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TCS Top 50 Angular Interview Questions and Answers

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TCS Top 50 Angular Interview Questions and Answers

Dev Kanungo

Table of Contents

This blog covers the top 50 Angular interview questions and answers to help you prepare for your TCS interview. Master Angular fundamentals, advanced topics, and coding challenges for a successful interview.

Basic Questions

1. What is Angular?

Angular is a TypeScript-based open-source web application framework developed and maintained by Google. It allows developers to build dynamic, single-page applications (SPAs) that run efficiently in a browser. Angular is designed to make the development process easier and faster with its component-based architecture, modular structure, and powerful tools for handling forms, routing, and HTTP requests.

2. What are the main features of Angular?

Some of the key features of Angular include:
Component-based Architecture: Angular applications are built using components that encapsulate logic, UI, and styles.
Data Binding: Angular supports powerful two-way data binding, allowing automatic synchronization between the model and view.
Dependency Injection: It promotes modularity by injecting services wherever needed, helping with better maintainability and scalability.
Directives: These extend HTML by adding new behaviors and functionality to the DOM.
Routing: Angular has a powerful routing module that helps developers build navigation between different pages and components in an application.
Cross-platform: It allows building web, desktop, and mobile apps.
Forms Handling: Angular simplifies form handling with its Reactive Forms and Template-driven Forms approach.

3. What is the difference between AngularJS and Angular?

Architecture: AngularJS is based on the Model-View-Controller (MVC) architecture, while Angular follows a component-based architecture.
Language: AngularJS uses JavaScript, while Angular (Angular 2 and above) uses TypeScript, a statically typed superset of JavaScript.
Mobile Support: Angular is optimized for mobile development, while AngularJS lacks this support.
Performance: Angular is faster and provides better performance due to improved data binding and component-based design. AngularJS uses dirty checking for data binding, which can lead to slower performance.
Dependency Injection: Angular has a robust dependency injection mechanism compared to AngularJS.
Routing: Angular has a more advanced and modular routing system than AngularJS.
Two-Way Data Binding: AngularJS has automatic two-way data binding, whereas Angular emphasizes unidirectional data flow with support for two-way binding when needed.

4. What is a component in Angular?

A component in Angular is a building block of the application, representing a part of the UI. Components consist of three main parts:
Template: Defines the HTML structure and appearance.
Class: Contains the logic and data that control the behavior of the component.
Styles: Includes the CSS specific to the component. Components interact with each other to form the overall user interface of the Angular application. They are reusable, maintainable, and modular.

5. What is a module in Angular?

An Angular module is a container that groups related components, directives, pipes, and services. It helps organize an application into cohesive blocks of functionality. The root module, AppModule, is required to bootstrap an Angular application, and additional feature modules can be created to organize the application further. Modules can be lazy-loaded for performance optimization, reducing the initial load time.

6. What are services in Angular?

Services in Angular are classes that encapsulate business logic, data access, or any other reusable functionality that is not tied directly to the UI. Services are typically injected into components or other services using Angular’s dependency injection system. They help maintain separation of concerns by keeping logic and data handling separate from the component classes, making the application easier to maintain.

7. What is dependency injection in Angular?

Dependency Injection (DI) in Angular is a design pattern that allows an object (like a service) to be injected into other objects (like components). It enables the decoupling of dependencies, allowing services to be reused across the application. Angular’s DI system automatically provides instances of services when they are needed, enhancing testability and modularity.

8. What is a directive in Angular?

A directive in Angular is a special type of class that modifies the behavior of elements in the DOM. There are three types of directives:
Component Directives: These are custom components that control a view.
Structural Directives: These change the structure of the DOM by adding or removing elements, e.g., *ngIf, *ngFor.
Attribute Directives: These change the appearance or behavior of an element, component, or another directive, e.g., ngClass, ngStyle.

9. What is data binding in Angular?

Data binding in Angular refers to the communication between the component and the DOM (view). It allows developers to coordinate the data flow between the model (data) and the view (UI), making it easier to handle user inputs and reflect changes dynamically in the UI.

10. What are the different types of data binding in Angular?

Angular supports the following types of data binding:
Interpolation (One-Way Data Binding): Used to display the data from the component to the view, e.g., {{ expression }}.
Property Binding (One-Way): Allows setting DOM element properties with component data, e.g., [property]="value".
Event Binding (One-Way): Allows listening to and handling events, e.g., (event)="handler()".
Two-Way Data Binding: Syncs data between the component and the view in both directions, e.g., [(ngModel)]="property". This is commonly used in form inputs.

Intermediate Questions

11. What is the purpose of the ngModule?

The ngModule is a core concept in Angular that defines the boundaries of an application or feature. It organizes the code by grouping related components, services, pipes, and directives into a cohesive block of functionality. An Angular application must have at least one module, called the root module (AppModule), to bootstrap the application. ngModule also helps configure providers (services) and define imports and exports of other modules, making it easier to structure and manage large applications by separating functionality into different modules.

12. How do you create a new Angular component?

To create a new Angular component, you can use Angular CLI, which simplifies the process. Run the following command:
```
ng generate component component-name
```
This command creates a new folder containing four files:
component-name.component.ts – Contains the component class and logic.
component-name.component.html – The template for the component’s view.
component-name.component.css – Styles specific to the component.
component-name.component.spec.ts – A test file for the component.
The newly created component is automatically registered in the declarations array of the module.

13. What is the lifecycle of a component in Angular?

Angular components go through a series of lifecycle events from creation to destruction. The main lifecycle hooks are:
ngOnChanges(): Called when input properties change.
ngOnInit(): Called once after the component is initialized.
ngDoCheck(): Called during every change detection run, after ngOnChanges.
ngAfterContentInit(): Called after content is projected into the component.
ngAfterContentChecked(): Called after the projected content is checked.
ngAfterViewInit(): Called after the component's view has been initialized.
ngAfterViewChecked(): Called after the view is checked.
ngOnDestroy(): Called right before the component is destroyed. It is typically used for cleanup.

14. What are pipes in Angular?

Pipes in Angular are used to transform the output in the template. They allow developers to format and transform data before displaying it. For example, they can be used to format dates, numbers, strings, or currency. Angular provides built-in pipes like DatePipe, CurrencyPipe, UpperCasePipe, etc. Custom pipes can also be created to handle specific transformations. Syntax for using a pipe is as follows:
```
{{ value | pipeName }}
```

15. How do you handle forms in Angular?

Angular provides two ways to handle forms:
Template-driven forms: These rely heavily on directives in the template (like ngModel) and are used for simpler forms. It is more declarative and easier to set up.
Reactive forms: These use a more programmatic approach, where the form’s structure is defined in the component class using FormGroup and FormControl. This approach offers more control and flexibility, making it suitable for complex forms with dynamic behavior.

16. What is reactive programming in Angular?

Reactive programming in Angular refers to working with asynchronous data streams and responding to changes over time. It is implemented using RxJS (Reactive Extensions for JavaScript), which provides tools like Observables to handle events, data streams, and asynchronous programming more efficiently. With reactive programming, you can build applications that react to a wide range of input sources such as user actions, HTTP requests, and real-time updates.

17. What is RxJS and how is it used in Angular?

RxJS (Reactive Extensions for JavaScript) is a library for handling asynchronous and event-based data using Observables. It provides powerful operators to manipulate data streams. In Angular, RxJS is heavily used for handling HTTP requests, routing events, form inputs, and other asynchronous operations. Common RxJS operators include map, filter, mergeMap, and switchMap. Observables in RxJS provide a more flexible way of handling asynchronous code compared to promises.

18. What is the purpose of the Angular router ?

The Angular router is a module that facilitates navigation between different views or components in a single-page application (SPA). It allows developers to define routes that map URL paths to specific components. The router also supports advanced features such as lazy loading, route guards, nested routes, and parameterized routes, enabling the creation of a dynamic and navigable web application.

19. How do you implement lazy loading in Angular?

Lazy loading in Angular is a technique to load specific modules only when they are needed, rather than loading the entire application upfront. This reduces the initial load time of the application. Lazy loading is implemented by using Angular's routing mechanism:

Create a feature module.
Configure the route with loadChildren to lazy-load the module when the route is accessed.

{
  path: 'feature',
  loadChildren: () => import('./feature/feature.module').then(m => m.FeatureModule)
}

With lazy loading, the module will be loaded only when the user navigates to the specified route.

20. What are observables and promises in Angular?

Both Observables and Promises are used to handle asynchronous operations in Angular, but they work differently:
Promises: A Promise represents a single future value. Once the promise is resolved or rejected, it cannot emit new values. Promises are used for one-time asynchronous operations like HTTP requests.
Observables: An Observable is a stream of multiple values over time. Observables can emit multiple values, handle errors, and be canceled. RxJS operators allow powerful manipulation of the data emitted by Observables. Observables are used extensively in Angular, especially for event handling, HTTP requests, and reactive programming with RxJS.

Advanced Questions

21. What is change detection in Angular?

Change detection in Angular is the mechanism by which the framework detects changes in the application's state and updates the view accordingly. Every time a component's state changes, Angular's change detection system determines whether the view needs to be updated. This is achieved by checking if any properties in the component have changed and then updating the DOM if necessary. Angular uses two strategies for change detection:
Default strategy: Checks every component in the tree, starting from the root, during each change detection cycle.
OnPush strategy: Optimizes performance by only checking components when their input properties change.

22. How does Angular handle state management?

Angular handles state management by using services and observables to manage data that multiple components share. While Angular does not provide built-in state management for large-scale applications, libraries like NgRx (inspired by Redux) and Akita are commonly used. These libraries help manage the state in a predictable manner by implementing the Flux pattern, where application state is stored in a single, centralized store, and components can dispatch actions to modify the state.

23. What are Angular guards?

Angular guards are functions that decide whether a route can be activated or deactivated based on specific conditions. They help secure and control access to routes in an Angular application. The different types of route guards include:
CanActivate: Determines if a route can be navigated to.
CanDeactivate: Determines if a user can leave the route.
CanLoad: Controls whether a module can be lazy-loaded.
CanActivateChild: Applies CanActivate checks to child routes.
Resolve: Pre-fetches data before a route is activated.

24. How do you optimize an Angular application?

There are several ways to optimize Angular applications for performance:
Lazy loading: Load only the necessary modules as users navigate through the application.
Ahead-of-Time (AOT) compilation: Precompile the application during the build phase to reduce the size of the application and improve load time.
Tree shaking: Remove unused code from the final bundle to reduce its size.
Use OnPush change detection: Reduce unnecessary change detection cycles by using the OnPush strategy.
Caching: Implement caching for HTTP requests using HttpClient interceptors or third-party libraries.
Minification and Uglification: Reduce the size of the JavaScript bundle by removing white spaces and renaming variables.

25. What is Ahead-of-Time (AOT) compilation?

Ahead-of-Time (AOT) compilation is a process in Angular where the application is compiled at build time instead of run time. The TypeScript and HTML templates are compiled into JavaScript during the build process, which reduces the size of the framework code and speeds up the application's initial load time. AOT helps catch template errors early and reduces the overhead of runtime compilation.

26. What is the difference between AOT and Just-in-Time (JIT) compilation?

AOT (Ahead-of-Time): Compilation happens during the build phase, so the browser directly loads the precompiled code. This reduces the application’s size and startup time. AOT is used in production builds.
JIT (Just-in-Time): Compilation happens in the browser when the application is loaded. This is usually slower and adds overhead during the initial load, as the browser compiles the application code at runtime. JIT is mostly used in development builds.

27. What are the advantages of using Angular Universal?

Angular Universal enables server-side rendering (SSR) of Angular applications. The benefits include:
Improved SEO: SSR ensures that search engine crawlers can easily index the content, enhancing the website's SEO.
Faster initial load: Pages are pre-rendered on the server, leading to faster first paint and a better user experience, especially on slow networks.
Social media previews: SSR helps generate meta tags and page content, making it easier to create rich previews when sharing pages on social platforms.

28. How do you create a custom directive in Angular?

To create a custom directive in Angular, follow these steps:
Use Angular CLI to generate a directive:
```
ng generate directive directive-name
```

Implement the directive logic in the generated class. A simple example of a custom directive that changes text color could look like this:

import { Directive, ElementRef, Renderer2 } from '@angular/core';

@Directive({
  selector: '[appHighlight]'
})
export class HighlightDirective {
  constructor(private el: ElementRef, private renderer: Renderer2) {
    this.renderer.setStyle(this.el.nativeElement, 'color', 'blue');
  }
}

Add the directive to the declarations array of your module, and use it in your templates as follows:
```
<p appHighlight>Highlighted Text</p>
```

29. What is the purpose of the HttpClient module?

The HttpClient module in Angular provides an API for making HTTP requests to a backend server. It supports various request methods such as GET, POST, PUT, DELETE, etc. It also provides features like:
Interceptors: Allows modifying requests or responses globally (e.g., for adding authentication tokens).
Automatic JSON Parsing: Responses are automatically parsed into JavaScript objects.
Observables: The requests return observables, allowing developers to easily handle asynchronous operations.

30. How do you handle errors in Angular applications?

Error handling in Angular can be managed at different levels:
Try-Catch Blocks: For basic synchronous code.

Error handling in services: For HTTP errors, you can use the catchError operator from RxJS in combination with the HttpClient service.

import { catchError } from 'rxjs/operators';
import { throwError } from 'rxjs';

this.http.get('api/data').pipe(
  catchError(this.handleError)
);

private handleError(error: HttpErrorResponse) {
  console.error('Server error:', error);
  return throwError('Something went wrong; please try again later.');
}

Global Error Handling: You can create a global error handler by implementing the ErrorHandler class.
Router Guards: Redirect users to an error page when certain conditions are not met.

Practical Questions

31. How do you perform unit testing in Angular?

Unit testing in Angular is performed using Jasmine for writing test cases and Karma as the test runner. Each component, service, or directive in Angular has a corresponding test file, typically with the .spec.ts extension. Angular CLI automatically generates these files when creating components or services. Unit tests focus on testing isolated pieces of logic. To perform unit testing:
Import the necessary modules and services.
Use the TestBed class to configure and set up the testing environment.
Write test cases with describe and it blocks provided by Jasmine.
Use assertions such as expect() to validate the outcomes.

Example:

describe('MyComponent', () => {
  let component: MyComponent;
  let fixture: ComponentFixture<MyComponent>;

  beforeEach(() => {
    TestBed.configureTestingModule({
      declarations: [MyComponent]
    });
    fixture = TestBed.createComponent(MyComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  it('should create the component', () => {
    expect(component).toBeTruthy();
  });
});

32. What tools do you use for testing Angular applications?

Jasmine: A behavior-driven development framework for writing tests.
Karma: A test runner that allows real-time testing in different browsers.
Protractor: A framework for end-to-end testing of Angular applications, often used with Selenium.
TestBed: A testing environment utility provided by Angular to set up and run unit tests.
Angular Testing Utilities: Provides utilities for mocking HTTP requests (via HttpClientTestingModule) and simulating user interactions.

33. How do you implement authentication in Angular?

Authentication in Angular is typically handled using a combination of services and guards. Here's how you can implement it:
Create an Authentication Service: The service will handle user login, logout, and token management (using JWT).
Use Angular Router Guards: Implement CanActivate to protect routes that require authentication.
Intercept HTTP Requests: Use an HttpInterceptor to add authentication tokens to outgoing requests.
Store Tokens Securely: Store JWT tokens in local storage or cookies.
Login and Logout Methods: In the authentication service, create methods to log in by sending credentials to the backend and store the received token.

Example:

@Injectable({ providedIn: 'root' })
export class AuthService {
  login(credentials: { username: string, password: string }): Observable<any> {
    return this.http.post('api/login', credentials).pipe(
      tap((response: any) => localStorage.setItem('token', response.token))
    );
  }

  logout() {
    localStorage.removeItem('token');
  }
}

34. What is NgRx and how is it used in Angular?

NgRx is a library for managing state in Angular applications, inspired by Redux. It provides a centralized store for the entire application state and allows components to dispatch actions to modify the state. NgRx is commonly used in large-scale applications to maintain a consistent state across the app.
Store: Holds the application state.
Actions: Describe the type of state changes.
Reducers: Functions that take the current state and action to return the new state.
Effects: Handle side effects, such as HTTP requests.

Example:

// Action
export const login = createAction('[Auth] Login', props<{ user: User }>());

// Reducer
const authReducer = createReducer(
  initialState,
  on(login, (state, { user }) => ({ ...state, user }))
);

// Selector
const selectAuthState = createFeatureSelector<AuthState>('auth');

35. How do you manage environment configurations in Angular?

Angular uses environment files to manage configurations for different environments like development, testing, and production. By default, the Angular CLI provides two environment files:
environment.ts: For development.
environment.prod.ts: For production.
You can define variables such as API URLs, feature flags, and third-party keys in these files and access them in your application. During the build process, Angular will replace the environment file based on the environment specified in the build command.

Example:
```
// environment.ts
export const environment = {
  production: false,
  apiUrl: 'http://localhost:3000/api'
};
```

36. What is the role of the Angular CLI?

The Angular CLI (Command Line Interface) is a powerful tool for automating common Angular development tasks. It helps in:
Creating Angular projects: ng new project-name.
Generating components, services, and directives: ng generate component component-name.
Building the application: ng build.
Running tests: ng test.
Serving the application: ng serve.
Linting and formatting code: ng lint.
Optimizing builds: It helps in producing optimized builds for production environments.

37. How do you serve an Angular application?

You can serve an Angular application locally using the Angular CLI with the command:
```
ng serve
```
This compiles the application and starts a local development server (usually on http://localhost:4200). The app will automatically reload when changes are made.

38. How do you deploy an Angular application?

To deploy an Angular application, you need to:
Build the app for production:
```
ng build --prod
```
This generates optimized and minified files in the dist folder.
Upload the contents of the dist folder to a web server or hosting platform such as Netlify, Firebase Hosting, AWS S3, Heroku, or GitHub Pages.
For Firebase hosting:
```
ng add @angular/fire
ng deploy
```

39. What are Angular animations?

Angular provides a built-in animation module, @angular/animations, which allows you to add dynamic transitions and animations to your application. Animations are defined in the component metadata using the trigger, state, and transition functions. It supports advanced animation features like keyframes, parallel animations, and easing.

Example:
```
@Component({
  selector: 'app-example',
  templateUrl: './example.component.html',
  styleUrls: ['./example.component.css'],
  animations: [
    trigger('fadeInOut', [
      state('void', style({ opacity: 0 })),
      transition(':enter, :leave', [animate('0.5s')])
    ])
  ]
})
```

40. How do you implement internationalization (i18n) in Angular?

Angular provides built-in support for internationalization (i18n) to localize the application for different languages and regions. To implement i18n:
Use Angular's i18n attribute in the templates for translatable content.
```
<p i18n="@@helloWorld">Hello, World!</p>
```
Extract translation messages using the Angular CLI:
```
ng extract-i18n
```
Translate the messages.xlf file into the desired languages.
Build the application for different locales:
```
ng build --localize
```
Configure the app to serve the appropriate locale based on the user’s browser settings or manually set the language.

Conceptual Questions

41. What is the purpose of the ngFor directive?

The ngFor directive in Angular is used to iterate over a list of items and dynamically generate HTML elements based on that list. It is similar to loops in programming languages, but it operates within the template. Each element of the array or collection is iterated and displayed in the DOM.

Example:
```
<ul>
  <li *ngFor="let item of items">{{ item }}</li>
</ul>
```
Here, the list of items will be rendered as a set of <li> elements.

42. What is the purpose of the ngIf directive?

The ngIf directive in Angular conditionally includes or removes an element from the DOM based on the evaluation of an expression. If the expression is true, the element is rendered; otherwise, it is removed.

Example:
```
<div *ngIf="isLoggedIn">Welcome back, user!</div>
```
This div will only be shown if isLoggedIn is true.

43. What is the difference between a component and a directive?

Component: A component is a combination of HTML, CSS, and JavaScript logic encapsulated into a self-contained unit. Components have a template, styles, and a class to control behavior. Components are the building blocks of an Angular application and are typically associated with a view.

Example:
```
@Component({
  selector: 'app-hello',
  template: '<h1>Hello World</h1>'
})
export class HelloComponent {}
```
Directive: A directive is a class that adds behavior to an existing DOM element. Directives can be attribute directives (which change the behavior or appearance of an element) or structural directives (which manipulate the DOM structure, like ngIf and ngFor).

Example of a custom directive:
```
@Directive({
  selector: '[appHighlight]'
})
export class HighlightDirective {
  constructor(el: ElementRef) {
    el.nativeElement.style.backgroundColor = 'yellow';
  }
}
```

44. How do you share data between components in Angular?

@Input() and @Output(): You can share data from a parent component to a child component using @Input() and from a child to a parent using @Output() with event emitters.

Example of passing data using @Input:
```
@Component({
  selector: 'app-child',
  template: '<p>{{ data }}</p>'
})
export class ChildComponent {
  @Input() data: string;
}
```
Service with Dependency Injection: Another way to share data between components that are not in a parent-child relationship is by using a shared service. The service stores the shared data, and components can access it through dependency injection.

45. What are the differences between template-driven and reactive forms?

Template-driven forms:
- Uses two-way data binding (ngModel).
- Simpler to implement, especially for small forms.
- Form logic is defined in the template (HTML).
- Less scalable for complex forms.
Reactive forms:
- Form control and validation logic are implemented programmatically in the component class.
- Uses FormControl, FormGroup, and FormBuilder.
- More suitable for complex forms.

46. How can you prevent memory leaks in Angular applications?

Unsubscribe from Observables: When you subscribe to an Observable, make sure to unsubscribe when the component is destroyed to prevent memory leaks. You can do this manually or use operators like takeUntil.
Use Async Pipe: Using the async pipe in templates automatically handles subscriptions and unsubscriptions for you.
Remove Event Listeners: Ensure you clean up any DOM event listeners or custom event handlers when a component is destroyed.
ngOnDestroy Lifecycle Hook: Use the ngOnDestroy lifecycle hook to perform cleanup tasks when the component is destroyed.

47. What is the purpose of the async pipe?

The async pipe in Angular is used to automatically subscribe to an Observable or a Promise and return the latest value emitted. It also takes care of unsubscribing from the Observable when the component is destroyed, which helps prevent memory leaks.

48. How do you use Angular Material in your application?

To use Angular Material in your application:
Install Angular Material:
```
ng add @angular/material
```

Import the desired Material components in your app.module.ts file, such as:

import { MatButtonModule, MatCardModule } from '@angular/material';
@NgModule({
  imports: [MatButtonModule, MatCardModule]
})
export class AppModule { }

Use Material components in the template:

<button mat-raised-button>Click Me</button>
<mat-card>Material Card Content</mat-card>

49. What is the role of the providers array in Angular?

The providers array in Angular is used to define the services or dependencies that a component or module needs. When a service is listed in the providers array, Angular creates a new instance of the service for that component or module.

50. How do you handle routing parameters in Angular?

You can handle routing parameters in Angular by using the ActivatedRoute service. This service allows you to access both static and dynamic parameters from the URL.

Example of accessing route parameters:
```
constructor(private route: ActivatedRoute) {}

ngOnInit() {
  this.route.params.subscribe(params => {
    const id = params['id'];
    console.log('Route parameter id:', id);
  });
}
```
For query parameters, you can use this.route.queryParams.