Angular JS Interview Questions and Answers

1. What is the role of ngZone in Angular?

Answer:
ngZone is a service in Angular that allows you to execute code inside or outside Angular's change detection cycle. By running code outside the zone, you can prevent unnecessary change detection, improving performance. It’s commonly used in performance optimization when dealing with third-party libraries or external events.

2. How does Angular handle change detection, and what is the difference between Default and OnPush strategies?

Answer:
Angular uses Change Detection to update the view whenever data changes.

• Default Change Detection: Angular checks all components in the component tree for changes whenever a user event or asynchronous operation occurs.

• OnPush Change Detection: Angular only checks the component if its input properties change or if an event (like user input) triggers it. This improves performance by reducing the number of checks.

3. What is the purpose of the Renderer2 service in Angular?

Answer:
The Renderer2 service provides an abstraction for DOM manipulation that ensures your application is platform-independent. It allows you to modify the DOM safely in a way that works across different environments, such as browsers or server-side rendering.

Example:

constructor(private renderer: Renderer2, private el: ElementRef) {} ngOnInit() { this.renderer.setStyle(this.el.nativeElement, 'background-color', 'red'); }

4. How does Angular handle internationalization (i18n)?

Answer:
Angular provides built-in support for internationalization (i18n) through the Angular i18n module. This involves translating static content, date formats, numbers, and currencies to support multiple languages and regions. Angular uses i18n attributes in templates, TranslateService for dynamic translations, and a translation file to define translations for each language.

5. What are dynamic components, and how can you load them dynamically in Angular?

Answer:
Dynamic components are components that are created and inserted into the DOM at runtime. They are typically used when you need to load components dynamically based on user input or external data.

You can load dynamic components using ComponentFactoryResolver:

import { ComponentFactoryResolver, ViewContainerRef } from '@angular/core'; constructor(private resolver: ComponentFactoryResolver, private viewContainerRef: ViewContainerRef) {} loadComponent() { const factory = this.resolver.resolveComponentFactory(DynamicComponent); this.viewContainerRef.createComponent(factory); }

6. How would you implement a custom form control in Angular?

Answer:
To implement a custom form control in Angular, you need to implement the ControlValueAccessor interface. This allows Angular to integrate the custom control with Angular’s reactive or template-driven forms.

Steps:

1. Implement writeValue(), registerOnChange(), registerOnTouched(), and setDisabledState() methods.

2. Use the NG_VALUE_ACCESSOR provider to register the custom control.

Example:

export class CustomInputComponent implements ControlValueAccessor { writeValue(value: any): void { ... } registerOnChange(fn: any): void { ... } registerOnTouched(fn: any): void { ... } setDisabledState?(isDisabled: boolean): void { ... } }

7. How does Angular handle server-side rendering (SSR)?

Answer:
Angular uses Angular Universal for server-side rendering. This allows the initial page to be rendered on the server and sent to the client as HTML. This improves performance and SEO because the client receives a fully rendered page rather than waiting for JavaScript to execute.

8. How does Angular handle lazy loading of modules?

Answer:
Lazy loading in Angular is implemented through the loadChildren property in the routing configuration. Modules are only loaded when the user navigates to the corresponding route, reducing the initial load time.

Example:

const routes: Routes = [ { path: 'feature', loadChildren: () => import('./feature/feature.module').then(m => m.FeatureModule) } ];

9. What is a Subject in RxJS, and how is it different from an Observable?

Answer:
A Subject is both an observable and an observer. Unlike a regular Observable, which can only be subscribed to, a Subject can multicast to many subscribers. Subjects are useful for sharing data between different parts of your application.

10. How would you implement custom authentication and authorization in Angular?

Answer:
Custom authentication and authorization can be implemented by creating an AuthService to handle login, logout, and token management. You can use HTTP interceptors to attach authentication tokens to outgoing requests and Guards to protect routes based on the user’s authentication state.

Example of Guard:

@Injectable({ providedIn: 'root' }) export class AuthGuard implements CanActivate { constructor(private authService: AuthService, private router: Router) {} canActivate(): boolean { if (this.authService.isAuthenticated()) { return true; } else { this.router.navigate(['/login']); return false; } } }

11. What are Web Workers, and how do you use them in Angular?

Answer:
Web Workers allow you to run JavaScript code in the background, without blocking the main thread. In Angular, you can create a Web Worker to perform heavy computations off the main thread, improving performance.

Example:

const worker = new Worker('./worker.js'); worker.postMessage('Hello Worker'); worker.onmessage = (message) => console.log(message.data);

12. How can you implement caching in Angular?

Answer:
Caching in Angular can be implemented using HttpInterceptor to intercept HTTP requests and cache responses. You can use the browser’s localStorage or sessionStorage to store cached data and serve it without making repeated API calls.

Example:

@Injectable() export class CacheInterceptor implements HttpInterceptor { intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> { const cachedData = localStorage.getItem(req.url); if (cachedData) { return of(new HttpResponse({ body: JSON.parse(cachedData) })); } return next.handle(req).pipe( tap(event => { if (event instanceof HttpResponse) { localStorage.setItem(req.url, JSON.stringify(event.body)); } }) ); } }

13. How can you optimize Angular's performance in large-scale applications?

Answer:
Performance can be optimized in large-scale Angular applications by:

• Lazy loading modules to reduce initial loading time.

• AOT (Ahead-of-Time) Compilation to compile Angular templates and components at build time.

• ChangeDetectionStrategy.OnPush to reduce the number of checks Angular performs.

• Tree-shaking to remove unused code during the build process.

• Using Web Workers for heavy computation tasks.

• Code-splitting to reduce the bundle size.

14. How can you integrate third-party libraries into Angular?

Answer:
Third-party libraries can be integrated into Angular by:

• Installing the library using npm or yarn.

• Importing the module or component into your Angular module.

• Using Angular’s wrapper components or directives for seamless integration.

Example:

import { ThirdPartyLibraryModule } from 'third-party-library'; @NgModule({ imports: [ThirdPartyLibraryModule] }) export class AppModule {}

15. What is a Service Worker, and how does it help in Angular?

Answer:
A Service Worker is a script that runs in the background to manage caching, background sync, and push notifications. It helps in building Progressive Web Apps (PWAs) by providing offline capabilities and improving performance by caching assets.

16. How would you implement file uploading in Angular with progress tracking?

Answer:
File uploading with progress tracking can be implemented using HttpClient and HttpEventType to monitor the upload progress. You can track the progress by listening to the upload events and updating the progress bar.

Example:

uploadFile(file: File): Observable<HttpEvent<any>> { const formData = new FormData(); formData.append('file', file, file.name); const req = new HttpRequest('POST', '/upload', formData, { headers: new HttpHeaders(), reportProgress: true }); return this.http.request(req).pipe( filter(event => event.type === HttpEventType.UploadProgress), map(event => { if (event instanceof HttpProgressEvent) { return Math.round((100 * event.loaded) / event.total); } }) ); }

17. What is a Resolver in Angular, and how is it used?

Answer:
A Resolver is used to pre-fetch data before a route is activated. It resolves data that is needed for the component before the route loads. It’s useful for ensuring that data is available when the component is rendered.

Example:

@Injectable({ providedIn: 'root' }) export class DataResolver implements Resolve<any> { constructor(private dataService: DataService) {} resolve(route: ActivatedRouteSnapshot, state: RouterStateSnapshot): Observable<any> { return this.dataService.fetchData(); } }

18. What are ng-template and ng-container in Angular?

Answer:

• ng-template is used to define template snippets that are rendered conditionally or lazily. It is not part of the DOM unless specifically rendered.

• ng-container is a container that doesn’t create an additional DOM element but can be used for structural directives.

19. What is Angular’s trackBy function, and why is it important for performance?

Answer:
`trackBy

` helps Angular track elements in an iterable list, optimizing the DOM updates. It prevents unnecessary re-renders by identifying unique items in the list and only updating those that have changed.

Example:

<div *ngFor="let item of items; trackBy: trackById"> {{ item.name }} </div> trackById(index: number, item: any): number { return item.id; }

20. What is the ngRx library, and how is it used for state management in Angular?

Answer:
NgRx is a state management library for Angular based on the Redux pattern. It helps manage application state in a predictable way using actions, reducers, and selectors. It uses RxJS for managing state updates and side effects.

21. How does Angular handle module bundling?

Answer:
Angular uses Webpack for module bundling. During the build process, Webpack bundles the application’s components, modules, services, and other resources into smaller chunks for optimization, allowing for lazy loading and code-splitting.

22. What is ngxs and how does it compare to ngRx?

Answer:
NGXS is another state management library for Angular that is simpler and less boilerplate than ngRx. It focuses on state as a class with actions, and provides a straightforward API for managing state and effects in Angular applications.

23. How would you optimize Angular's startup time?

Answer:
Optimizing Angular’s startup time can be done by:

• Using Lazy loading to load only essential modules initially.

• Enabling AOT Compilation to reduce runtime compilation.

• Optimizing the change detection strategy.

• Tree shaking unused code and dependencies.

• Compressing assets and splitting bundles effectively.

24. What are the benefits of ngOnDestroy in Angular, and when is it used?

Answer:
ngOnDestroy is a lifecycle hook that is called before Angular destroys the component. It is typically used for cleanup tasks such as unsubscribing from observables, removing event listeners, and releasing resources to prevent memory leaks.

25. How does Angular handle authentication tokens in HTTP requests?

Answer:
Angular can manage authentication tokens by using HTTP Interceptors to modify the request before it is sent. The token is typically added to the request headers using the Authorization header.

Example:

@Injectable() export class AuthInterceptor implements HttpInterceptor { intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> { const authToken = localStorage.getItem('authToken'); const clonedRequest = req.clone({ headers: req.headers.set('Authorization', `Bearer ${authToken}`) }); return next.handle(clonedRequest); } }

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