How do you pass headers for HTTP client?

You can directly pass object map for http client or create HttpHeaders class to supply the headers.

constructor(private _http: HttpClient) {}
this._http.get('someUrl',{
   headers: {'header1':'value1','header2':'value2'}
});

(or)
let headers = new HttpHeaders().set('header1', headerValue1); // create header object
headers = headers.append('header2', headerValue2); // add a new header, creating a new object
headers = headers.append('header3', headerValue3); // add another header

let params = new HttpParams().set('param1', value1); // create params object
params = params.append('param2', value2); // add a new param, creating a new object
params = params.append('param3', value3); // add another param

return this._http.get<any[]>('someUrl', { headers: headers, params: params })

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What is the purpose of differential loading in CLI?

From Angular8 release onwards, the applications are built using differential loading strategy from CLI to build two separate bundles as part of your deployed application.

1. The first build contains ES2015 syntax which takes the advantage of built-in support in modern browsers, ships less polyfills, and results in a smaller bundle size.
2. The second build contains old ES5 syntax to support older browsers with all necessary polyfills. But this results in a larger bundle size.

Note: This strategy is used to support multiple browsers but it only load the code that the browser needs.

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Is Angular supports dynamic imports?

Yes, Angular 8 supports dynamic imports in router configuration. i.e, You can use the import statement for lazy loading the module using loadChildren method and it will be understood by the IDEs(VSCode and WebStorm), webpack, etc. Previously, you have been written as below to lazily load the feature module. By mistake, if you have typo in the module name it still accepts the string and throws an error during build time.

{path: ‘user’, loadChildren: ‘./users/user.module#UserModulee’},

This problem is resolved by using dynamic imports and IDEs are able to find it during compile time itself.

{path: ‘user’, loadChildren: () => import(‘./users/user.module’).then(m => m.UserModule)};

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What is lazy loading?

Lazy loading is one of the most useful concepts of Angular Routing. It helps us to download the web pages in chunks instead of downloading everything in a big bundle. It is used for lazy loading by asynchronously loading the feature module for routing whenever required using the property loadChildren. Let's load both Customer and Order feature modules lazily as below,

const routes: Routes = [
  {
    path: 'customers',
    loadChildren: () => import('./customers/customers.module').then(module => module.CustomersModule)
  },
  {
    path: 'orders',
    loadChildren: () => import('./orders/orders.module').then(module => module.OrdersModule)
  },
  {
    path: '',
    redirectTo: '',
    pathMatch: 'full'
  }
];

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What are workspace APIs?

Angular 8.0 release introduces Workspace APIs to make it easier for developers to read and modify the angular.json file instead of manually modifying it. Currently, the only supported storage3 format is the JSON-based format used by the Angular CLI. You can enable or add optimization option for build target as below,

import { NodeJsSyncHost } from '@angular-devkit/core/node';
import { workspaces } from '@angular-devkit/core';

async function addBuildTargetOption() {
    const host = workspaces.createWorkspaceHost(new NodeJsSyncHost());
    const workspace = await workspaces.readWorkspace('path/to/workspace/directory/', host);

    const project = workspace.projects.get('my-app');
    if (!project) {
      throw new Error('my-app does not exist');
    }

    const buildTarget = project.targets.get('build');
    if (!buildTarget) {
      throw new Error('build target does not exist');
    }

    buildTarget.options.optimization = true;

    await workspaces.writeWorkspace(workspace, host);
}

addBuildTargetOption();

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How do you upgrade angular version?

The Angular upgrade is quite easier using Angular CLI ng update command as mentioned below. For example, if you upgrade from Angular 7 to 8 then your lazy loaded route imports will be migrated to the new import syntax automatically.

$ ng update @angular/cli @angular/core

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What is Angular Material?

Angular Material is a collection of Material Design components for Angular framework following the Material Design spec. You can apply Material Design very easily using Angular Material. The installation can be done through npm or yarn,

npm install --save @angular/material @angular/cdk @angular/animations
(OR)
yarn add @angular/material @angular/cdk @angular/animations

It supports the most recent two versions of all major browsers. The latest version of Angular material is 8.1.1

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How do you upgrade location service of angularjs?

If you are using $location service in your old AngularJS application, now you can use LocationUpgradeModule(unified location service) which puts the responsibilities of $location service to Location service in Angular. Let's add this module to AppModule as below,

// Other imports ...
import { LocationUpgradeModule } from '@angular/common/upgrade';

@NgModule({
  imports: [
    // Other NgModule imports...
    LocationUpgradeModule.config()
  ]
})
export class AppModule {}

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What is NgUpgrade?

NgUpgrade is a library put together by the Angular team, which you can use in your applications to mix and match AngularJS and Angular components and bridge the AngularJS and Angular dependency injection systems.

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How do you test Angular application using CLI?

Angular CLI downloads and install everything needed with the Jasmine Test framework. You just need to run ng test to see the test results. By default this command builds the app in watch mode, and launches the Karma test runner. The output of test results would be as below,

10% building modules 1/1 modules 0 active
...INFO [karma]: Karma v1.7.1 server started at http://0.0.0.0:9876/
...INFO [launcher]: Launching browser Chrome ...
...INFO [launcher]: Starting browser Chrome
...INFO [Chrome ...]: Connected on socket ...
Chrome ...: Executed 3 of 3 SUCCESS (0.135 secs / 0.205 secs)

Note: A chrome browser also opens and displays the test output in the "Jasmine HTML Reporter".

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How to use polyfills in Angular application?

The Angular CLI provides support for polyfills officially. When you create a new project with the ng new command, a src/polyfills.ts configuration file is created as part of your project folder. This file includes the mandatory and many of the optional polyfills as JavaScript import statements. Let's categorize the polyfills,

1. Mandatory polyfills: These are installed automatically when you create your project with ng new command and the respective import statements enabled in 'src/polyfills.ts' file.
2. Optional polyfills: You need to install its npm package and then create import statement in 'src/polyfills.ts' file. For example, first you need to install below npm package for adding web animations (optional) polyfill. bash npm install --save web-animations-js and create import statement in polyfill file. javascript import 'web-animations-js';

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What are the ways to trigger change detection in Angular?

You can inject either ApplicationRef or NgZone, or ChangeDetectorRef into your component and apply below specific methods to trigger change detection in Angular. i.e, There are 3 possible ways,

1. ApplicationRef.tick(): Invoke this method to explicitly process change detection and its side-effects. It check the full component tree.
2. NgZone.run(callback): It evaluate the callback function inside the Angular zone.
3. ChangeDetectorRef.detectChanges(): It detects only the components and it's children.

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What are the differences of various versions of Angular?

There are different versions of Angular framework. Let's see the features of all the various versions,

1. Angular 1:
   • Angular 1 (AngularJS) is the first angular framework released in the year 2010.
   • AngularJS is not built for mobile devices.
   • It is based on controllers with MVC architecture.
2. Angular 2:
   • Angular 2 was released in the year 2016. Angular 2 is a complete rewrite of Angular1 version.
   • The performance issues that Angular 1 version had has been addressed in Angular 2 version.
   • Angular 2 is built from scratch for mobile devices unlike Angular 1 version.
   • Angular 2 is components based.
3. Angular 3:
   • The following are the different package versions in Angular 2:
     • @angular/core v2.3.0
     • @angular/compiler v2.3.0
     • @angular/http v2.3.0
     • @angular/router v3.3.0
   • The router package is already versioned 3 so to avoid confusion switched to Angular 4 version and skipped 3 version.
4. Angular 4:
   • The compiler generated code file size in AOT mode is very much reduced.
   • With Angular 4 the production bundles size is reduced by hundreds of KB’s.
   • Animation features are removed from angular/core and formed as a separate package.
   • Supports Typescript 2.1 and 2.2.
   • Angular Universal
   • New HttpClient
5. Angular 5:
   • Angular 5 makes angular faster. It improved the loading time and execution time.
   • Shipped with new build optimizer.
   • Supports Typescript 2.5.
   • Service Worker
6. Angular 6:
   • It is released in May 2018.
   • Includes Angular Command Line Interface (CLI), Component Development KIT (CDK), Angular Material Package, Angular Elements.
   • Service Worker bug fixes.
   • i18n
   • Experimental mode for Ivy.
   • RxJS 6.0
   • Tree Shaking
7. Angular 7:
   • It is released in October 2018.
   • TypeScript 3.1
   • RxJS 6.3
   • New Angular CLI
   • CLI Prompts capability provide an ability to ask questions to the user before they run. It is like interactive dialog between the user and the CLI
   • With the improved CLI Prompts capability, it helps developers to make the decision. New ng commands ask users for routing and CSS styles types(SCSS) and ng add @angular/material asks for themes and gestures or animations.
8. Angular 8:
   • It is released in May 2019.
   • TypeScript 3.4
9. Angular 9:
   • It is released in February 2020.
   • TypeScript 3.7
   • Ivy enabled by default
10. Angular 10:
    • It is released in June 2020.
    • TypeScript 3.9
    • TSlib 2.0

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What are the security principles in angular?

Below are the list of security principles in angular,

1. You should avoid direct use of the DOM APIs.
2. You should enable Content Security Policy (CSP) and configure your web server to return appropriate CSP HTTP headers.
3. You should Use the offline template compiler.
4. You should Use Server Side XSS protection.
5. You should Use DOM Sanitizer.
6. You should Preventing CSRF or XSRF attacks.

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What is the reason to deprecate Web Tracing Framework?

Angular has supported the integration with the Web Tracing Framework (WTF) for the purpose of performance testing. Since it is not well maintained and failed in majority of the applications, the support is deprecated in latest releases.

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What is the reason to deprecate web worker packages?

Both @angular/platform-webworker and @angular/platform-webworker-dynamic are officially deprecated, the Angular team realized it's not good practice to run the Angular application on Web worker

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How do you find angular CLI version?

Angular CLI provides it's installed version using below different ways using ng command,

ng v
ng version
ng -v
ng --version

and the output would be as below,

Angular CLI: 1.6.3
Node: 8.11.3
OS: darwin x64
Angular:
...

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What is the browser support for Angular?

Angular supports most recent browsers which includes both desktop and mobile browsers.

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What is schematic?

It's a scaffolding library that defines how to generate or transform a programming project by creating, modifying, refactoring, or moving files and code. It defines rules that operate on a virtual file system called a tree.

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What is rule in Schematics?

In schematics world, it's a function that operates on a file tree to create, delete, or modify files in a specific manner.

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What is Schematics CLI?

Schematics come with their own command-line tool known as Schematics CLI. It is used to install the schematics executable, which you can use to create a new schematics collection with an initial named schematic. The collection folder is a workspace for schematics. You can also use the schematics command to add a new schematic to an existing collection, or extend an existing schematic. You can install Schematic CLI globally as below,

npm install -g @angular-devkit/schematics-cli

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What are the best practices for security in angular?

Below are the best practices of security in angular,

1. Use the latest Angular library releases
2. Don't modify your copy of Angular
3. Avoid Angular APIs marked in the documentation as “Security Risk.”

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What is Angular security model for preventing XSS attacks?

Angular treats all values as untrusted by default. i.e, Angular sanitizes and escapes untrusted values When a value is inserted into the DOM from a template, via property, attribute, style, class binding, or interpolation.

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What is the role of template compiler for prevention of XSS attacks?

The offline template compiler prevents vulnerabilities caused by template injection, and greatly improves application performance. So it is recommended to use offline template compiler in production deployments without dynamically generating any template.

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What are the various security contexts in Angular?

Angular defines the following security contexts for sanitization,

1. HTML: It is used when interpreting a value as HTML such as binding to innerHtml.
2. Style: It is used when binding CSS into the style property.
3. URL: It is used for URL properties such as <a href>.
4. Resource URL: It is a URL that will be loaded and executed as code such as <script src>.

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What is Sanitization? Is angular supports it?

Sanitization is the inspection of an untrusted value, turning it into a value that's safe to insert into the DOM. Yes, Angular suppports sanitization. It sanitizes untrusted values for HTML, styles, and URLs but sanitizing resource URLs isn't possible because they contain arbitrary code.

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What is the purpose of innerHTML?

The innerHtml is a property of HTML-Elements, which allows you to set it's html-content programmatically. Let's display the below html code snippet in a <div> tag as below using innerHTML binding,

<div [innerHTML]="htmlSnippet"></div>

and define the htmlSnippet property from any component

export class myComponent {
  htmlSnippet: string = '<b>Hello World</b>, Angular';
}

Unfortunately this property could cause Cross Site Scripting (XSS) security bugs when improperly handled.

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What is the difference between interpolated content and innerHTML?

The main difference between interpolated and innerHTML code is the behavior of code interpreted. Interpolated content is always escaped i.e, HTML isn't interpreted and the browser displays angle brackets in the element's text content. Where as in innerHTML binding, the content is interpreted i.e, the browser will convert < and > characters as HTMLEntities. For example, the usage in template would be as below,

<p>Interpolated value:</p>
<div >{{htmlSnippet}}</div>
<p>Binding of innerHTML:</p>
<div [innerHTML]="htmlSnippet"></div>

and the property defined in a component.

export class InnerHtmlBindingComponent {
  htmlSnippet = 'Template <script>alert("XSS Attack")</script> <b>Code attached</b>';
}

Even though innerHTML binding create a chance of XSS attack, Angular recognizes the value as unsafe and automatically sanitizes it.

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How do you prevent automatic sanitization?

Sometimes the applications genuinely need to include executable code such as displaying <iframe> from an URL. In this case, you need to prevent automatic sanitization in Angular by saying that you inspected a value, checked how it was generated, and made sure it will always be secure. Basically it involves 2 steps,

1. Inject DomSanitizer: You can inject DomSanitizer in component as parameter in constructor

2. Mark the trusted value by calling some of the below methods

   1. bypassSecurityTrustHtml
   2. bypassSecurityTrustScript
   3. bypassSecurityTrustStyle
   4. bypassSecurityTrustUrl
   5. bypassSecurityTrustResourceUrl

For example,The usage of dangerous url to trusted url would be as below,

constructor(private sanitizer: DomSanitizer) {
  this.dangerousUrl = 'javascript:alert("XSS attack")';
  this.trustedUrl = sanitizer.bypassSecurityTrustUrl(this.dangerousUrl);

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Is safe to use direct DOM API methods in terms of security?

No,the built-in browser DOM APIs or methods don't automatically protect you from security vulnerabilities. In this case it is recommended to use Angular templates instead of directly interacting with DOM. If it is unavoidable then use the built-in Angular sanitization functions.

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What is DOM sanitizer?

DomSanitizer is used to help preventing Cross Site Scripting Security bugs (XSS) by sanitizing values to be safe to use in the different DOM contexts.

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How do you support server side XSS protection in Angular application?

The server-side XSS protection is supported in an angular application by using a templating language that automatically escapes values to prevent XSS vulnerabilities on the server. But don't use a templating language to generate Angular templates on the server side which creates a high risk of introducing template-injection vulnerabilities.

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Is angular prevents http level vulnerabilities?

Angular has built-in support for preventing http level vulnerabilities such as as cross-site request forgery (CSRF or XSRF) and cross-site script inclusion (XSSI). Even though these vulnerabilities need to be mitigated on server-side, Angular provides helpers to make the integration easier on the client side.

1. HttpClient supports a token mechanism used to prevent XSRF attacks
2. HttpClient library recognizes the convention of prefixed JSON responses(which non-executable js code with ")]}',\n" characters) and automatically strips the string ")]}',\n" from all responses before further parsing

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What are Http Interceptors?

Http Interceptors are part of @angular/common/http, which inspect and transform HTTP requests from your application to the server and vice-versa on HTTP responses. These interceptors can perform a variety of implicit tasks, from authentication to logging.

The syntax of HttpInterceptor interface looks like as below,

interface HttpInterceptor {
  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>>
}

You can use interceptors by declaring a service class that implements the intercept() method of the HttpInterceptor interface.

@Injectable()
export class MyInterceptor implements HttpInterceptor {
    constructor() {}
    intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
        ...
    }
}

After that you can use it in your module,

@NgModule({
    ...
    providers: [
        {
            provide: HTTP_INTERCEPTORS,
            useClass: MyInterceptor,
            multi: true
        }
    ]
    ...
})
export class AppModule {}

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What are the applications of HTTP interceptors?

The HTTP Interceptors can be used for different variety of tasks,

1. Authentication
2. Logging
3. Caching
4. Fake backend
5. URL transformation
6. Modifying headers

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Is multiple interceptors supported in Angular?

Yes, Angular supports multiple interceptors at a time. You could define multiple interceptors in providers property:

providers: [
  { provide: HTTP_INTERCEPTORS, useClass: MyFirstInterceptor, multi: true },
  { provide: HTTP_INTERCEPTORS, useClass: MySecondInterceptor, multi: true }
],

The interceptors will be called in the order in which they were provided. i.e, MyFirstInterceptor will be called first in the above interceptors configuration.

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How can I use interceptor for an entire application?

You can use same instance of HttpInterceptors for the entire app by importing the HttpClientModule only in your AppModule, and add the interceptors to the root application injector. For example, let's define a class that is injectable in root application.

@Injectable()
export class MyInterceptor implements HttpInterceptor {
  intercept(
    req: HttpRequest<any>,
    next: HttpHandler
  ): Observable<HttpEvent<any>> {

    return next.handle(req).do(event => {
      if (eventt instanceof HttpResponse) {
           // Code goes here
      }
    });

  }
}

After that import HttpClientModule in AppModule

@NgModule({
  declarations: [AppComponent],
  imports: [BrowserModule, HttpClientModule],
  providers: [
    { provide: HTTP_INTERCEPTORS, useClass: MyInterceptor, multi: true }
  ],
  bootstrap: [AppComponent]
})
export class AppModule {}

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How does Angular simplifies Internationalization?

Angular simplifies the below areas of internationalization,

1. Displaying dates, number, percentages, and currencies in a local format.
2. Preparing text in component templates for translation.
3. Handling plural forms of words.
4. Handling alternative text.

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How do you manually register locale data?

By default, Angular only contains locale data for en-US which is English as spoken in the United States of America . But if you want to set to another locale, you must import locale data for that new locale. After that you can register using registerLocaleData method and the syntax of this method looks like below,

registerLocaleData(data: any, localeId?: any, extraData?: any): void

For example, let us import German locale and register it in the application

import { registerLocaleData } from '@angular/common';
import localeDe from '@angular/common/locales/de';

registerLocaleData(localeDe, 'de');

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What are the four phases of template translation?

The i18n template translation process has four phases:

1. Mark static text messages in your component templates for translation: You can place i18n on every element tag whose fixed text is to be translated. For example, you need i18n attribue for heading as below,

   <h1 i18n>Hello i18n!</h1>

2. Create a translation file: Use the Angular CLI xi18n command to extract the marked text into an industry-standard translation source file. i.e, Open terminal window at the root of the app project and run the CLI command xi18n.

   The above command creates a file named messages.xlf in your project's root directory.

   Note: You can supply command options to change the format, the name, the location, and the source locale of the extracted file.

3. Edit the generated translation file: Translate the extracted text into the target language. In this step, create a localization folder (such as locale)under root directory(src) and then create target language translation file by copying and renaming the default messages.xlf file. You need to copy source text node and provide the translation under target tag. For example, create the translation file(messages.de.xlf) for German language

   <trans-unit id="greetingHeader" datatype="html">
     <source>Hello i18n!</source>
     <target>Hallo i18n !</target>
     <note priority="1" from="description">A welcome header for this sample</note>
     <note priority="1" from="meaning">welcome message</note>
   </trans-unit>

4. Merge the completed translation file into the app: You need to use Angular CLI build command to compile the app, choosing a locale-specific configuration, or specifying the following command options.

   1. --i18nFile=path to the translation file
   2. --i18nFormat=format of the translation file
   3. --i18nLocale= locale id

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What is the purpose of i18n attribute?

The Angular i18n attribute marks translatable content. It is a custom attribute, recognized by Angular tools and compilers. The compiler removes it after translation.

Note: Remember that i18n is not an Angular directive.

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What is the purpose of custom id?

When you change the translatable text, the Angular extractor tool generates a new id for that translation unit. Because of this behavior, you must then update the translation file with the new id every time.

For example, the translation file messages.de.xlf.html has generated trans-unit for some text message as below

<trans-unit id="827wwe104d3d69bf669f823jjde888" datatype="html">

You can avoid this manual update of id attribute by specifying a custom id in the i18n attribute by using the prefix @@.

<h1 i18n="@@welcomeHeader">Hello i18n!</h1>

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What happens if the custom id is not unique?

You need to define custom ids as unique. If you use the same id for two different text messages then only the first one is extracted. But its translation is used in place of both original text messages.

For example, let's define same custom id myCustomId for two messages,

<h2 i18n="@@myCustomId">Good morning</h3>
<!-- ... -->
<h2 i18n="@@myCustomId">Good night</p>

and the translation unit generated for first text in for German language as

<trans-unit id="myId" datatype="html">
  <source>Good morning</source>
  <target state="new">Guten Morgen</target>
</trans-unit>

Since custom id is the same, both of the elements in the translation contain the same text as below

<h2>Guten Morgen</h2>
<h2>Guten Morgen</h2>

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Can I translate text without creating an element?

Yes, you can achieve using <ng-container> attribute. Normally you need to wrap a text content with i18n attribute for the translation. But if you don't want to create a new DOM element just for the sake of translation, you can wrap the text in an element.

<ng-container i18n>I'm not using any DOM element for translation</ng-container>

Remember that <ng-container> is transformed into an html comment

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How can I translate attribute?

You can translate attributes by attaching i18n-x attribute where x is the name of the attribute to translate. For example, you can translate image title attribute as below,

<img [src]="example" i18n-title title="Internationlization" />

By the way, you can also assign meaning, description and id with the i18n-x="|@@" syntax.

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List down the pluralization categories?

Pluralization has below categories depending on the language.

1. =0 (or any other number)
2. zero
3. one
4. two
5. few
6. many
7. other

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What is select ICU expression?

ICU expression is is similar to the plural expressions except that you choose among alternative translations based on a string value instead of a number. Here you define those string values.

Let's take component binding with residenceStatus property which has "citizen", "permanent resident" and "foreigner" possible values and the message maps those values to the appropriate translations.

<span i18n>The person is {residenceStatus, select, citizen {citizen} permanent resident {permanentResident} foreigner {foreigner}}</span>

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How do you report missing translations?

By default, When translation is missing, it generates a warning message such as "Missing translation for message 'somekey'". But you can configure with a different level of message in Angular compiler as below,

1. Error: It throws an error. If you are using AOT compilation, the build will fail. But if you are using JIT compilation, the app will fail to load.
2. Warning (default): It shows a 'Missing translation' warning in the console or shell.
3. Ignore: It doesn't do anything.

If you use AOT compiler then you need to perform changes in configurations section of your Angular CLI configuration file, angular.json.

"configurations": {
  ...
  "de": {
    ...
    "i18nMissingTranslation": "error"
  }
}

If you use the JIT compiler, specify the warning level in the compiler config at bootstrap by adding the 'MissingTranslationStrategy' property as below,

import { MissingTranslationStrategy } from '@angular/core';
import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';
import { AppModule } from './app/app.module';

platformBrowserDynamic().bootstrapModule(AppModule, {
  missingTranslation: MissingTranslationStrategy.Error,
  providers: [
    // ...
  ]
});

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How do you provide build configuration for multiple locales?

You can provide build configuration such as translation file path, name, format and application url in configuration settings of Angular.json file. For example, the German version of your application configured the build as follows,

"configurations": {
  "de": {
    "aot": true,
    "outputPath": "dist/my-project-de/",
    "baseHref": "/fr/",
    "i18nFile": "src/locale/messages.de.xlf",
    "i18nFormat": "xlf",
    "i18nLocale": "de",
    "i18nMissingTranslation": "error",
  }

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What is an angular library?

An Angular library is an Angular project that differs from an app in that it cannot run on its own. It must be imported and used in an app. For example, you can import or add service worker library to an Angular application which turns an application into a Progressive Web App (PWA).

Note: You can create own third party library and publish it as npm package to be used in an Application.

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What is AOT compiler?

The AOT compiler is part of a build process that produces a small, fast, ready-to-run application package, typically for production. It converts your Angular HTML and TypeScript code into efficient JavaScript code during the build phase before the browser downloads and runs that code.

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How do you select an element in component template?

You can control any DOM element via ElementRef by injecting it into your component's constructor. i.e, The component should have constructor with ElementRef parameter,

constructor(myElement: ElementRef) {
   el.nativeElement.style.backgroundColor = 'yellow';
}

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What is TestBed?

TestBed is an api for writing unit tests for Angular applications and it's libraries. Even though We still write our tests in Jasmine and run using Karma, this API provides an easier way to create components, handle injection, test asynchronous behaviour and interact with our application.

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What is protractor?

Protractor is an end-to-end test framework for Angular and AngularJS applications. It runs tests against your application running in a real browser, interacting with it as a user would.

npm install -g protractor

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What is collection?

Collection is a set of related schematics collected in an npm package. For example, @schematics/angular collection is used in Angular CLI to apply transforms to a web-app project. You can create your own schematic collection for customizing angular projects.

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How do you create schematics for libraries?

You can create your own schematic collections to integrate your library with the Angular CLI. These collections are classified as 3 main schematics,

1. Add schematics: These schematics are used to install library in an Angular workspace using ng add command. For example, @angular/material schematic tells the add command to install and set up Angular Material and theming.
2. Generate schematics: These schematics are used to modify projects, add configurations and scripts, and scaffold artifacts in library using ng generate command. For example, @angular/material generation schematic supplies generation schematics for the UI components. Let's say the table component is generated using ng generate @angular/material:table .
3. Update schematics: These schematics are used to update library's dependencies and adjust for breaking changes in a new library release using ng update command. For example, @angular/material update schematic updates material and cdk dependencies using ng update @angular/material command.

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How do you use jquery in Angular?

You can use jquery in Angular using 3 simple steps,

1. Install the dependency: At first, install the jquery dependency using npm

      npm install --save jquery

2. Add the jquery script: In Angular-CLI project, add the relative path to jquery in the angular.json file.

   "scripts": [
      "node_modules/jquery/dist/jquery.min.js"
   ]

3. Start using jquery: Define the element in template. Whereas declare the jquery variable and apply CSS classes on the element.

   <div id="elementId">
     <h1>JQuery integration</h1>
   </div>

   import {Component, OnInit} from '@angular/core';
   
   declare var $: any; // (or) import * as $ from 'jquery';
   
   @Component({
     selector: 'app-root',
     templateUrl: './app.component.html',
     styleUrls: ['./app.component.css']
   })
   export class AppComponent implements OnInit {
     ngOnInit(): void {
       $(document).ready(() => {
         $('#elementId').css({'text-color': 'blue', 'font-size': '150%'});
       });
     }
   }

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What is the reason for No provider for HTTP exception?

This exception is due to missing HttpClientModule in your module. You just need to import in module as below,

import { HttpClientModule } from '@angular/common/http';

@NgModule({
  imports: [
    BrowserModule,
    HttpClientModule,
  ],
  declarations: [ AppComponent ],
  bootstrap:    [ AppComponent ]
})
export class AppModule { }

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What is router state?

The RouteState is an interface which represents the state of the router as a tree of activated routes.

interface RouterState extends Tree {
  snapshot: RouterStateSnapshot
  toString(): string
}

You can access the current RouterState from anywhere in the Angular app using the Router service and the routerState property.

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How can I use SASS in angular project?

When you are creating your project with angular cli, you can use ng newcommand. It generates all your components with predefined sass files.

ng new My_New_Project --style=sass

But if you are changing your existing style in your project then use ng set command,

ng set defaults.styleExt scss

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What is the purpose of hidden property?

The hidden property is used to show or hide the associated DOM element, based on an expression. It can be compared close to ng-show directive in AngularJS. Let's say you want to show user name based on the availability of user using hidden property.

<div [hidden]="!user.name">
  My name is: {{user.name}}
</div>

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What is the difference between ngIf and hidden property?

The main difference is that *ngIf will remove the element from the DOM, while [hidden] actually plays with the CSS style by setting display:none. Generally it is expensive to add and remove stuff from the DOM for frequent actions.

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What is slice pipe?

The slice pipe is used to create a new Array or String containing a subset (slice) of the elements. The syntax looks like as below,

{{ value_expression | slice : start [ : end ] }}

For example, you can provide 'hello' list based on a greeting array,

@Component({
  selector: 'list-pipe',
  template: `<ul>
    <li *ngFor="let i of greeting | slice:0:5">{{i}}</li>
  </ul>`
})
export class PipeListComponent {
  greeting: string[] = ['h', 'e', 'l', 'l', 'o', 'm','o', 'r', 'n', 'i', 'n', 'g'];
}

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What is index property in ngFor directive?

The index property of the NgFor directive is used to return the zero-based index of the item in each iteration. You can capture the index in a template input variable and use it in the template.

For example, you can capture the index in a variable named indexVar and displays it with the todo's name using ngFor directive as below.

<div *ngFor="let todo of todos; let i=index">{{i + 1}} - {{todo.name}}</div>

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What is the purpose of ngFor trackBy?

The main purpose of using *ngFor with trackBy option is performance optimization. Normally if you use NgFor with large data sets, a small change to one item by removing or adding an item, can trigger a cascade of DOM manipulations. In this case, Angular sees only a fresh list of new object references and to replace the old DOM elements with all new DOM elements. You can help Angular to track which items added or removed by providing a trackBy function which takes the index and the current item as arguments and needs to return the unique identifier for this item.

For example, lets set trackBy to the trackByTodos() method

<div *ngFor="let todo of todos; trackBy: trackByTodos">
  ({{todo.id}}) {{todo.name}}
</div>

and define the trackByTodos method,

trackByTodos(index: number, item: Todo): number { return todo.id; }

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What is the purpose of ngSwitch directive?

NgSwitch directive is similar to JavaScript switch statement which displays one element from among several possible elements, based on a switch condition. In this case only the selected element placed into the DOM. It has been used along with NgSwitch, NgSwitchCase and NgSwitchDefault directives.

For example, let's display the browser details based on selected browser using ngSwitch directive.

<div [ngSwitch]="currentBrowser.name">
  <chrome-browser    *ngSwitchCase="'chrome'"    [item]="currentBrowser"></chrome-browser>
  <firefox-browser   *ngSwitchCase="'firefox'"     [item]="currentBrowser"></firefox-browser>
  <opera-browser     *ngSwitchCase="'opera'"  [item]="currentBrowser"></opera-browser>
  <safari-browser     *ngSwitchCase="'safari'"   [item]="currentBrowser"></safari-browser>
  <ie-browser  *ngSwitchDefault           [item]="currentItem"></ie-browser>
</div>

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Is it possible to do aliasing for inputs and outputs?

Yes, it is possible to do aliasing for inputs and outputs in two ways.

1. Aliasing in metadata: The inputs and outputs in the metadata aliased using a colon-delimited (:) string with the directive property name on the left and the public alias on the right. i.e. It will be in the format of propertyName:alias.

   inputs: ['input1: buyItem'],
   outputs: ['outputEvent1: completedEvent']

2. Aliasing with @Input()/@Output() decorator: The alias can be specified for the property name by passing the alias name to the @Input()/@Output() decorator.i.e. It will be in the form of @Input(alias) or @Output(alias).

   @Input('buyItem') input1: string;
   @Output('completedEvent') outputEvent1 = new EventEmitter<string>();

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What is safe navigation operator?

The safe navigation operator(?)(or known as Elvis Operator) is used to guard against null and undefined values in property paths when you are not aware whether a path exists or not. i.e. It returns value of the object path if it exists, else it returns the null value.

For example, you can access nested properties of a user profile easily without null reference errors as below,

<p>The user firstName is: {{user?.fullName.firstName}}</p>

Using this safe navigation operator, Angular framework stops evaluating the expression when it hits the first null value and renders the view without any errors.

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Is any special configuration required for Angular9?

You don't need any special configuration. In Angular9, the Ivy renderer is the default Angular compiler. Even though Ivy is available Angular8 itself, you had to configure it in tsconfig.json file as below,

"angularCompilerOptions": {    "enableIvy": true  }

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What are type safe TestBed API changes in Angular9?

Angular 9 provides type safe changes in TestBed API changes by replacing the old get function with the new inject method. Because TestBed.get method is not type-safe. The usage would be as below,

TestBed.get(ChangeDetectorRef) // returns any. It is deprecated now.

TestBed.inject(ChangeDetectorRef) // returns ChangeDetectorRef

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Is mandatory to pass static flag for ViewChild?

In Angular 8, the static flag is required for ViewChild. Whereas in Angular9, you no longer need to pass this property. Once you updated to Angular9 using ng update, the migration will remove { static: false } script everywhere.

@ViewChild(ChildDirective) child: ChildDirective; // Angular9 usage
@ViewChild(ChildDirective, { static: false }) child: ChildDirective; //Angular8 usage

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What are the list of template expression operators?

The Angular template expression language supports three special template expression operators.

1. Pipe operator
2. Safe navigation operator
3. Non-null assertion operator

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What is the precedence between pipe and ternary operators?

The pipe operator has a higher precedence than the ternary operator (?:). For example, the expression first ? second : third | fourth is parsed as first ? second : (third | fourth).

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What is an entry component?

An entry component is any component that Angular loads imperatively(i.e, not referencing it in the template) by type. Due to this behavior, they can’t be found by the Angular compiler during compilation. These components created dynamically with ComponentFactoryResolver.

Basically, there are two main kinds of entry components which are following -

1. The bootstrapped root component
2. A component you specify in a route

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What is a bootstrapped component?

A bootstrapped component is an entry component that Angular loads into the DOM during the bootstrap process or application launch time. Generally, this bootstrapped or root component is named as AppComponent in your root module using bootstrap property as below.

@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    BrowserModule,
    FormsModule,
    HttpClientModule,
    AppRoutingModule
  ],
  providers: [],
  bootstrap: [AppComponent] // bootstrapped entry component need to be declared here
})

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How do you manually bootstrap an application?

You can use ngDoBootstrap hook for a manual bootstrapping of the application instead of using bootstrap array in @NgModule annotation. This hook is part of DoBootstap interface.

interface DoBootstrap {
  ngDoBootstrap(appRef: ApplicationRef): void
}

The module needs to be implement the above interface to use the hook for bootstrapping.

class AppModule implements DoBootstrap {
  ngDoBootstrap(appRef: ApplicationRef) {
    appRef.bootstrap(AppComponent); // bootstrapped entry component need to be passed
  }
}

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Is it necessary for bootstrapped component to be entry component?

Yes, the bootstrapped component needs to be an entry component. This is because the bootstrapping process is an imperative process.

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What is a routed entry component?

The components referenced in router configuration are called as routed entry components. This routed entry component defined in a route definition as below,

const routes: Routes = [
  {
    path: '',
    component: TodoListComponent // router entry component
  }
];

Since router definition requires you to add the component in two places (router and entryComponents), these components are always entry components.

Note: The compilers are smart enough to recognize a router definition and automatically add the router component into entryComponents.

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Why is not necessary to use entryComponents array every time?

Most of the time, you don't need to explicity to set entry components in entryComponents array of ngModule decorator. Because angular adds components from both @NgModule.bootstrap and route definitions to entry components automatically.

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Do I still need to use entryComponents array in Angular9?

No. In previous angular releases, the entryComponents array of ngModule decorator is used to tell the compiler which components would be created and inserted dynamically in the view. In Angular9, this is not required anymore with Ivy.

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Is it all components generated in production build?

No, only the entry components and template components appears in production builds. If a component isn't an entry component and isn't found in a template, the tree shaker will throw it away. Due to this reason, make sure to add only true entry components to reduce the bundle size.

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What is Angular compiler?

The Angular compiler is used to convert the application code into JavaScript code. It reads the template markup, combines it with the corresponding component class code, and emits component factories which creates JavaScript representation of the component along with elements of @Component metadata.

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What is the role of ngModule metadata in compilation process?

The @NgModule metadata is used to tell the Angular compiler what components to be compiled for this module and how to link this module with other modules.

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How does angular finds components, directives and pipes?

The Angular compiler finds a component or directive in a template when it can match the selector of that component or directive in that template. Whereas it finds a pipe if the pipe's name appears within the pipe syntax of the template HTML.

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Give few examples for NgModules?

The Angular core libraries and third-party libraries are available as NgModules.

1. Angular libraries such as FormsModule, HttpClientModule, and RouterModule are NgModules.
2. Many third-party libraries such as Material Design, Ionic, and AngularFire2 are NgModules.

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What are feature modules?

Feature modules are NgModules, which are used for the purpose of organizing code. The feature module can be created with Angular CLI using the below command in the root directory,

ng generate module MyCustomFeature //

Angular CLI creates a folder called my-custom-feature with a file inside called my-custom-feature.module.ts with the following contents

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';

@NgModule({
  imports: [
    CommonModule
  ],
  declarations: []
})
export class MyCustomFeature { }

Note: The "Module" suffix shouldn't present in the name because the CLI appends it.

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What are the imported modules in CLI generated feature modules?

In the CLI generated feature module, there are two JavaScript import statements at the top of the file

1. NgModule: InOrder to use the @NgModule decorator
2. CommonModule: It provides many common directives such as ngIf and ngFor.

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What are the differences between ngmodule and javascript module?

Below are the main differences between Angular NgModule and javascript module,

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What are the possible errors with declarations?

There are two common possible errors with declarations array,

1. If you use a component without declaring it, Angular returns an error message.
2. If you try to declare the same class in more than one module then compiler emits an error.

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What are the steps to use declaration elements?

Below are the steps to be followed to use declaration elements.

1. Create the element(component, directive and pipes) and export it from the file where you wrote it
2. Import it into the appropriate module.
3. Declare it in the @NgModule declarations array.

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What happens if browserModule used in feature module?

If you do import BrowserModule into a lazy loaded feature module, Angular returns an error telling you to use CommonModule instead. Because BrowserModule’s providers are for the entire app so it should only be in the root module, not in feature module. Whereas Feature modules only need the common directives in CommonModule.

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What are the types of feature modules?

Below are the five categories of feature modules,

1. Domain: Deliver a user experience dedicated to a particular application domain(For example, place an order, registration etc)
2. Routed: These are domain feature modules whose top components are the targets of router navigation routes.
3. Routing: It provides routing configuration for another module.
4. Service: It provides utility services such as data access and messaging(For example, HttpClientModule)
5. Widget: It makes components, directives, and pipes available to external modules(For example, third-party libraries such as Material UI)

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What is a provider?

A provider is an instruction to the Dependency Injection system on how to obtain a value for a dependency(aka services created). The service can be provided using Angular CLI as below,

ng generate service my-service

The created service by CLI would be as below,

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root', //Angular provide the service in root injector
})
export class MyService {
}

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What is the recommendation for provider scope?

You should always provide your service in the root injector unless there is a case where you want the service to be available only if you import a particular @NgModule.

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How do you restrict provider scope to a module?

It is possible to restrict service provider scope to a specific module instead making available to entire application. There are two possible ways to do it.

1. Using providedIn in service:

   import { Injectable } from '@angular/core';
   import { SomeModule } from './some.module';
   
   @Injectable({
     providedIn: SomeModule,
   })
   export class SomeService {
   }

2. Declare provider for the service in module:

   import { NgModule } from '@angular/core';
   
   import { SomeService } from './some.service';
   
   @NgModule({
     providers: [SomeService],
   })
   export class SomeModule {
   }

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How do you provide a singleton service?

There are two possible ways to provide a singleton service.

1. Set the providedIn property of the @Injectable() to "root". This is the preferred way(starting from Angular 6.0) of creating a singleton service since it makes your services tree-shakable.

   import { Injectable } from '@angular/core';
   
   @Injectable({
     providedIn: 'root',
   })
   export class MyService {
   }

2. Include the service in root module or in a module that is only imported by root module. It has been used to register services before Angular 6.0.

   @NgModule({
     ...
     providers: [MyService],
     ...
   })

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What are the different ways to remove duplicate service registration?

If a module defines provides and declarations then loading the module in multiple feature modules will duplicate the registration of the service. Below are the different ways to prevent this duplicate behavior.

1. Use the providedIn syntax instead of registering the service in the module.
2. Separate your services into their own module.
3. Define forRoot() and forChild() methods in the module.

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How does forRoot method helpful to avoid duplicate router instances?

If the RouterModule module didn’t have forRoot() static method then each feature module would instantiate a new Router instance, which leads to broken application due to duplicate instances. After using forRoot() method, the root application module imports RouterModule.forRoot(...) and gets a Router, and all feature modules import RouterModule.forChild(...) which does not instantiate another Router.

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What is a shared module?

The Shared Module is the module in which you put commonly used directives, pipes, and components into one module that is shared(import it) throughout the application.

For example, the below shared module imports CommonModule, FormsModule for common directives and components, pipes and directives based on the need,

import { CommonModule } from '@angular/common';
import { NgModule } from '@angular/core';
import { FormsModule } from '@angular/forms';
import { UserComponent } from './user.component';
import { NewUserDirective } from './new-user.directive';
import { OrdersPipe } from './orders.pipe';

@NgModule({
 imports:      [ CommonModule ],
 declarations: [ UserComponent, NewUserDirective, OrdersPipe ],
 exports:      [ UserComponent, NewUserDirective, OrdersPipe,
                 CommonModule, FormsModule ]
})
export class SharedModule { }

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Can I share services using modules?

No, it is not recommended to share services by importing module. i.e Import modules when you want to use directives, pipes, and components only. The best approach to get a hold of shared services is through 'Angular dependency injection' because importing a module will result in a new service instance.

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How do you get current direction for locales?

In Angular 9.1, the API method getLocaleDirection can be used to get the current direction in your app. This method is useful to support Right to Left locales for your Internationalization based applications.

import { getLocaleDirection, registerLocaleData } from '@angular/common';
import { LOCALE_ID } from '@angular/core';
import localeAr from '@angular/common/locales/ar';

  ...

  constructor(@Inject(LOCALE_ID) locale) {

    const directionForLocale = getLocaleDirection(locale); // Returns 'rtl' or 'ltr' based on the current locale
    registerLocaleData(localeAr, 'ar-ae');
    const direction = getLocaleDirection('ar-ae'); // Returns 'rtl'

    // Current direction is used to provide conditional logic here
  }

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What is ngcc?

The ngcc(Angular Compatibility Compiler) is a tool which upgrades node_module compiled with non-ivy ngc into ivy compliant format. The postinstall script from package.json will make sure your node_modules will be compatible with the Ivy renderer.

"scripts": {
   "postinstall": "ngcc"
}

Whereas, Ivy compiler (ngtsc), which compiles Ivy-compatible code.

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What classes should not be added to declarations?

The below class types shouldn't be added to declarations

1. A class which is already declared in any another module.
2. Directives imported from another module.
3. Module classes.
4. Service classes.
5. Non-Angular classes and objects, such as strings, numbers, functions, entity models, configurations, business logic, and helper classes.

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What is NgZone?

Angular provides a service called NgZone which creates a zone named angular to automatically trigger change detection when the following conditions are satisfied.

1. When a sync or async function is executed.
2. When there is no microTask scheduled.

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What is NoopZone?

Zone is loaded/required by default in Angular applications and it helps Angular to know when to trigger the change detection. This way, it make sures developers focus on application development rather core part of Angular. You can also use Angular without Zone but the change detection need to be implemented on your own and noop zone need to be configured in bootstrap process. Let's follow the below two steps to remove zone.js,

1. Remove the zone.js import from polyfills.ts.

   /***************************************************************************************************
    * Zone JS is required by default for Angular itself.
    */
   // import 'zone.js/dist/zone';  // Included with Angular CLI.

2. Bootstrap Angular with noop zone in src/main.ts.

   platformBrowserDynamic().bootstrapModule(AppModule, {ngZone: 'noop'})
     .catch(err => console.error(err));

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How do you create displayBlock components?

By default, Angular CLI creates components in an inline displayed mode(i.e, display:inline). But it is possible to create components with display: block style using displayBlock option,

ng generate component my-component --displayBlock

(OR) the option can be turned on by default in Angular.json with schematics.@schematics/angular:component.displayBlock key value as true.

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What are the possible data update scenarios for change detection?

The change detection works in the following scenarios where the data changes needs to update the application HTML.

1. Component initialization: While bootstrapping the Angular application, Angular triggers the ApplicationRef.tick() to call change detection and View Rendering.
2. Event listener: The DOM event listener can update the data in an Angular component and trigger the change detection too.

   @Component({
     selector: 'app-event-listener',
     template: `
       <button (click)="onClick()">Click</button>
       {{message}}`
   })
   export class EventListenerComponent {
     message = '';
   
     onClick() {
       this.message = 'data updated';
     }
   }

3. HTTP Data Request: You can get data from a server through an HTTP request

   data = 'default value';
   constructor(private httpClient: HttpClient) {}
   
     ngOnInit() {
       this.httpClient.get(this.serverUrl).subscribe(response => {
         this.data = response.data; // change detection will happen automatically
       });
     }

4. Macro tasks setTimeout() or setInterval(): You can update the data in the callback function of setTimeout or setInterval

   data = 'default value';
   
     ngOnInit() {
       setTimeout(() => {
         this.data = 'data updated'; // Change detection will happen automatically
       });
     }

5. Micro tasks Promises: You can update the data in the callback function of promise

   data = 'initial value';
   
     ngOnInit() {
       Promise.resolve(1).then(v => {
         this.data = v; // Change detection will happen automatically
       });
     }

6. Async operations like Web sockets and Canvas: The data can be updated asynchronously using WebSocket.onmessage() and Canvas.toBlob().

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What is a zone context?

Execution Context is an abstract concept that holds information about the environment within the current code being executed. A zone provides an execution context that persists across asynchronous operations is called as zone context. For example, the zone context will be same in both outside and inside setTimeout callback function,

zone.run(() => {
  // outside zone
  expect(zoneThis).toBe(zone);
  setTimeout(function() {
    // the same outside zone exist here
    expect(zoneThis).toBe(zone);
  });
});

The current zone is retrieved through Zone.current.

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What are the lifecycle hooks of a zone?

There are four lifecycle hooks for asynchronous operations from zone.js.

1. onScheduleTask: This hook triggers when a new asynchronous task is scheduled. For example, when you call setTimeout()

   onScheduleTask: function(delegate, curr, target, task) {
       console.log('new task is scheduled:', task.type, task.source);
       return delegate.scheduleTask(target, task);
     }

2. onInvokeTask: This hook triggers when an asynchronous task is about to execute. For example, when the callback of setTimeout() is about to execute.

   onInvokeTask: function(delegate, curr, target, task, applyThis, applyArgs) {
       console.log('task will be invoked:', task.type, task.source);
       return delegate.invokeTask(target, task, applyThis, applyArgs);
     }

3. onHasTask: This hook triggers when the status of one kind of task inside a zone changes from stable(no tasks in the zone) to unstable(a new task is scheduled in the zone) or from unstable to stable.

     onHasTask: function(delegate, curr, target, hasTaskState) {
       console.log('task state changed in the zone:', hasTaskState);
       return delegate.hasTask(target, hasTaskState);
     }

4. onInvoke: This hook triggers when a synchronous function is going to execute in the zone.

   onInvoke: function(delegate, curr, target, callback, applyThis, applyArgs) {
       console.log('the callback will be invoked:', callback);
       return delegate.invoke(target, callback, applyThis, applyArgs);
     }

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What are the methods of NgZone used to control change detection?

NgZone service provides a run() method that allows you to execute a function inside the angular zone. This function is used to execute third party APIs which are not handled by Zone and trigger change detection automatically at the correct time.

export class AppComponent implements OnInit {
  constructor(private ngZone: NgZone) {}
  ngOnInit() {
    // use ngZone.run() to make the asynchronous operation in the angular zone
    this.ngZone.run(() => {
      someNewAsyncAPI(() => {
        // update the data of the component
      });
    });
  }
}

Whereas runOutsideAngular() method is used when you don't want to trigger change detection.

export class AppComponent implements OnInit {
  constructor(private ngZone: NgZone) {}
  ngOnInit() {
    // Use this method when you know no data will be updated
    this.ngZone.runOutsideAngular(() => {
      setTimeout(() => {
        // update component data and don't trigger change detection
      });
    });
  }
}

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How do you change the settings of zonejs?

You can change the settings of zone by configuring them in a separate file and import it just after zonejs import. For example, you can disable the requestAnimationFrame() monkey patch to prevent change detection for no data update as one setting and prevent DOM events(a mousemove or scroll event) to trigger change detection. Let's say the new file named zone-flags.js,

// disable patching requestAnimationFrame
(window as any).__Zone_disable_requestAnimationFrame = true;

// disable patching specified eventNames
(window as any).__zone_symbol__UNPATCHED_EVENTS = ['scroll', 'mousemove'];

The above configuration file can be imported in a polyfill.ts file as below,

/***************************************************************************************************
 * Zone JS is required by default for Angular.
 */
import `./zone-flags`;
import 'zone.js/dist/zone';  // Included with Angular CLI.

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How do you trigger an animation?

Angular provides a trigger() function for animation in order to collect the states and transitions with a specific animation name, so that you can attach it to the triggering element in the HTML template. This function watch for changes and trigger initiates the actions when a change occurs. For example, let's create trigger named upDown, and attach it to the button element.

content_copy
@Component({
  selector: 'app-up-down',
  animations: [
    trigger('upDown', [
      state('up', style({
        height: '200px',
        opacity: 1,
        backgroundColor: 'yellow'
      })),
      state('down', style({
        height: '100px',
        opacity: 0.5,
        backgroundColor: 'green'
      })),
      transition('up => down', [
        animate('1s')
      ]),
      transition('down => up', [
        animate('0.5s')
      ]),
    ]),
  ],
  templateUrl: 'up-down.component.html',
  styleUrls: ['up-down.component.css']
})
export class UpDownComponent {
  isUp = true;

  toggle() {
    this.isUp = !this.isUp;
  }

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How do you configure injectors with providers at different levels?

You can configure injectors with providers at different levels of your application by setting a metadata value. The configuration can happen in one of three places,

1. In the @Injectable() decorator for the service itself
2. In the @NgModule() decorator for an NgModule
3. In the @Component() decorator for a component

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Is it mandatory to use injectable on every service class?

No. The @Injectable() decorator is not strictly required if the class has other Angular decorators on it or does not have any dependencies. But the important thing here is any class that is going to be injected with Angular is decorated. i.e, If we add the decorator, the metadata design:paramtypes is added, and the dependency injection can do it's job. That is the exact reason to add the @Injectable() decorator on a service if this service has some dependencies itself. For example, Let's see the different variations of AppService in a root component,

1. The below AppService can be injected in AppComponent without any problems. This is because there are no dependency services inside AppService.

   export class AppService {
     constructor() {
       console.log('A new app service');
     }
   }

2. The below AppService with dummy decorator and httpService can be injected in AppComponent without any problems. This is because meta information is generated with dummy decorator.

   function SomeDummyDecorator() {
     return (constructor: Function) => console.log(constructor);
   }
   
   @SomeDummyDecorator()
   export class AppService {
     constructor(http: HttpService) {
       console.log(http);
     }
   }

and the generated javascript code of above service has meta information about HttpService, js var AppService = (function () { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], AppService); return AppService; }()); exports.AppService = AppService; 3. The below AppService with @injectable decorator and httpService can be injected in AppComponent without any problems. This is because meta information is generated with Injectable decorator. js @Injectable({ providedIn: 'root', }) export class AppService { constructor(http: HttpService) { console.log(http); } } ⬆ Back to Top

What is an optional dependency?

The optional dependency is a parameter decorator to be used on constructor parameters, which marks the parameter as being an optional dependency. Due to this, the DI framework provides null if the dependency is not found. For example, If you don't register a logger provider anywhere, the injector sets the value of logger(or logger service) to null in the below class.

import { Optional } from '@angular/core';

constructor(@Optional() private logger?: Logger) {
  if (this.logger) {
    this.logger.log('This is an optional dependency message');
  } else {
    console.log('The logger is not registered');
  }
}

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What are the types of injector hierarchies?

There are two types of injector hierarchies in Angular

1. ModuleInjector hierarchy: It configure on a module level using an @NgModule() or @Injectable() annotation.
2. ElementInjector hierarchy: It created implicitly at each DOM element. Also it is empty by default unless you configure it in the providers property on @Directive() or @Component().

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What are reactive forms?

Reactive forms is a model-driven approach for creating forms in a reactive style(form inputs changes over time). These are built around observable streams, where form inputs and values are provided as streams of input values. Let's follow the below steps to create reactive forms,

1. Register the reactive forms module which declares reactive-form directives in your app

   import { ReactiveFormsModule } from '@angular/forms';
   
   @NgModule({
     imports: [
       // other imports ...
       ReactiveFormsModule
     ],
   })
   export class AppModule { }

2. Create a new FormControl instance and save it in the component.

   import { Component } from '@angular/core';
   import { FormControl } from '@angular/forms';
   
   @Component({
     selector: 'user-profile',
     styleUrls: ['./user-profile.component.css']
   })
   export class UserProfileComponent {
     userName = new FormControl('');
   }

3. Register the FormControl in the template.

   <label>
     User name:
     <input type="text" [formControl]="userName">
   </label>

Finally, the component with reactive form control appears as below, ```js import { Component } from '@angular/core'; import { FormControl } from '@angular/forms';

@Component({
  selector: 'user-profile',
  styleUrls: ['./user-profile.component.css']
  template: `
     <label>
       User name:
       <input type="text" [formControl]="userName">
     </label>
  `
})
export class UserProfileComponent {
  userName = new FormControl('');
}
```

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What are dynamic forms?

Dynamic forms is a pattern in which we build a form dynamically based on metadata that describes a business object model. You can create them based on reactive form API. ⬆ Back to Top

What are template driven forms?

Template driven forms are model-driven forms where you write the logic, validations, controls etc, in the template part of the code using directives. They are suitable for simple scenarios and uses two-way binding with [(ngModel)] syntax. For example, you can create register form easily by following the below simple steps,

1. Import the FormsModule into the Application module's imports array

      import { BrowserModule } from '@angular/platform-browser';
      import { NgModule } from '@angular/core';
      import {FormsModule} from '@angular/forms'
      import { RegisterComponent } from './app.component';
      @NgModule({
        declarations: [
          RegisterComponent,
        ],
        imports: [
          BrowserModule,
          FormsModule
        ],
        providers: [],
        bootstrap: [RegisterComponent]
      })
      export class AppModule { }

2. Bind the form from template to the component using ngModel syntax

   <input type="text" class="form-control" id="name"
          required
          [(ngModel)]="model.name" name="name">

3. Attach NgForm directive to the tag in order to create FormControl instances and register them

   <form #registerForm="ngForm">

4. Apply the validation message for form controls

   <label for="name">Name</label>
   <input type="text" class="form-control" id="name"
          required
          [(ngModel)]="model.name" name="name"
          #name="ngModel">
   <div [hidden]="name.valid || name.pristine"
        class="alert alert-danger">
     Please enter your name
   </div>

5. Let's submit the form with ngSubmit directive and add type="submit" button at the bottom of the form to trigger form submit.

   <form (ngSubmit)="onSubmit()" #heroForm="ngForm">
   // Form goes here
   <button type="submit" class="btn btn-success" [disabled]="!registerForm.form.valid">Submit</button>

Finally, the completed template-driven registration form will be appeared as follow.

```html
<div class="container">
    <h1>Registration Form</h1>
    <form (ngSubmit)="onSubmit()" #registerForm="ngForm">
      <div class="form-group">
        <label for="name">Name</label>
        <input type="text" class="form-control" id="name"
               required
               [(ngModel)]="model.name" name="name"
               #name="ngModel">
        <div [hidden]="name.valid || name.pristine"
             class="alert alert-danger">
          Please enter your name
        </div>
      </div>
            <button type="submit" class="btn btn-success" [disabled]="!registerForm.form.valid">Submit</button>
    </form>
</div>
```

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What are the differences between reactive forms and template driven forms?

Below are the main differences between reactive forms and template driven forms

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What are the different ways to group form controls?

Reactive forms provide two ways of grouping multiple related controls.

1. FormGroup: It defines a form with a fixed set of controls those can be managed together in an one object. It has same properties and methods similar to a FormControl instance. This FormGroup can be nested to create complex forms as below.

   import { Component } from '@angular/core';
   import { FormGroup, FormControl } from '@angular/forms';
   
   @Component({
     selector: 'user-profile',
     templateUrl: './user-profile.component.html',
     styleUrls: ['./user-profile.component.css']
   })
   export class UserProfileComponent {
     userProfile = new FormGroup({
       firstName: new FormControl(''),
       lastName: new FormControl(''),
       address: new FormGroup({
             street: new FormControl(''),
             city: new FormControl(''),
             state: new FormControl(''),
             zip: new FormControl('')
           })
     });
   
     onSubmit() {
       // Store this.userProfile.value in DB
     }
   }

   <form [formGroup]="userProfile" (ngSubmit)="onSubmit()">
   
     <label>
       First Name:
       <input type="text" formControlName="firstName">
     </label>
   
     <label>
       Last Name:
       <input type="text" formControlName="lastName">
     </label>
   
     <div formGroupName="address">
       <h3>Address</h3>
   
       <label>
         Street:
         <input type="text" formControlName="street">
       </label>
   
       <label>
         City:
         <input type="text" formControlName="city">
       </label>
   
       <label>
         State:
         <input type="text" formControlName="state">
       </label>
   
       <label>
         Zip Code:
         <input type="text" formControlName="zip">
       </label>
      </div>
       <button type="submit" [disabled]="!userProfile.valid">Submit</button>
   
   </form>

2. FormArray: It defines a dynamic form in an array format, where you can add and remove controls at run time. This is useful for dynamic forms when you don’t know how many controls will be present within the group.

    import { Component } from '@angular/core';
    import { FormArray, FormControl } from '@angular/forms';
   
    @Component({
      selector: 'order-form',
      templateUrl: './order-form.component.html',
      styleUrls: ['./order-form.component.css']
    })
    export class OrderFormComponent {
      constructor () {
        this.orderForm = new FormGroup({
          firstName: new FormControl('John', Validators.minLength(3)),
          lastName: new FormControl('Rodson'),
          items: new FormArray([
            new FormControl(null)
          ])
        });
      }
   
      onSubmitForm () {
        // Save the items this.orderForm.value in DB
      }
   
      onAddItem () {
        this.orderForm.controls
        .items.push(new FormControl(null));
      }
   
      onRemoveItem (index) {
        this.orderForm.controls['items'].removeAt(index);
      }
    }

   <form [formControlName]="orderForm" (ngSubmit)="onSubmit()">
   
     <label>
       First Name:
       <input type="text" formControlName="firstName">
     </label>
   
     <label>
       Last Name:
       <input type="text" formControlName="lastName">
     </label>
   
     <div>
     <p>Add items</p>
     <ul formArrayName="items">
       <li *ngFor="let item of orderForm.controls.items.controls; let i = index">
         <input type="text" formControlName="{{i}}">
         <button type="button" title="Remove Item" (click)="onRemoveItem(i)">Remove</button>
       </li>
     </ul>
     <button type="button" (click)="onAddItem">
       Add an item
     </button>
    </div>

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How do you update specific properties of a form model?

You can use patchValue() method to update specific properties defined in the form model. For example,you can update the name and street of certain profile on click of the update button as shown below.

updateProfile() {
  this.userProfile.patchValue({
    firstName: 'John',
    address: {
      street: '98 Crescent Street'
    }
  });
}

  <button (click)="updateProfile()">Update Profile</button>

You can also use setValue method to update properties.

Note: Remember to update the properties against the exact model structure.

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What is the purpose of FormBuilder?

FormBuilder is used as syntactic sugar for easily creating instances of a FormControl, FormGroup, or FormArray. This is helpful to reduce the amount of boilerplate needed to build complex reactive forms. It is available as an injectable helper class of the @angular/forms package.

For example, the user profile component creation becomes easier as shown here.

export class UserProfileComponent {
  profileForm = this.formBuilder.group({
    firstName: [''],
    lastName: [''],
    address: this.formBuilder.group({
      street: [''],
      city: [''],
      state: [''],
      zip: ['']
    }),
  });
  constructor(private formBuilder: FormBuilder) { }
  }

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How do you verify the model changes in forms?

You can add a getter property(let's say, diagnostic) inside component to return a JSON representation of the model during the development. This is useful to verify whether the values are really flowing from the input box to the model and vice versa or not.

export class UserProfileComponent {

  model = new User('John', 29, 'Writer');

  // TODO: Remove after the verification
  get diagnostic() { return JSON.stringify(this.model); }
}

and add diagnostic binding near the top of the form

{{diagnostic}}
<div class="form-group">
  // FormControls goes here
</div>

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What are the state CSS classes provided by ngModel?

The ngModel directive updates the form control with special Angular CSS classes to reflect it's state. Let's find the list of classes in a tabular format,

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How do you reset the form?

In a model-driven form, you can reset the form just by calling the function reset() on our form model. For example, you can reset the form model on submission as follows,

onSubmit() {
  if (this.myform.valid) {
    console.log("Form is submitted");
    // Perform business logic here
    this.myform.reset();
  }
}

Now, your form model resets the form back to its original pristine state.

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What are the types of validator functions?

In reactive forms, the validators can be either synchronous or asynchronous functions,

1. Sync validators: These are the synchronous functions which take a control instance and immediately return either a set of validation errors or null. Also, these functions passed as second argument while instantiating the form control. The main use cases are simple checks like whether a field is empty, whether it exceeds a maximum length etc.
2. Async validators: These are the asynchronous functions which take a control instance and return a Promise or Observable that later emits a set of validation errors or null. Also, these functions passed as second argument while instantiating the form control. The main use cases are complex validations like hitting a server to check the availability of a username or email.

The representation of these validators looks like below

this.myForm = formBuilder.group({
    firstName: ['value'],
    lastName: ['value', *Some Sync validation function*],
    email: ['value', *Some validation function*, *Some asynchronous validation function*]
});

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Can you give an example of built-in validators?

In reactive forms, you can use built-in validator like required and minlength on your input form controls. For example, the registration form can have these validators on name input field

this.registrationForm = new FormGroup({
    'name': new FormControl(this.hero.name, [
      Validators.required,
      Validators.minLength(4),
    ])
  });

Whereas in template-driven forms, both required and minlength validators available as attributes.

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How do you optimize the performance of async validators?

Since all validators run after every form value change, it creates a major impact on performance with async validators by hitting the external API on each keystroke. This situation can be avoided by delaying the form validity by changing the updateOn property from change (default) to submit or blur. The usage would be different based on form types,

1. Template-driven forms: Set the property on ngModelOptions directive

   <input [(ngModel)]="name" [ngModelOptions]="{updateOn: 'blur'}">

2. Reactive-forms: Set the property on FormControl instance

   name = new FormControl('', {updateOn: 'blur'});

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How to set ngFor and ngIf on the same element?

Sometimes you may need to both ngFor and ngIf on the same element but unfortunately you are going to encounter below template error.

 Template parse errors: Can't have multiple template bindings on one element.

In this case, You need to use either ng-container or ng-template. Let's say if you try to loop over the items only when the items are available, the below code throws an error in the browser

<ul *ngIf="items" *ngFor="let item of items">
  <li></li>
</ul>

and it can be fixed by

<ng-container *ngIf="items">
  <ul *ngFor="let item of items">
    <li></li>
  </ul>
</ng-container>

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What is host property in css?

The :host pseudo-class selector is used to target styles in the element that hosts the component. Since the host element is in a parent component's template, you can't reach the host element from inside the component by other means. For example, you can create a border for parent element as below,

//Other styles for app.component.css
//...
:host {
  display: block;
  border: 1px solid black;
  padding: 20px;
}

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How do you get the current route?

In Angular, there is an url property of router package to get the current route. You need to follow the below few steps,

1. Import Router from @angular/router

  import { Router } from '@angular/router';

2. Inject router inside constructor

constructor(private router: Router ) {

}

3. Access url parameter

  console.log(this.router.url); //  /routename

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What is Component Test Harnesses?

A component harness is a testing API around an Angular directive or component to make tests simpler by hiding implementation details from test suites. This can be shared between unit tests, integration tests, and end-to-end tests. The idea for component harnesses comes from the PageObject pattern commonly used for integration testing.

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What is the benefit of Automatic Inlining of Fonts?

During compile time, Angular CLI will download and inline the fonts that your application is using. This performance update speed up the first contentful paint(FCP) and this feature is enabled by default in apps built with version 11.

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What is content projection?

Content projection is a pattern in which you insert, or project, the content you want to use inside another component.

What is ng-content and its purpose?

The ng-content is used to insert the content dynamically inside the component that helps to increase component reusability.