the-reason-for-no-provider-for-http-exception)|
|198| [What is router state?](#what-is-router-state)|
|199| [How can I use SASS in angular project?](#how-can-i-use-sass-in-angular-project)|
|200| [What is the purpose of hidden property?](#what-is-the-purpose-of-hidden-property)|
|201| [What is the difference between ngIf and hidden property?](#what-is-the-difference-between-ngif-and-hidden-property)|
|202| [What is slice pipe?](#what-is-slice-pipe)|
|203| [What is index property in ngFor directive?](#what-is-index-property-in-ngfor-directive)|
|204| [What is the purpose of ngFor trackBy?](#what-is-the-purpose-of-ngfor-trackby)|
|205| [What is the purpose of ngSwitch directive?](#what-is-the-purpose-of-ngswitch-directive)|
|206| [Is it possible to do aliasing for inputs and outputs?](#is-it-possible-to-do-aliasing-for-inputs-and-outputs)|
|207| [What is safe navigation operator?](#what-is-safe-navigation-operator)|
|208| [Is any special configuration required for Angular9?](#is-any-special-configuration-required-for-angular9)|
|209| [What are type safe TestBed API changes in Angular9?](#what-are-type-safe-testbed-api-changes-in-angular9)|
|210| [Is mandatory to pass static flag for ViewChild?](#is-mandatory-to-pass-static-flag-for-viewchild)|
|211| [What are the list of template expression operators?](#what-are-the-list-of-template-expression-operators)
|212| [What is the precedence between pipe and ternary operators?](#what-is-the-precedence-between-pipe-and-ternary-operators)
|213| [What is an entry component?](#what-is-an-entry-component)|
|214| [What is a bootstrapped component?](#what-is-a-bootstrapped-component)|
|215| [How do you manually bootstrap an application?](#how-do-you-manually-bootstrap-an-application)|
|216| [Is it necessary for bootstrapped component to be entry component?](#is-it-necessary-for-bootstrapped-component-to-be-entry-component)|
|217| [What is a routed entry component?](#what-is-a-routed-entry-component#)|
|218| [Why is not necessary to use entryComponents array every time?](#why-is-not-necessary-to-use-entrycomponents-array-every-time)|
|219| [Do I still need to use entryComponents array in Angular9?](#do-i-still-need-to-use-entrycomponents-array-in-angular9#)|
|220| [Is it all components generated in production build?](#is-it-all-components-generated-in-production-build)|
|221| [What is Angular compiler?](#what-is-angular-compiler)|
|222| [What is the role of ngModule metadata in compilation process?](#what-is-the-role-of-ngmodule-metadata-in-compilation-process)|
|223| [How does angular finds components, directives and pipes?](#how-does-angular-finds-components-directives-and-pipes)|
|224| [Give few examples for NgModules?](#give-few-examples-for-ngmodules)|
|225| [What are feature modules?](#what-are-feature-modules)|
|226| [What are the imported modules in CLI generated feature modules?](#what-are-the-imported-modules-in-cli-generated-feature-modules)|
|227| [What are the differences between ngmodule and javascript module?](#what-are-the-differences-between-ngmodule-and-javascript-module)|
|228| [What are the possible errors with declarations?](#what-are-the-possible-errors-with-declarations)|
|229| [What are the steps to use declaration elements?](#what-are-the-steps-to-use-declaration-elements)|
|230| [What happens if browserModule used in feature module?](#what-happens-if-browsermodule-used-in-feature-module)|
|231| [What are the types of feature modules?](#what-are-the-types-of-feature-modules)|
|232| [What is a provider?](#what-is-a-provider)|
|233| [What is the recommendation for provider scope?](#what-is-the-recommendation-for-provider-scope#)|
|234| [How do you restrict provider scope to a module?](#how-do-you-restrict-provider-scope-to-a-module)|
|235| [How do you provide a singleton service?](#how-do-you-provide-a-singleton-service)|
|236| [What are the different ways to remove duplicate service registration?](#what-are-the-different-ways-to-remove-duplicate-service-registration)|
|237| [How does forRoot method helpful to avoid duplicate router instances?](#how-does-forroot-method-helpful-to-avoid-duplicate-router-instances)|
|238| [What is a shared module?](#what-is-a-shared-module)|
|239| [Can I share services using modules?](#can-i-share-services-using-modules)|
|240| [How do you get current direction for locales??](#how-do-you-get-current-direction-for-locales)|
|241| [What is ngcc?](#what-is-ngcc)|
|242| [What classes should not be added to declarations?](#what-classes-should-not-be-added-to-declarations)|
|243| [What is ngzone?](#what-is-ngzone)|
|244| [What is NoopZone?](#what-is-noopzone)|
|245| [How do you create displayBlock components?](#how-do-you-create-displayblock-components)|
|246| [What are the possible data change scenarios for change detection?](#what-are-the-possible-data-change-scenarios-for-change-detection)|
|247| [What is a zone context?](#what-is-a-zone-context)|
|248| [What are the lifecycle hooks of a zone?](#what-are-the-lifecycle-hooks-of-a-zone)|
|249| [Which are the methods of NgZone used to control change detection?](#which-are-the-methods-of-ngzone-used-to-control-change-detection)|
|250| [How do you change the settings of zonejs?](#how-do-you-change-the-settings-of-zonejs)|
|251| [How do you trigger an animation?](#how-do-you-trigger-an-animation)|
|252| [How do you configure injectors with providers at different levels?](#how-do-you-configure-injectors-with-providers-at-different-levels)|
|253| [Is it mandatory to use injectable on every service class?](#is-it-mandatory-to-use-injectable-on-every-service-class)|
|254| [What is an optional dependency?](#what-is-an-optional-dependency)|
|255| [What are the types of injector hierarchies?](#what-are-the-types-of-injector-hierarchies)|
|256| [What are reactive forms?](#what-are-reactive-forms)|
|257| [What are dynamic forms?](#what-are-dynamic-forms)|
|258| [What are template driven forms?](#what-are-template-driven-forms)|
|259| [What are the differences between reactive forms and template driven forms?](#what-are-the-differences-between-reactive-forms-and-template-driven-forms)|
|260| [What are the different ways to group form controls?](#what-are-the-different-ways-to-group-form-controls)|
|261| [How do you update specific properties of a form model?](#how-do-you-update-specific-properties-of-a-form-model)|
|262| [What is the purpose of FormBuilder?](#what-is-the-purpose-of-formbuilder)|
|263| [How do you verify the model changes in forms?](#how-do-you-verify-the-model-changes-in-forms)|
|264| [What are the state CSS classes provided by ngModel?](#what-are-the-state-css-classes-provided-by-ngmodel)|
|265| [How do you reset the form?](#how-do-you-reset-the-form)|
|266| [What are the types of validator functions?](#what-are-the-types-of-validator-functions)|
|267| [Can you give an example of built-in validators?](#can-you-give-an-example-of-built-in-validators)|
|268| [How do you optimize the performance of async validators?](#how-do-you-optimize-the-performance-of-async-validators)|
|269| [How to set ngFor and ngIf on the same element?](#how-to-set-ngfor-and-ngif-on-the-same-element)|
|270| [What is host property in css?](#what-is-host-property-in-css)|
|271| [How do you get the current route?](#how-do-you-get-the-current-route)|
|272| [What is Component Test Harnesses?](#what-is-component-test-harnesses)|
|273| [What is the benefit of Automatic Inlining of Fonts?](#what-is-the-benefit-of-automatic-inlining-of-fonts)|
|274| [What is content projection?](#what-is-content-projection)|
|275| [What is ng-content and its purpose?](#what-is-ng-content-and-its-purpose)|
|276| [What is standalone component?](#what-is-standalone-component)|
|277| [How to create a standalone component uing CLI command?](#how-to-create-a-standalone-component-uing-cli-command)
|278| [How to create a standalone component manually?](#how-to-create-a-standalone-component-manually)
|279| [What is hydration ?](#what-is-hydration)
|280| [What are Angular Signals?](#what-are-angular-signals)
|281| [Explain Angular Signals with an example](#explain-angular-signals-with-an-example)
|282| [What are the Route Parameters? Could you explain each of them?](#what-are-the-route-parameters-could-you-explain-each-of-them)
|283| [](#)
1. ### What is Angular Framework?
Angular is a **TypeScript-based open-source** front-end platform that makes it easy to build web, mobile and desktop applications. The major features of this framework include declarative templates, dependency injection, end to end tooling which ease application development.
**[⬆ Back to Top](#table-of-contents)**
2. ### What is the difference between AngularJS and Angular?
AngularJS (version 1.x) is a JavaScript framework, whereas Angular (version 2+) is a complete rewrite of AngularJS using TypeScript, providing better performance, mobile support, modularity, and a more modern architecture.
Here are some of the major differences in tabular format:-
| AngularJS | Angular |
|---- | ---------
| It is based on MVC architecture| This is based on Service/Controller|
| It uses JavaScript to build the application| Uses TypeScript to build the application|
| Based on controllers concept| This is a component based UI approach|
| No support for mobile platforms| Fully supports mobile platforms|
| Difficult to build SEO friendly application| Ease to build SEO friendly applications|
**[⬆ Back to Top](#table-of-contents)**
3. ### What is TypeScript?
TypeScript is a strongly typed superset of JavaScript created by Microsoft that adds optional types, classes, async/await and many other features, and compiles to plain JavaScript. Angular is written entirely in TypeScript as a primary language.
You can install TypeScript globally as
```cmd
npm install -g typescript
```
Let's see a simple example of TypeScript usage:-
```typescript
function greeter(person: string) {
return "Hello, " + person;
}
let user = "Sudheer";
document.body.innerHTML = greeter(user);
```
The greeter method allows only string type as argument.
**[⬆ Back to Top](#table-of-contents)**
4. ### Write a pictorial diagram of Angular architecture?
The main building blocks of an Angular application are shown in the diagram below:-
**[⬆ Back to Top](#table-of-contents)**
5. ### What are the key components of Angular?
Angular has the key components below,
1. **Component:** These are the basic building blocks of an Angular application to control HTML views.
2. **Modules:** An Angular module is a set of angular basic building blocks like components, directives, services etc. An application is divided into logical pieces and each piece of code is called as "module" which perform a single task.
3. **Templates:** These represent the views of an Angular application.
4. **Services:** Are used to create components which can be shared across the entire application.
5. **Metadata:** This can be used to add more data to an Angular class.
**[⬆ Back to Top](#table-of-contents)**
6. ### What are directives?
Directives add behaviour to an existing DOM element or an existing component instance.
```typescript
import { Directive, ElementRef, Input } from '@angular/core';
@Directive({ selector: '[myHighlight]' })
export class HighlightDirective {
constructor(el: ElementRef) {
el.nativeElement.style.backgroundColor = 'yellow';
}
}
```
Now this directive extends HTML element behavior with a yellow background as below
```html
Highlight me!
```
**[⬆ Back to Top](#table-of-contents)**
7. ### What are components?
Components are the most basic UI building block of an Angular app, which form a tree of Angular components. These components are a subset of directives. Unlike directives, components always have a template, and only one component can be instantiated per element in a template.
Let's see a simple example of Angular component
```typescript
import { Component } from '@angular/core';
@Component ({
selector: 'my-app',
template: `
{{title}}
Learn Angular6 with examples
`,
})
export class AppComponent {
title: string = 'Welcome to Angular world';
}
```
**[⬆ Back to Top](#table-of-contents)**
8. ### What are the differences between Component and Directive?
In a short note, A component(@component) is a directive-with-a-template.
Some of the major differences are mentioned in a tabular form
| Component | Directive |
|---- | ---------
| To register a component we use @Component meta-data annotation | To register a directive we use @Directive meta-data annotation |
| Components are typically used to create UI widgets| Directives are used to add behavior to an existing DOM element |
| Component is used to break down the application into smaller components| Directive is used to design re-usable components|
| Only one component can be present per DOM element | Many directives can be used per DOM element |
| @View decorator or templateurl/template are mandatory | Directive doesn't use View|
**[⬆ Back to Top](#table-of-contents)**
9. ### What is a template?
A template is a HTML view where you can display data by binding controls to properties of an Angular component. You can store your component's template in one of two places. You can define it inline using the template property, or you can define the template in a separate HTML file and link to it in the component metadata using the @Component decorator's templateUrl property.
**Using inline template with template syntax,**
```typescript
import { Component } from '@angular/core';
@Component ({
selector: 'my-app',
template: '
{{title}}
Learn Angular
'
})
export class AppComponent {
title: string = 'Hello World';
}
```
**Using separate template file such as app.component.html**
```typescript
import { Component } from '@angular/core';
@Component ({
selector: 'my-app',
templateUrl: 'app/app.component.html'
})
export class AppComponent {
title: string = 'Hello World';
}
```
**[⬆ Back to Top](#table-of-contents)**
10. ### What is a module?
Modules are logical boundaries in your application and the application is divided into separate modules to separate the functionality of your application.
Lets take an example of **app.module.ts** root module declared with **@NgModule** decorator as below,
```typescript
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { AppComponent } from './app.component';
@NgModule ({
imports: [ BrowserModule ],
declarations: [ AppComponent ],
bootstrap: [ AppComponent ],
providers: []
})
export class AppModule { }
```
The NgModule decorator has five important (among all) options:
1. The imports option is used to import other dependent modules. The BrowserModule is required by default for any web based angular application.
2. The declarations option is used to define components in the respective module.
3. The bootstrap option tells Angular which Component to bootstrap in the application.
4. The providers option is used to configure a set of injectable objects that are available in the injector of this module.
5. The entryComponents option is a set of components dynamically loaded into the view.
**[⬆ Back to Top](#table-of-contents)**
11. ### What are lifecycle hooks available?
Angular application goes through an entire set of processes or has a lifecycle right from its initiation to the end of the application.
The representation of lifecycle in pictorial representation as follows,
The description of each lifecycle method is as below,
1. **ngOnChanges:** When the value of a data bound property changes, then this method is called.
2. **ngOnInit:** This is called whenever the initialization of the directive/component after Angular first displays the data-bound properties happens.
3. **ngDoCheck:** This is for the detection and to act on changes that Angular can't or won't detect on its own.
4. **ngAfterContentInit:** This is called in response after Angular projects external content into the component's view.
5. **ngAfterContentChecked:** This is called in response after Angular checks the content projected into the component.
6. **ngAfterViewInit:** This is called in response after Angular initializes the component's views and child views.
7. **ngAfterViewChecked:** This is called in response after Angular checks the component's views and child views.
8. **ngOnDestroy:** This is the cleanup phase just before Angular destroys the directive/component.
**[⬆ Back to Top](#table-of-contents)**
12. ### What is a data binding?
Data binding is a core concept in Angular and allows to define communication between a component and the DOM, making it very easy to define interactive applications without worrying about pushing and pulling data. There are four forms of data binding(divided as 3 categories) which differ in the way the data is flowing.
1. **From the Component to the DOM:**
**Interpolation:** {{ value }}: Adds the value of a property from the component
```html
```
**Property binding:** [property]=”value”: The value is passed from the component to the specified property or simple HTML attribute
```html
```
2. **From the DOM to the Component:**
**Event binding: (event)=”function”:** When a specific DOM event happens (eg.: click, change, keyup), call the specified method in the component
```html
```
3. **Two-way binding:**
**Two-way data binding:** [(ngModel)]=”value”: Two-way data binding allows to have the data flow both ways. For example, in the below code snippet, both the email DOM input and component email property are in sync
```html
```
**[⬆ Back to Top](#table-of-contents)**
13. ### What is metadata?
Metadata is used to decorate a class so that it can configure the expected behavior of the class. The metadata is represented by decorators
1. **Class decorators**, e.g. @Component and @NgModule
```typescript
import { NgModule, Component } from '@angular/core';
@Component({
selector: 'my-component',
template: '
})
export class MyComponent {
constructor() {
console.log('Hey I am a component!');
}
}
@NgModule({
imports: [],
declarations: [],
})
export class MyModule {
constructor() {
console.log('Hey I am a module!');
}
}
```
2. **Property decorators** Used for properties inside classes, e.g. @Input and @Output
```typescript
import { Component, Input } from '@angular/core';
@Component({
selector: 'my-component',
template: '
})
export class MyComponent {
@Input()
title: string;
}
```
3. **Method decorators** Used for methods inside classes, e.g. @HostListener
```typescript
import { Component, HostListener } from '@angular/core';
@Component({
selector: 'my-component',
template: '
})
export class MyComponent {
@HostListener('click', ['$event'])
onHostClick(event: Event) {
// clicked, `event` available
}
}
```
4. **Parameter decorators** Used for parameters inside class constructors, e.g. @Inject, @Optional
```typescript
import { Component, Inject } from '@angular/core';
import { MyService } from './my-service';
@Component({
selector: 'my-component',
template: '
})
export class MyComponent {
constructor(@Inject(MyService) myService) {
console.log(myService); // MyService
}
}
// Angular v18 and above
import { Component, inject } from '@angular/core';
import { MyService } from './my-service';
@Component({
selector: 'my-component',
template: '
})
export class MyComponent {
myService: MyService = inject(MyService)
constructor() {
console.log(myService); // MyService
}
}
```
**[⬆ Back to Top](#table-of-contents)**
14. ### What is angular CLI?
Angular CLI(**Command Line Interface**) is a command line interface to scaffold and build angular apps using nodejs style (commonJs) modules.
You need to install using below npm command,
```
npm install @angular/cli@latest
```
Below are the list of few commands, which will come handy while creating angular projects
1. **Creating New Project:** ng new
2. **Generating Components, Directives & Services:** ng generate/g
The different types of commands would be,
* ng generate class my-new-class: add a class to your application
* ng generate component my-new-component: add a component to your application
* ng generate directive my-new-directive: add a directive to your application
* ng generate enum my-new-enum: add an enum to your application
* ng generate module my-new-module: add a module to your application
* ng generate pipe my-new-pipe: add a pipe to your application
* ng generate service my-new-service: add a service to your application
3. **Running the Project:** ng serve
**[⬆ Back to Top](#table-of-contents)**
15. ### What is the difference between constructor and ngOnInit?
The **Constructor** is a default method of the class that is executed when the class is instantiated and ensures proper initialisation of fields in the class and its subclasses. Angular, or better Dependency Injector (DI), analyses the constructor parameters and when it creates a new instance by calling new MyClass() it tries to find providers that match the types of the constructor parameters, resolves them and passes them to the constructor.
**ngOnInit** is a life cycle hook called by Angular to indicate that Angular is done creating the component.
Mostly we use ngOnInit for all the initialization/declaration and avoid stuff to work in the constructor. The constructor should only be used to initialize class members but shouldn't do actual "work".
So you should use constructor() to setup Dependency Injection and not much else. ngOnInit() is better place to "start" - it's where/when components' bindings are resolved.
```typescript
export class App implements OnInit{
constructor(private myService: MyService){
//called first time before the ngOnInit()
}
ngOnInit(){
//called after the constructor and called after the first ngOnChanges()
//e.g. http call...
}
}
```
**[⬆ Back to Top](#table-of-contents)**
16. ### What is a service?
A service is used when a common functionality needs to be provided to various modules. Services allow for greater separation of concerns for your application and better modularity by allowing you to extract common functionality out of components.
Let's create a repoService which can be used across components,
```typescript
import { Injectable } from '@angular/core';
import { Http } from '@angular/http';
@Injectable({ // The Injectable decorator is required for dependency injection to work
// providedIn option registers the service with a specific NgModule
providedIn: 'root', // This declares the service with the root app (AppModule)
})
export class RepoService{
constructor(private http: Http){
}
fetchAll(){
return this.http.get('https://api.github.com/repositories');
}
}
```
The above service uses Http service as a dependency.
**[⬆ Back to Top](#table-of-contents)**
17. ### What is dependency injection in Angular?
Dependency injection (DI), is an important application design pattern in which a class asks for dependencies from external sources rather than creating them itself. Angular comes with its own dependency injection framework for resolving dependencies( services or objects that a class needs to perform its function).So you can have your services depend on other services throughout your application.
**[⬆ Back to Top](#table-of-contents)**
18. ### How is Dependency Hierarchy formed?
Injectors in Angular have rules that can be leveraged to achieve the desired visibility of injectables in your applications. By understanding these rules, you can determine in which NgModule, Component, or Directive you should declare a provider.
#### Angular has two injector hierarchies:
#### Module injector
When angular starts, it creates a root injector where the services will be registered, these are provided via injectable annotation. All services provided in the `ng-model` property are called providers (if those modules are not lazy-loaded).
Angular recursively goes through all models which are being used in the application and creates instances for provided services in the root injector. If you provide some service in an eagerly-loaded model, the service will be added to the root injector, which makes it available across the whole application.
#### Platform Module
During application bootstrapping angular creates a few more injectors, above the root injector goes the platform injector, this one is created by the platform browser dynamic function inside the `main.ts` file, and it provides some platform-specific features like `DomSanitizer`.
#### NullInjector()
At the very top, the next parent injector in the hierarchy is the `NullInjector()`.The responsibility of this injector is to throw the error if something tries to find dependencies there, unless you've used `@Optional()` because ultimately, everything ends at the `NullInjector()` and it returns an error or, in the case of `@Optional()`, `null`.
#### ElementInjector
Angular creates `ElementInjector` hierarchies implicitly for each DOM element. `ElementInjector` injector is being created for any tag that matches the angular component, or any tag on which directive is applied, and you can configure it in component and directive annotations inside the provider's property, thus, it creates its own hierarchy likewise the upper one.
**[⬆ Back to Top](#table-of-contents)**
19. ### What is the purpose of async pipe?
The AsyncPipe subscribes to an observable or promise and returns the latest value it has emitted. When a new value is emitted, the pipe marks the component to be checked for changes.
Let's take a time observable which continuously updates the view for every 2 seconds with the current time.
```typescript
@Component({
selector: 'async-observable-pipe',
template: `
observable|async:
Time: {{ time | async }}
})
export class AsyncObservablePipeComponent {
time: Observable;
constructor() {
this.time = new Observable((observer) => {
setInterval(() => {
observer.next(new Date().toString());
}, 2000);
});
}
}
```
**[⬆ Back to Top](#table-of-contents)**
20. ### What is the option to choose between inline and external template file?
You can store your component's template in one of two places. You can define it inline using the **template** property, or you can define the template in a separate HTML file and link to it in the component metadata using the **@Component** decorator's **templateUrl** property.
The choice between inline and separate HTML is a matter of taste, circumstances, and organization policy. But normally we use inline template for small portion of code and external template file for bigger views. By default, the Angular CLI generates components with a template file. But you can override that with the below command,
```
ng generate component hero -it
```
**[⬆ Back to Top](#table-of-contents)**
21. ### What is the purpose of `*ngFor` directive?
We use Angular `*ngFor` directive in the template to display each item in the list. For example, here we can iterate over a list of users:
```html
{{ user }}
```
The user variable in the `*ngFor` double-quoted instruction is a **template input variable**.
**[⬆ Back to Top](#table-of-contents)**
22. ### What is the purpose of `*ngIf` directive?
Sometimes an app needs to display a view or a portion of a view only under specific circumstances. The Angular `*ngIf` directive inserts or removes an element based on a truthy/falsy condition. Let's take an example to display a message if the user age is more than 18:
```html
18">You are not eligible for student pass!
```
**Note:** Angular isn't showing and hiding the message. It is adding and removing the paragraph element from the DOM. That improves performance, especially in the larger projects with many data bindings.
**[⬆ Back to Top](#table-of-contents)**
23. ### What happens if you use script tag inside template?
Angular recognizes the value as unsafe and automatically sanitizes it, which removes the `script` tag but keeps safe content such as the text content of the `script` tag. This way it eliminates the risk of script injection attacks. If you still use it then it will be ignored and a warning appears in the browser console.
Let's take an example of innerHtml property binding which causes XSS vulnerability,
```typescript
export class InnerHtmlBindingComponent {
// For example, a user/attacker-controlled value from a URL.
htmlSnippet = 'Template alert("0wned") Syntax';
}
```
**[⬆ Back to Top](#table-of-contents)**
24. ### What is interpolation?
Interpolation is a special syntax that Angular converts into property binding. It’s a convenient alternative to property binding. It is represented by double curly braces({{}}). The text between the braces is often the name of a component property. Angular replaces that name with the string value of the corresponding component property.
Let's take an example,
```html
{{title}}
```
In the example above, Angular evaluates the title and url properties and fills in the blanks, first displaying a bold application title and then a URL.
**[⬆ Back to Top](#table-of-contents)**
25. ### What are template expressions?
A template expression produces a value similar to any Javascript expression. Angular executes the expression and assigns it to a property of a binding target; the target might be an HTML element, a component, or a directive. In the property binding, a template expression appears in quotes to the right of the = symbol as in `[property]="expression"`.
In interpolation syntax, the template expression is surrounded by double curly braces. For example, in the below interpolation, the template expression is `{{username}}`,
```html
{{username}}, welcome to Angular
```
The below javascript expressions are prohibited in template expression
1. assignments (=, +=, -=, ...)
2. new
3. chaining expressions with ; or ,
4. increment and decrement operators (++ and --)
----------------------------------
**[⬆ Back to Top](#table-of-contents)**
26. ### What are template statements?
A template statement responds to an event raised by a binding target such as an element, component, or directive. The template statements appear in quotes to the right of the = symbol like `(event)="statement"`.
Let's take an example of button click event's statement
```html
Edit Profile
```
In the above expression, editProfile is a template statement. The below JavaScript syntax expressions are not allowed.
1. new
2. increment and decrement operators, ++ and --
3. operator assignment, such as += and -=
4. the bitwise operators | and &
5. the template expression operators
--------------------------------------
**[⬆ Back to Top](#table-of-contents)**
27. ### How do you categorize data binding types?
Binding types can be grouped into three categories distinguished by the direction of data flow. They are listed as below,
1. From the source-to-view
2. From view-to-source
3. View-to-source-to-view
The possible binding syntax can be tabularized as below,
| Data direction | Syntax | Type |
|---- | --------- | ---- |
| From the source-to-view(One-way) | 1. {{expression}} 2. [target]="expression" 3. bind-target="expression" | Interpolation, Property, Attribute, Class, Style|
| From view-to-source(One-way) | 1. (target)="statement" 2. on-target="statement" | Event |
| View-to-source-to-view(Two-way)| 1. [(target)]="expression" 2. bindon-target="expression"| Two-way |
**[⬆ Back to Top](#table-of-contents)**
28. ### What are pipes?
Pipes are simple functions that use [template expressions](#what-are-template-expressions) to accept data as input and transform it into a desired output. For example, let us take a pipe to transform a component's birthday property into a human-friendly date using **date** pipe.
```javascript
import { Component } from '@angular/core';
@Component({
selector: 'app-birthday',
template: `
Birthday is {{ birthday | date }}
`})
export class BirthdayComponent {
birthday = new Date(1987, 6, 18); // June 18, 1987
}
```
**[⬆ Back to Top](#table-of-contents)**
29. ### What is a parameterized pipe?
A pipe can accept any number of optional parameters to fine-tune its output. The parameterized pipe can be created by declaring the pipe name with a colon ( : ) and then the parameter value. If the pipe accepts multiple parameters, separate the values with colons. Let's take a birthday example with a particular format(dd/MM/yyyy):
```javascript
import { Component } from '@angular/core';
@Component({
selector: 'app-birthday',
template: `
Birthday is {{ birthday | date:'dd/MM/yyyy'}}
` // 18/06/1987})
export class BirthdayComponent {
birthday = new Date(1987, 6, 18);
}
```
**Note:** The parameter value can be any valid template expression, such as a string literal or a component property.
**[⬆ Back to Top](#table-of-contents)**
30. ### How do you chain pipes?
You can chain pipes together in potentially useful combinations as per the needs. Let's take a birthday property which uses date pipe(along with parameter) and uppercase pipes as below
```javascript
import { Component } from '@angular/core';
@Component({
selector: 'app-birthday',
template: `
Birthday is {{ birthday | date:'fullDate' | uppercase}}
` // THURSDAY, JUNE 18, 1987})
export class BirthdayComponent {
birthday = new Date(1987, 6, 18);
}
```
**[⬆ Back to Top](#table-of-contents)**
31. ### What is a custom pipe?
Apart from built-in pipes, you can write your own custom pipe with the below key characteristics:
1. A pipe is a class decorated with pipe metadata `@Pipe` decorator, which you import from the core Angular library
For example,
```javascript
@Pipe({name: 'myCustomPipe'})
```
2. The pipe class implements the **PipeTransform** interface's transform method that accepts an input value followed by optional parameters and returns the transformed value.
The structure of `PipeTransform` would be as below,
```javascript
interface PipeTransform {
transform(value: any, ...args: any[]): any
}
```
3. The `@Pipe` decorator allows you to define the pipe name that you'll use within template expressions. It must be a valid JavaScript identifier.
```javascript
template: `{{someInputValue | myCustomPipe: someOtherValue}}`
```
**[⬆ Back to Top](#table-of-contents)**
32. ### Give an example of custom pipe?
You can create custom reusable pipes for the transformation of existing value. For example, let us create a custom pipe for finding file size based on an extension,
```javascript
import { Pipe, PipeTransform } from '@angular/core';
@Pipe({name: 'customFileSizePipe'})
export class FileSizePipe implements PipeTransform {
transform(size: number, extension: string = 'MB'): string {
return (size / (1024 * 1024)).toFixed(2) + extension;
}
}
```
Now you can use the above pipe in template expression as below,
```javascript
template: `
Find the size of a file
Size: {{288966 | customFileSizePipe: 'GB'}}
`
```
**[⬆ Back to Top](#table-of-contents)**
33. ### What is the difference between pure and impure pipe?
A pure pipe is only called when Angular detects a change in the value or the parameters passed to a pipe. For example, any changes to a primitive input value (String, Number, Boolean, Symbol) or a changed object reference (Date, Array, Function, Object). An impure pipe is called for every change detection cycle no matter whether the value or parameters changes. i.e, An impure pipe is called often, as often as every keystroke or mouse-move.
**[⬆ Back to Top](#table-of-contents)**
34. ### What is a bootstrapping module?
Every application has at least one Angular module, the root module that you bootstrap to launch the application is called as bootstrapping module. It is commonly known as `AppModule`. The default structure of `AppModule` generated by AngularCLI would be as follows:
```javascript
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { FormsModule } from '@angular/forms';
import { HttpClientModule } from '@angular/common/http';
import { AppComponent } from './app.component';
/* the AppModule class with the @NgModule decorator */
@NgModule({
declarations: [
AppComponent
],
imports: [
BrowserModule,
FormsModule,
HttpClientModule
],
providers: [],
bootstrap: [AppComponent]
})
export class AppModule { }
```
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35. ### What are observables?
Observables are declarative which provide support for passing messages between publishers and subscribers in your application. They are mainly used for event handling, asynchronous programming, and handling multiple values. In this case, you define a function for publishing values, but it is not executed until a consumer subscribes to it. The subscribed consumer then receives notifications until the function completes, or until they unsubscribe.
**[⬆ Back to Top](#table-of-contents)**
36. ### What is HttpClient and its benefits?
Most of the Front-end applications communicate with backend services over `HTTP` protocol using either `XMLHttpRequest` interface or the `fetch()` API. Angular provides a simplified client HTTP API known as `HttpClient` which is based on top of `XMLHttpRequest` interface. This client is available from `@angular/common/http` package.
You can import in your root module as below:
```javascript
import { HttpClientModule } from '@angular/common/http';
```
The major advantages of HttpClient can be listed as below,
1. Contains testability features
2. Provides typed request and response objects
3. Intercept request and response
4. Supports Observable APIs
5. Supports streamlined error handling
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37. ### Explain on how to use `HttpClient` with an example?
Below are the steps need to be followed for the usage of `HttpClient`.
1. Import `HttpClient` into root module:
```javascript
import { HttpClientModule } from '@angular/common/http';
@NgModule({
imports: [
BrowserModule,
// import HttpClientModule after BrowserModule.
HttpClientModule,
],
......
})
export class AppModule {}
```
2. Inject the `HttpClient` into the application:
Let's create a userProfileService(`userprofile.service.ts`) as an example. It also defines get method of `HttpClient`:
```javascript
import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
const userProfileUrl: string = 'assets/data/profile.json';
@Injectable()
export class UserProfileService {
constructor(private http: HttpClient) { }
getUserProfile() {
return this.http.get(this.userProfileUrl);
}
}
```
3. Create a component for subscribing service:
Let's create a component called UserProfileComponent(`userprofile.component.ts`), which injects `UserProfileService` and invokes the service method:
```javascript
fetchUserProfile() {
this.userProfileService.getUserProfile()
.subscribe((data: User) => this.user = {
id: data['userId'],
name: data['firstName'],
city: data['city']
});
}
```
Since the above service method returns an Observable which needs to be subscribed in the component.
**[⬆ Back to Top](#table-of-contents)**
38. ### How can you read full response?
The response body doesn't or may not return full response data because sometimes servers also return special headers or status code, which are important for the application workflow. In order to get the full response, you should use `observe` option from `HttpClient`:
```javascript
getUserResponse(): Observable> {
return this.http.get(
this.userUrl, { observe: 'response' });
}
```
Now `HttpClient.get()` method returns an Observable of typed `HttpResponse` rather than just the `JSON` data.
**[⬆ Back to Top](#table-of-contents)**
39. ### How do you perform Error handling?
If the request fails on the server or fails to reach the server due to network issues, then `HttpClient` will return an error object instead of a successful response. In this case, you need to handle in the component by passing `error` object as a second callback to `subscribe()` method.
Let's see how it can be handled in the component with an example,
```javascript
fetchUser() {
this.userService.getProfile()
.subscribe(
(data: User) => this.userProfile = { ...data }, // success path
error => this.error = error // error path
);
}
```
It is always a good idea to give the user some meaningful feedback instead of displaying the raw error object returned from `HttpClient`.
**[⬆ Back to Top](#table-of-contents)**
40. ### What is RxJS?
RxJS is a library for composing asynchronous and callback-based code in a functional, reactive style using Observables. Many APIs such as HttpClient produce and consume RxJS Observables and also uses operators for processing observables.
For example, you can import observables and operators for using HttpClient as below,
```javascript
import { Observable, throwError } from 'rxjs';
import { catchError, retry } from 'rxjs/operators';
```
**[⬆ Back to Top](#table-of-contents)**
41. ### What is subscribing?
An Observable instance begins publishing values only when someone subscribes to it. So you need to subscribe by calling the `subscribe()` method of the instance, passing an observer object to receive the notifications.
Let's take an example of creating and subscribing to a simple observable, with an observer that logs the received message to the console.
```javascript
// Creates an observable sequence of 5 integers, starting from 1
const source = range(1, 5);
// Create observer object
const myObserver = {
next: x => console.log('Observer got a next value: ' + x),
error: err => console.error('Observer got an error: ' + err),
complete: () => console.log('Observer got a complete notification'),
};
// Execute with the observer object and Prints out each item
source.subscribe(myObserver);
// => Observer got a next value: 1
// => Observer got a next value: 2
// => Observer got a next value: 3
// => Observer got a next value: 4
// => Observer got a next value: 5
// => Observer got a complete notification
```
**[⬆ Back to Top](#table-of-contents)**
42. ### What is an observable?
An Observable is a unique Object similar to a Promise that can help manage async code. Observables are not part of the JavaScript language so we need to rely on a popular Observable library called RxJS.
The observables are created using new keyword.
Let see the simple example of observable,
```javascript
import { Observable } from 'rxjs';
const observable = new Observable(observer => {
setTimeout(() => {
observer.next('Hello from a Observable!');
}, 2000);
});
```
**[⬆ Back to Top](#table-of-contents)**
43. ### What is an observer?
Observer is an interface for a consumer of push-based notifications delivered by an Observable. It has below structure,
```javascript
interface Observer {
closed?: boolean;
next: (value: T) => void;
error: (err: any) => void;
complete: () => void;
}
```
A handler that implements the Observer interface for receiving observable notifications will be passed as a parameter for observable as below,
```javascript
myObservable.subscribe(myObserver);
```
**Note:** If you don't supply a handler for a notification type, the observer ignores notifications of that type.
**[⬆ Back to Top](#table-of-contents)**
44. ### What is the difference between promise and observable?
Below are the list of differences between promise and observable:
| Observable | Promise |
|---- | --------- |
| Declarative: Computation does not start until subscription, so they can run whenever you need the result | Executes immediately on creation|
| Provides multiple values over time | Provides only one |
| Subscribe method is used for error handling that facilitates centralized and predictable error handling | Push errors to the child promises |
| Provides chaining and subscription to handle complex applications | Uses only `.then()` clause |
**[⬆ Back to Top](#table-of-contents)**
45. ### What is multicasting?
Multi-casting is the practice of broadcasting to a list of multiple subscribers in a single execution.
Let's demonstrate the multi-casting feature:
```javascript
var source = Rx.Observable.from([1, 2, 3]);
var subject = new Rx.Subject();
var multicasted = source.multicast(subject);
// These are, under the hood, `subject.subscribe({...})`:
multicasted.subscribe({
next: (v) => console.log('observerA: ' + v)
});
multicasted.subscribe({
next: (v) => console.log('observerB: ' + v)
});
// This is, under the hood, `s
```
**[⬆ Back to Top](#table-of-contents)**
46. ### How do you perform error handling in observables?
You can handle errors by specifying an **error callback** on the observer instead of relying on `try`/`catch`, which are ineffective in asynchronous environment.
For example, you can define error callback as below,
```javascript
myObservable.subscribe({
next(num) { console.log('Next num: ' + num)},
error(err) { console.log('Received an error: ' + err)}
});
```
**[⬆ Back to Top](#table-of-contents)**
47. ### What is the shorthand notation for subscribe method?
The `subscribe()` method can accept callback function definitions in line, for `next`, `error`, and `complete` handlers. It is known as shorthand notation or Subscribe method with positional arguments.
For example, you can define subscribe method as below,
```javascript
myObservable.subscribe(
x => console.log('Observer got a next value: ' + x),
err => console.error('Observer got an error: ' + err),
() => console.log('Observer got a complete notification')
);
```
**[⬆ Back to Top](#table-of-contents)**
48. ### What are the utility functions provided by RxJS?
The RxJS library also provides below utility functions for creating and working with observables.
1. Converting existing code for async operations into observables
2. Iterating through the values in a stream
3. Mapping values to different types
4. Filtering streams
5. Composing multiple streams
**[⬆ Back to Top](#table-of-contents)**
49. ### What are observable creation functions?
RxJS provides creation functions for the process of creating observables from promises, events, timers and Ajax requests. Let us explain each of them with an example:
1. Create an observable from a promise
```javascript
import { from } from 'rxjs'; // from function
const data = from(fetch('/api/endpoint')); //Created from Promise
data.subscribe({
next(response) { console.log(response); },
error(err) { console.error('Error: ' + err); },
complete() { console.log('Completed'); }
});
```
2. Create an observable that creates an AJAX request
```javascript
import { ajax } from 'rxjs/ajax'; // ajax function
const apiData = ajax('/api/data'); // Created from AJAX request
// Subscribe to create the request
apiData.subscribe(res => console.log(res.status, res.response));
```
3. Create an observable from a counter
```javascript
import { interval } from 'rxjs'; // interval function
const secondsCounter = interval(1000); // Created from Counter value
secondsCounter.subscribe(n =>
console.log(`Counter value: ${n}`));
```
4. Create an observable from an event
```javascript
import { fromEvent } from 'rxjs';
const el = document.getElementById('custom-element');
const mouseMoves = fromEvent(el, 'mousemove');
const subscription = mouseMoves.subscribe((e: MouseEvent) => {
console.log(`Coordnitaes of mouse pointer: ${e.clientX} * ${e.clientY}`);
});
```
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50. ### What will happen if you do not supply handler for the observer?
Usually, an observer object can define any combination of `next`, `error`, and `complete` notification type handlers. If you don't supply a handler for a notification type, the observer just ignores notifications of that type.
**[⬆ Back to Top](#table-of-contents)**
51. ### What are Angular elements?
Angular elements are Angular components packaged as **custom elements** (a web standard for defining new HTML elements in a framework-agnostic way). Angular Elements host an Angular component, providing a bridge between the data and the logic defined in the component and the standard DOM APIs, thus, providing a way to use Angular components in `non-Angular environments`.
**[⬆ Back to Top](#table-of-contents)**
52. ### What is the browser support of Angular Elements?
Since Angular elements are packaged as custom elements the browser support of angular elements is same as custom elements support.
This feature is is currently supported natively in a number of browsers and pending for other browsers.
| Browser | Angular Element Support |
|---- | --------- |
| Chrome | Natively supported|
| Opera | Natively supported |
| Safari| Natively supported |
| Firefox | Natively supported from 63 version onwards. You need to enable dom.webcomponents.enabled and dom.webcomponents.customelements.enabled in older browsers |
| Edge| Currently it is in progress|
**[⬆ Back to Top](#table-of-contents)**
53. ### What are custom elements?
Custom elements (or Web Components) are a Web Platform feature which extends HTML by allowing you to define a tag whose content is created and controlled by JavaScript code. The browser maintains a `CustomElementRegistry` of defined custom elements, which maps an instantiable JavaScript class to an HTML tag. Currently this feature is supported by Chrome, Firefox, Opera, and Safari, and available in other browsers through polyfills.
**[⬆ Back to Top](#table-of-contents)**
54. ### Do I need to bootstrap custom elements?
No, custom elements bootstrap (or start) automatically when they are added to the DOM, and are automatically destroyed when removed from the DOM. Once a custom element is added to the DOM for any page, it looks and behaves like any other HTML element, and does not require any special knowledge of Angular.
**[⬆ Back to Top](#table-of-contents)**
55. ### Explain how custom elements works internally?
Below are the steps in an order about custom elements functionality,
1. **App registers custom element with browser:** Use the `createCustomElement()` function to convert a component into a class that can be registered with the browser as a custom element.
2. **App adds custom element to DOM:** Add custom element just like a built-in HTML element directly into the DOM.
3. **Browser instantiate component based class:** Browser creates an instance of the registered class and adds it to the DOM.
4. **Instance provides content with data binding and change detection:** The content with in template is rendered using the component and DOM data.
The flow chart of the custom elements functionality would be as follows,
**[⬆ Back to Top](#table-of-contents)**
56. ### How to transfer components to custom elements?
Transforming components to custom elements involves **two** major steps,
1. **Build custom element class:** Angular provides the `createCustomElement()` function for converting an Angular component (along with its dependencies) to a custom element. The conversion process implements `NgElementConstructor` interface, and creates a constructor class which is used to produce a self-bootstrapping instance of Angular component.
2. **Register element class with browser:** It uses `customElements.define()` JS function, to register the configured constructor and its associated custom-element tag with the browser's `CustomElementRegistry`. When the browser encounters the tag for the registered element, it uses the constructor to create a custom-element instance.
The detailed structure would be as follows,
**[⬆ Back to Top](#table-of-contents)**
57. ### What are the mapping rules between Angular component and custom element?
The Component properties and logic maps directly into HTML attributes and the browser's event system. Let us describe them in two steps,
1. The createCustomElement() API parses the component input properties with corresponding attributes for the custom element. For example, component @Input('myInputProp') converted as custom element attribute `my-input-prop`.
2. The Component outputs are dispatched as HTML Custom Events, with the name of the custom event matching the output name. For example, component @Output() valueChanged = new EventEmitter() converted as custom element with dispatch event as "valueChanged".
**[⬆ Back to Top](#table-of-contents)**
58. ### How do you define typings for custom elements?
You can use the `NgElement` and `WithProperties` types exported from @angular/elements.
Let's see how it can be applied by comparing with Angular component.
1. The simple container with input property would be as below,
```javascript
@Component(...)
class MyContainer {
@Input() message: string;
}
```
2. After applying types typescript validates input value and their types,
```javascirpt
const container = document.createElement('my-container') as NgElement & WithProperties<{message: string}>;
container.message = 'Welcome to Angular elements!';
container.message = true; // <-- ERROR: TypeScript knows this should be a string.
container.greet = 'News'; // <-- ERROR: TypeScript knows there is no `greet` property on `container`.
```
**[⬆ Back to Top](#table-of-contents)**
59. ### What are dynamic components?
Dynamic components are the components in which the component's location in the application is not defined at build time i.e. they are not used in any angular template. Instead, the component is instantiated and placed in the application at runtime.
**[⬆ Back to Top](#table-of-contents)**
60. ### What are the various kinds of directives?
There are mainly three kinds of directives:
1. **Components** — These are directives with a template.
2. **Structural directives** — These directives change the DOM layout by adding and removing DOM elements.
3. **Attribute directives** — These directives change the appearance or behavior of an element, component, or another directive.
**[⬆ Back to Top](#table-of-contents)**
61. ### How do you create directives using CLI?
You can use CLI command `ng generate directive` to create the directive class file. It creates the source file(`src/app/components/directivename.directive.ts`), the respective test file `.spec.ts` and declare the directive class file in root module.
**[⬆ Back to Top](#table-of-contents)**
62. ### Give an example for attribute directives?
Let's take simple highlighter behavior as a example directive for DOM element. You can create and apply the attribute directive using below step:
1. Create HighlightDirective class with the file name `src/app/highlight.directive.ts`. In this file, we need to import **Directive** from core library to apply the metadata and **ElementRef** in the directive's constructor to inject a reference to the host DOM element ,
```javascript
import { Directive, ElementRef } from '@angular/core';
@Directive({
selector: '[appHighlight]'
})
export class HighlightDirective {
constructor(el: ElementRef) {
el.nativeElement.style.backgroundColor = 'red';
}
}
```
2. Apply the attribute directive as an attribute to the host element(for example,
)
```javascript
Highlight me!
```
3. Run the application to see the highlight behavior on paragraph element
```javascript
ng serve
```
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63. ### What is Angular Router?
Angular Router is a mechanism in which navigation happens from one view to the next as users perform application tasks. It borrows the concepts or model of browser's application navigation. It enables developers to build Single Page Applications with multiple views and allow navigation between these views.
**[⬆ Back to Top](#table-of-contents)**
64. ### What is the purpose of base href tag?
The routing application should add element to the index.html as the first child in the tag in order to indicate how to compose navigation URLs. If app folder is the application root then you can set the href value as below
```html
```
**[⬆ Back to Top](#table-of-contents)**
65. ### What are the router imports?
The Angular Router which represents a particular component view for a given URL is not part of Angular Core. It is available in library named `@angular/router` to import required router components. For example, we import them in app module as below,
```javascript
import { RouterModule, Routes } from '@angular/router';
```
**[⬆ Back to Top](#table-of-contents)**
66. ### What is router outlet?
The RouterOutlet is a directive from the router library and it acts as a placeholder that marks the spot in the template where the router should display the components for that outlet. Router outlet is used like a component,
```html
```
**[⬆ Back to Top](#table-of-contents)**
67. ### What are router links?
The RouterLink is a directive on the anchor tags give the router control over those elements. Since the navigation paths are fixed, you can assign string values to router-link directive as below,
```html
Angular Router
List of todos
Completed todos
```
**[⬆ Back to Top](#table-of-contents)**
68. ### What are active router links?
RouterLinkActive is a directive that toggles css classes for active RouterLink bindings based on the current RouterState. i.e, The Router will add CSS classes when this link is active and remove when the link is inactive. For example, you can add them to RouterLinks as below.
```html
Angular Router
List of todos
Completed todos
```
**[⬆ Back to Top](#table-of-contents)**
69. ### What is router state?
RouterState is a tree of activated routes. Every node in this tree knows about the "consumed" URL segments, the extracted parameters, and the resolved data. You can access the current RouterState from anywhere in the application using the `Router service` and the `routerState` property.
```javascript
@Component({templateUrl:'template.html'})
class MyComponent {
constructor(router: Router) {
const state: RouterState = router.routerState;
const root: ActivatedRoute = state.root;
const child = root.firstChild;
const id: Observable = child.params.map(p => p.id);
//...
}
}
```
**[⬆ Back to Top](#table-of-contents)**
70. ### What are router events?
During each navigation, the Router emits navigation events through the Router.events property allowing you to track the lifecycle of the route.
The sequence of router events is as below,
1. NavigationStart,
2. RouteConfigLoadStart,
3. RouteConfigLoadEnd,
4. RoutesRecognized,
5. GuardsCheckStart,
6. ChildActivationStart,
7. ActivationStart,
8. GuardsCheckEnd,
9. ResolveStart,
10. ResolveEnd,
11. ActivationEnd
12. ChildActivationEnd
13. NavigationEnd,
14. NavigationCancel,
15. NavigationError
16. Scroll
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71. ### What is activated route?
ActivatedRoute contains the information about a route associated with a component loaded in an outlet. It can also be used to traverse the router state tree. The ActivatedRoute will be injected as a router service to access the information. In the below example, you can access route path and parameters,
```javascript
@Component({...})
class MyComponent {
constructor(route: ActivatedRoute) {
const id: Observable = route.params.pipe(map(p => p.id));
const url: Observable = route.url.pipe(map(segments => segments.join('')));
// route.data includes both `data` and `resolve`
const user = route.data.pipe(map(d => d.user));
}
}
```
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72. ### How do you define routes?
A router must be configured with a list of route definitions. You configures the router with routes via the `RouterModule.forRoot()` method, and adds the result to the AppModule's `imports` array.
```javascript
const appRoutes: Routes = [
{ path: 'todo/:id', component: TodoDetailComponent },
{
path: 'todos',
component: TodosListComponent,
data: { title: 'Todos List' }
},
{ path: '',
redirectTo: '/todos',
pathMatch: 'full'
},
{ path: '**', component: PageNotFoundComponent }
];
@NgModule({
imports: [
RouterModule.forRoot(
appRoutes,
{ enableTracing: true } // <-- debugging purposes only
)
// other imports here
],
...
})
export class AppModule { }
```
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73. ### What is the purpose of Wildcard route?
If the URL doesn't match any predefined routes then it causes the router to throw an error and crash the app. In this case, you can use wildcard route. A wildcard route has a path consisting of two asterisks to match every URL.
For example, you can define PageNotFoundComponent for wildcard route as below
```javascript
{ path: '**', component: PageNotFoundComponent }
```
**[⬆ Back to Top](#table-of-contents)**
74. ### Do I need a Routing Module always?
No, the Routing Module is a design choice. You can skip routing Module (for example, AppRoutingModule) when the configuration is simple and merge the routing configuration directly into the companion module (for example, AppModule). But it is recommended when the configuration is complex and includes specialized guard and resolver services.
**[⬆ Back to Top](#table-of-contents)**
75. ### What is Angular Universal?
Angular Universal is a server-side rendering module for Angular applications in various scenarios. This is a community driven project and available under @angular/platform-server package. Recently Angular Universal is integrated with Angular CLI.
**[⬆ Back to Top](#table-of-contents)**
76. ### What are different types of compilation in Angular?
Angular offers two ways to compile your application,
1. Just-in-Time (JIT)
2. Ahead-of-Time (AOT)
**[⬆ Back to Top](#table-of-contents)**
77. ### What is JIT?
Just-in-Time (JIT) is a type of compilation that compiles your app in the browser at runtime. JIT compilation was the default until Angular 8, now default is AOT. When you run the ng build (build only) or ng serve (build and serve locally) CLI commands, the type of compilation (JIT or AOT) depends on the value of the aot property in your build configuration specified in angular.json. By default, aot is set to true.
**[⬆ Back to Top](#table-of-contents)**
78. ### What is AOT?
Ahead-of-Time (AOT) is a type of compilation that compiles your app at build time. This is the default starting in Angular 9. When you run the ng build (build only) or ng serve (build and serve locally) CLI commands, the type of compilation (JIT or AOT) depends on the value of the aot property in your build configuration specified in angular.json. By default, aot is set to true.
```cmd
ng build
ng serve
```
**[⬆ Back to Top](#table-of-contents)**
79. ### Why do we need compilation process?
The Angular components and templates cannot be understood by the browser directly. Due to that Angular applications require a compilation process before they can run in a browser. For example, In AOT compilation, both Angular HTML and TypeScript code converted into efficient JavaScript code during the build phase before browser runs it.
**[⬆ Back to Top](#table-of-contents)**
80. ### What are the advantages with AOT?
Below are the list of AOT benefits,
1. **Faster rendering:** The browser downloads a pre-compiled version of the application. So it can render the application immediately without compiling the app.
2. **Fewer asynchronous requests:** It inlines external HTML templates and CSS style sheets within the application javascript which eliminates separate ajax requests.
3. **Smaller Angular framework download size:** Doesn't require downloading the Angular compiler. Hence it dramatically reduces the application payload.
4. **Detect template errors earlier:** Detects and reports template binding errors during the build step itself
5. **Better security:** It compiles HTML templates and components into JavaScript. So there won't be any injection attacks.
**[⬆ Back to Top](#table-of-contents)**
81. ### What are the ways to control AOT compilation?
You can control your app compilation in two ways,
1. By providing template compiler options in the `tsconfig.json` file
2. By configuring Angular metadata with decorators
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82. ### What are the restrictions of metadata?
In Angular, You must write metadata with the following general constraints,
1. Write expression syntax with in the supported range of javascript features
2. The compiler can only reference symbols which are exported
3. Only call the functions supported by the compiler
4. Decorated and data-bound class members must be public.
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83. ### What are the three phases of AOT?
The AOT compiler works in three phases,
1. **Code Analysis:** The compiler records a representation of the source
2. **Code generation:** It handles the interpretation as well as places restrictions on what it interprets.
3. **Validation:** In this phase, the Angular template compiler uses the TypeScript compiler to validate the binding expressions in templates.
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84. ### Can I use arrow functions in AOT?
No, Arrow functions or lambda functions can’t be used to assign values to the decorator properties. For example, the following snippet is invalid:
```javascript
@Component({
providers: [{
provide: MyService, useFactory: () => getService()
}]
})
```
To fix this, it has to be changed as following exported function:
```javascript
function getService(){
return new MyService();
}
@Component({
providers: [{
provide: MyService, useFactory: getService
}]
})
```
If you still use arrow function, it generates an error node in place of the function. When the compiler later interprets this node, it reports an error to turn the arrow function into an exported function.
**Note:** From Angular5 onwards, the compiler automatically performs this rewriting while emitting the .js file.
**[⬆ Back to Top](#table-of-contents)**
85. ### What is the purpose of metadata json files?
The metadata.json file can be treated as a diagram of the overall structure of a decorator's metadata, represented as an abstract syntax tree(AST). During the analysis phase, the AOT collector scan the metadata recorded in the Angular decorators and outputs metadata information in .metadata.json files, one per .d.ts file.
**[⬆ Back to Top](#table-of-contents)**
86. ### Can I use any javascript feature for expression syntax in AOT?
No, the AOT collector understands a subset of (or limited) JavaScript features. If an expression uses unsupported syntax, the collector writes an error node to the .metadata.json file. Later point of time, the compiler reports an error if it needs that piece of metadata to generate the application code.
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87. ### What is folding?
The compiler can only resolve references to exported symbols in the metadata. Where as some of the non-exported members are folded while generating the code. i.e Folding is a process in which the collector evaluate an expression during collection and record the result in the .metadata.json instead of the original expression.
For example, the compiler couldn't refer selector reference because it is not exported
```javascript
let selector = 'app-root';
@Component({
selector: selector
})
```
Will be folded into inline selector
```javascript
@Component({
selector: 'app-root'
})
```
Remember that the compiler can’t fold everything. For example, spread operator on arrays, objects created using new keywords and function calls.
**[⬆ Back to Top](#table-of-contents)**
88. ### What are macros?
The AOT compiler supports macros in the form of functions or static methods that return an expression in a `single return expression`.
For example, let us take a below macro function,
```javascript
export function wrapInArray(value: T): T[] {
return [value];
}
```
You can use it inside metadata as an expression,
```javascript
@NgModule({
declarations: wrapInArray(TypicalComponent)
})
export class TypicalModule {}
```
The compiler treats the macro expression as it written directly
```javascript
@NgModule({
declarations: [TypicalComponent]
})
export class TypicalModule {}
```
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89. ### Give an example of few metadata errors?
Below are some of the errors encountered in metadata,
1. **Expression form not supported:** Some of the language features outside of the compiler's restricted expression syntax used in angular metadata can produce this error.
Let's see some of these examples,
```javascript
1. export class User { ... }
const prop = typeof User; // typeof is not valid in metadata
2. { provide: 'token', useValue: { [prop]: 'value' } }; // bracket notation is not valid in metadata
```
2. **Reference to a local (non-exported) symbol:** The compiler encountered a referenced to a locally defined symbol that either wasn't exported or wasn't initialized.
Let's take example of this error,
```javascript
// ERROR
let username: string; // neither exported nor initialized
@Component({
selector: 'my-component',
template: ... ,
providers: [
{ provide: User, useValue: username }
]
})
export class MyComponent {}
```
You can fix this by either exporting or initializing the value,
```javascript
export let username: string; // exported
(or)
let username = 'John'; // initialized
```
3. **Function calls are not supported:** The compiler does not currently support function expressions or lambda functions. For example, you cannot set a provider's useFactory to an anonymous function or arrow function as below.
```javascript
providers: [
{ provide: MyStrategy, useFactory: function() { ... } },
{ provide: OtherStrategy, useFactory: () => { ... } }
]
```
You can fix this with exported function
```javascript
export function myStrategy() { ... }
export function otherStrategy() { ... }
... // metadata
providers: [
{ provide: MyStrategy, useFactory: myStrategy },
{ provide: OtherStrategy, useFactory: otherStrategy },
```
4. **Destructured variable or constant not supported:** The compiler does not support references to variables assigned by destructuring.
For example, you cannot write something like this:
```javascript
import { user } from './user';
// destructured assignment to name and age
const {name, age} = user;
... //metadata
providers: [
{provide: Name, useValue: name},
{provide: Age, useValue: age},
]
```
You can fix this by non-destructured values
```javascript
import { user } from './user';
... //metadata
providers: [
{provide: Name, useValue: user.name},
{provide: Age, useValue: user.age},
]
```
**[⬆ Back to Top](#table-of-contents)**
90. ### What is metadata rewriting?
Metadata rewriting is the process in which the compiler converts the expression initializing the fields such as useClass, useValue, useFactory, and data into an exported variable, which replaces the expression. Remember that the compiler does this rewriting during the emit of the .js file but not in definition files( .d.ts file).
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91. ### How do you provide configuration inheritance?
Angular Compiler supports configuration inheritance through extends in the tsconfig.json on angularCompilerOptions. i.e, The configuration from the base file(for example, tsconfig.base.json) are loaded first, then overridden by those in the inheriting config file.
```javascript
{
"extends": "../tsconfig.base.json",
"compilerOptions": {
"experimentalDecorators": true,
...
},
"angularCompilerOptions": {
"fullTemplateTypeCheck": true,
"preserveWhitespaces": true,
...
}
}
```
**[⬆ Back to Top](#table-of-contents)**
92. ### How do you specify angular template compiler options?
The angular template compiler options are specified as members of the **angularCompilerOptions** object in the tsconfig.json file. These options will be specified adjacent to typescript compiler options.
```javascript
{
"compilerOptions": {
"experimentalDecorators": true,
...
},
"angularCompilerOptions": {
"fullTemplateTypeCheck": true,
"preserveWhitespaces": true,
...
}
}
```
**[⬆ Back to Top](#table-of-contents)**
93. ### How do you enable binding expression validation?
You can enable binding expression validation explicitly by adding the compiler option **fullTemplateTypeCheck** in the "angularCompilerOptions" of the project's tsconfig.json. It produces error messages when a type error is detected in a template binding expression.
For example, consider the following component:
```javascript
@Component({
selector: 'my-component',
template: '{{user.contacts.email}}'
})
class MyComponent {
user?: User;
}
```
This will produce the following error:
```javascript
my.component.ts.MyComponent.html(1,1): : Property 'contacts' does not exist on type 'User'. Did you mean 'contact'?
```
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94. ### What is the purpose of any type cast function?
You can disable binding expression type checking using $any() type cast function(by surrounding the expression). In the following example, the error Property contacts does not exist is suppressed by casting user to the any type.
```javascript
template:
'{{ $any(user).contacts.email }}'
```
The $any() cast function also works with this to allow access to undeclared members of the component.
```javascript
template:
'{{ $any(this).contacts.email }}'
```
**[⬆ Back to Top](#table-of-contents)**
95. ### What is Non null type assertion operator?
You can use the non-null type assertion operator to suppress the Object is possibly 'undefined' error. In the following example, the user and contact properties are always set together, implying that contact is always non-null if user is non-null. The error is suppressed in the example by using contact!.email.
```javascript
@Component({
selector: 'my-component',
template: ' {{user.name}} contacted through {{contact!.email}} '
})
class MyComponent {
user?: User;
contact?: Contact;
setData(user: User, contact: Contact) {
this.user = user;
this.contact = contact;
}
}
```
**[⬆ Back to Top](#table-of-contents)**
96. ### What is type narrowing?
The expression used in an ngIf directive is used to narrow type unions in the Angular template compiler similar to if expression in typescript. So *ngIf allows the typeScript compiler to infer that the data used in the binding expression will never be undefined.
```javascript
@Component({
selector: 'my-component',
template: ' {{user.contact.email}}