Top 25+ Angular Interview Questions & Answers | Personalized AI Guidance

Angular lifecycle refers to the series of events that happen from the creation to the destruction of a component or directive. Angular provides lifecycle hooks that allow developers to tap into these events and perform actions at specific points.

Lifecycle Hooks:

• ngOnChanges() : Called when an input/output binding value changes.
• ngOnInit() : Called once after the first ngOnChanges() . Ideal for component initialization.
• ngDoCheck() : Called during every change detection run, allowing for custom change detection.
• ngAfterContentInit() : Called once after the first ngDoCheck() when content is projected into the component.
• ngAfterContentChecked() : Called after ngAfterContentInit() and after every subsequent ngDoCheck() .
• ngAfterViewInit() : Called once after the first ngAfterContentChecked() when the component's view (and child views) is initialized.
• ngAfterViewChecked() : Called after ngAfterViewInit() and after every subsequent ngAfterContentChecked() .
• ngOnDestroy() : Called just before the component or directive is destroyed. Ideal for cleanup tasks.

Angular Routing allows navigation from one view to another within a single-page application. It enables the application to display different components based on the URL.

Setting Up Angular Routing:

1. Import RouterModule and Routes: Import these from @angular/router in your main module file.

import { RouterModule, Routes } from '@angular/router';

2. Define Routes: Create an array of routes where each route maps a URL path to a component.

const routes: Routes = [
  { path: '', component: HomeComponent },
  { path: 'about', component: AboutComponent },
  { path: '**', component: NotFoundComponent }
];

3. Add RouterModule to NgModule: Add RouterModule.forRoot(routes) to the imports array in your module.

@NgModule({
  imports: [RouterModule.forRoot(routes)],
  exports: [RouterModule]
})
export class AppRoutingModule {}

4. Use Router Outlet: Add <router-outlet></router-outlet> in your main HTML template where you want the routed components to be displayed.

<router-outlet></router-outlet>

Angular Forms are used to handle user input and validation. There are two types of forms in Angular:

Template-driven Forms:

• Simple and easy to use.
• Suitable for simpler forms.
• Form logic is defined in the template using directives.
• Relies on Angular's two-way data binding ( ngModel ).

Example:

<form #form="ngForm">
  <input name="name" ngModel>
</form>

Reactive Forms:

• More powerful and scalable.
• Suitable for complex forms.
• Form logic is defined in the component class.
• Uses FormControl and FormGroup for managing form state and validation.

Example:

import { FormGroup, FormControl } from '@angular/forms';

this.form = new FormGroup({
  name: new FormControl('')
});

<form [formGroup]="form">
  <input formControlName="name">
</form>

Angular Modules (NgModules) are containers for a cohesive block of code dedicated to an application domain, a workflow, or a closely related set of capabilities. An Angular app is modularized into multiple modules, and each module defines a compilation context for a set of components, directives, and pipes.

Importance of Angular Modules:

• Organizing Code: Modules help organize an application into cohesive blocks of functionality.
• Separation of Concerns: Promotes separation of concerns by grouping related code together.
• Lazy Loading: Enables lazy loading, which improves performance by loading modules only when needed.
• Reusability: Facilitates code reusability across different parts of the application.

Example:

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { AppComponent } from './app.component';

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

Angular CLI (Command Line Interface) is a powerful tool to initialize, develop, scaffold, and maintain Angular applications. It provides a set of commands that simplify various development tasks.

Benefits of Angular CLI:

• Scaffolding: Quickly generates boilerplate code for components, services, modules, and more using commands like ng generate component .
• Building: Handles the build process with commands like ng build , optimizing the application for production.
• Development Server: Provides a development server with live reload capabilities using ng serve .
• Testing: Simplifies running unit tests and end-to-end tests with commands like ng test and ng e2e .
• Configuration: Manages configuration for different environments, making it easier to switch between development, staging, and production settings.

Example Commands:

# Create a new Angular application
ng new my-angular-app

# Serve the application locally
cd my-angular-app
ng serve

# Generate a new component
ng generate component my-component

A Pipe in Angular is a way to transform data in templates. Pipes can be used to format strings, dates, numbers, and more.

Creating a Custom Pipe:

1. Generate the Pipe:

ng generate pipe custom

2. Define the Pipe:

import { Pipe, PipeTransform } from '@angular/core';

@Pipe({
  name: 'custom'
})
export class CustomPipe implements PipeTransform {
  transform(value: string, ...args: any[]): string {
    // Transformation logic
    return value.toUpperCase();
  }
}

3. Use the Pipe in a Template:

<p>{{ 'hello world' | custom }}</p>

In this example, the CustomPipe transforms the input string to uppercase.

Angular Dependency Injection (DI) is a design pattern that allows a class to receive its dependencies from external sources rather than creating them itself. DI is used to increase modularity and testability of the code.

How DI Works:

1. Injectable Decorator: Services and other classes can be marked as injectable using the @Injectable decorator.

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

@Injectable({
  providedIn: 'root'
})
export class MyService {
  constructor() { }
}

2. Provider Configuration: Services can be provided in modules, components, or other services.

@NgModule({
  providers: [MyService]
})
export class AppModule { }

3. Injection via Constructor: Dependencies are injected via the constructor.

import { Component } from '@angular/core';
import { MyService } from './my-service.service';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  constructor(private myService: MyService) { }
}

Angular Change Detection is a mechanism that automatically synchronizes the model and the view. Angular's change detection process checks the component's data-bound properties for any changes and updates the DOM accordingly.

How Change Detection Works:

Change Detection Strategy: Angular provides two change detection strategies:

• Default: Checks the entire component tree.
• OnPush: Checks only when an input property changes or an event occurs.

import { ChangeDetectionStrategy, Component } from '@angular/core';

@Component({
  selector: 'app-my-component',
  changeDetection: ChangeDetectionStrategy.OnPush,
  templateUrl: './my-component.component.html'
})
export class MyComponent {
  // Component logic
}

Change Detection Cycle: Angular performs change detection in a top-down manner, starting from the root component down to the child components.

Zone.js: Angular uses Zone.js to detect asynchronous operations and trigger change detection automatically.

Angular Universal is a technology for server-side rendering (SSR) of Angular applications. It allows the application to be rendered on the server and then sent to the client, improving the initial load time and SEO.

Benefits of Angular Universal:

• Improved Performance: Faster initial load times because the server sends the pre-rendered HTML to the client.
• SEO Optimization: Better search engine indexing as search engines can crawl the pre-rendered HTML.
• Better User Experience: Users can see the content faster, reducing the perceived load time.

Setting Up Angular Universal:

1. Install Angular Universal:

ng add @nguniversal/express-engine

2. Build the Application:

npm run build:ssr

3. Serve the Application:

npm run serve:ssr

NgZone is a service in Angular that provides a way to execute code inside or outside of the Angular zone. This can be useful for optimizing performance by running heavy operations outside the Angular zone to prevent triggering change detection unnecessarily.

Using NgZone:

1. Inject NgZone:

import { Component, NgZone } from '@angular/core';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  constructor(private ngZone: NgZone) {}

  runOutsideAngular() {
    this.ngZone.runOutsideAngular(() => {
      // Code to run outside Angular's zone
    });
  }

  runInsideAngular() {
    this.ngZone.run(() => {
      // Code to run inside Angular's zone
    });
  }
}

2. Use NgZone.runOutsideAngular : Perform operations that do not need to trigger change detection outside the Angular zone.

3. Use NgZone.run : Bring the code back into the Angular zone if it needs to update the UI.

Angular Guards are used to control access to routes in an Angular application. They are functions that can control whether a user can navigate to a particular route or not. There are four types of guards:

• CanActivate: Determines if a route can be activated.
• CanActivateChild: Determines if a child route can be activated.
• CanDeactivate: Determines if a route can be deactivated.
• CanLoad: Determines if a module can be loaded lazily.

Using CanActivate Guard:

1. Create a Guard:

ng generate guard auth

2. Implement the Guard:

import { Injectable } from '@angular/core';
import { CanActivate, ActivatedRouteSnapshot, RouterStateSnapshot, UrlTree } from '@angular/router';
import { Observable } from 'rxjs';

@Injectable({
  providedIn: 'root'
})
export class AuthGuard implements CanActivate {
  canActivate(
    next: ActivatedRouteSnapshot,
    state: RouterStateSnapshot): Observable | Promise | boolean | UrlTree {
    // Logic to determine if the route can be activated
    return true; // or false
  }
}

3. Apply the Guard to a Route:

const routes: Routes = [
  {
    path: 'protected-route',
    component: ProtectedComponent,
    canActivate: [AuthGuard]
  }
];

Both Promises and Observables are used to handle asynchronous operations, but they have some key differences:

1. Promises:

• Single Value: A Promise resolves or rejects a single value.
• Eager Execution: A Promise starts executing immediately when it is created.
• Not Cancellable: Once created, a Promise cannot be cancelled.
• API Methods: then() , catch() , finally() .

const promise = new Promise((resolve, reject) => {
  // asynchronous operation
});

promise.then(value => {
  // handle resolved value
}).catch(error => {
  // handle error
});

2. Observables:

• Multiple Values: An Observable can emit multiple values over time.
• Lazy Execution: An Observable does not start emitting values until it is subscribed to.
• Cancellable: Observables can be cancelled using unsubscribe() .
• API Methods: subscribe() , unsubscribe() , operators like map , filter , merge .

import { Observable } from 'rxjs';

const observable = new Observable(observer => {
  // asynchronous operation
  observer.next(value);
  observer.complete();
});

const subscription = observable.subscribe({
  next(value) {
    // handle emitted value
  },
  error(err) {
    // handle error
  },
  complete() {
    // handle completion
  }
});

// To cancel the subscription
subscription.unsubscribe();

Angular's HttpClient is a service used to make HTTP requests to remote servers. It is part of the @angular/common/http package and provides a simplified API for making HTTP calls.

Using HttpClient:

1. Import HttpClientModule :

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

@NgModule({
  imports: [HttpClientModule],
})
export class AppModule {}

2. Inject HttpClient:

import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';
import { Observable } from 'rxjs';

@Injectable({
  providedIn: 'root'
})
export class DataService {
  private apiUrl = 'https://api.example.com/data';

  constructor(private http: HttpClient) {}

  getData(): Observable<any> {
    return this.http.get<any>(this.apiUrl);
  }
}

3. Make HTTP Requests:

import { Component, OnInit } from '@angular/core';
import { DataService } from './data.service';

@Component({
  selector: 'app-data',
  template: `<div *ngFor="let item of data">{{ item }}</div>`
})
export class DataComponent implements OnInit {
  data: any[];

  constructor(private dataService: DataService) {}

  ngOnInit() {
    this.dataService.getData().subscribe(data => {
      this.data = data;
    });
  }
}

Ahead-of-Time (AOT) Compilation is a process in Angular where the Angular compiler compiles the application during the build time, before the browser downloads and runs the code. This is opposed to Just-in-Time (JIT) compilation, which happens in the browser during runtime.

Benefits of AOT Compilation:

• Faster Rendering: Since the application is compiled during build time, the browser can render the application faster.
• Smaller Bundle Size: The compiler is not included in the application bundle, resulting in a smaller bundle size.
• Earlier Error Detection: Errors are caught during the build phase, making debugging easier and faster.
• Enhanced Security: AOT compilation prevents injection attacks by compiling templates to JavaScript, which makes it difficult to manipulate the application structure.

Using AOT Compilation: AOT compilation can be enabled by running the build command with the --aot flag:

ng build --aot

Both Renderer2 and ElementRef are used to interact with the DOM in Angular, but they serve different purposes and have different use cases.

1. ElementRef:

• Direct Access: Provides direct access to the DOM element. It is generally discouraged to use ElementRef for direct DOM manipulation due to potential security risks and cross-platform compatibility issues.

import { Component, ElementRef, OnInit } from '@angular/core';

@Component({
  selector: 'app-sample',
  template: `<div #myDiv>Sample</div>`
})
export class SampleComponent implements OnInit {
  constructor(private el: ElementRef) {}

  ngOnInit() {
    this.el.nativeElement.querySelector('#myDiv').style.color = 'blue';
  }
}

2. Renderer2:

• Abstraction Layer: Provides an abstraction layer for interacting with the DOM, making the code more secure and compatible across different platforms.

import { Component, OnInit, Renderer2 } from '@angular/core';

@Component({
  selector: 'app-sample',
  template: `<div #myDiv>Sample</div>`
})
export class SampleComponent implements OnInit {
  constructor(private renderer: Renderer2, private el: ElementRef) {}

  ngOnInit() {
    const div = this.el.nativeElement.querySelector('#myDiv');
    this.renderer.setStyle(div, 'color', 'blue');
  }
}

Renderer2 is generally preferred for DOM manipulation to ensure compatibility and security.