Angular Interview Questions

1. What is Angular, and How Does it Differ from AngularJS?

Angular is a powerful, open-source framework developed by Google for building dynamic and scalable single-page applications (SPAs). It is designed to enhance web development by offering a structured and modular approach. Unlike its predecessor, AngularJS, which is based on JavaScript, Angular is built using TypeScript—a statically typed superset of JavaScript—allowing for better maintainability and error detection.

One of the major differences between Angular and AngularJS is the shift from a controller-based architecture to a component-based structure, which improves reusability, scalability, and organization. Angular also offers enhanced performance due to features like Ahead-of-Time (AOT) compilation and a more efficient change detection mechanism. Additionally, Angular provides better tooling, including the Angular CLI, which simplifies project setup, development, and deployment.

While both frameworks support two-way data binding, Angular’s implementation is more optimized, reducing unnecessary DOM manipulations and improving efficiency. Dependency injection in Angular is more advanced, allowing for better management of services and modularity. AngularJS lacks many of these modern optimizations, making Angular a more suitable choice for large-scale and complex applications. The transition from AngularJS to Angular reflects the need for more scalable, maintainable, and high-performance web applications.

2. Key Features of Angular

Angular offers a comprehensive set of features that make it a robust framework for modern web development:

  1. Component-Based Architecture – Angular uses reusable and self-contained components, making development more structured and modular. This improves code organization, scalability, and maintainability.

  2. Two-Way Data Binding – Angular ensures real-time synchronization between the data model and the user interface, reducing the need for manual DOM manipulation and improving development efficiency.

  3. Dependency Injection (DI) – The DI system in Angular simplifies service management, making applications more scalable and modular. This approach enhances code reusability and testing.

  4. Angular CLI (Command Line Interface) – The Angular CLI provides powerful tools for creating, configuring, and deploying applications. It automates repetitive tasks like generating components, services, and modules, improving developer productivity.

  5. Ahead-of-Time (AOT) Compilation – Angular compiles TypeScript and HTML templates into optimized JavaScript before runtime, leading to faster loading times and better performance.

  6. RxJS and Reactive Programming – Angular integrates RxJS (Reactive Extensions for JavaScript), enabling efficient handling of asynchronous data streams and event-driven programming. This is especially useful for real-time applications.

  7. Lazy Loading – This feature improves performance by loading only the required modules when needed, reducing the initial load time and enhancing user experience.

  8. Robust Routing System – Angular provides a built-in routing module that enables navigation between views, including support for route guards, lazy-loaded modules, and nested routing.

  9. Cross-Platform Development – With Angular, developers can build progressive web applications (PWAs), native mobile apps (using frameworks like Ionic), and even desktop applications.

  10. Built-in Testing Tools – Angular supports unit and end-to-end testing with tools like Jasmine and Karma, ensuring high code quality and reducing debugging efforts.

  11. Security Features – Angular includes built-in security measures, such as sanitization of user inputs and protection against cross-site scripting (XSS) attacks, making applications more secure.

These features collectively make Angular a highly efficient and versatile framework for building dynamic, scalable, and high-performance web applications.

3. What is a component in Angular, and how is it structured?

A component in Angular is a fundamental building block of the application, representing a reusable piece of the user interface. It consists of three main parts: the template (HTML), the class (TypeScript), and the metadata (decorator). The template defines the UI structure, the class contains the logic and data, and the metadata (using the @Component decorator) provides configuration details like the selector, template URL, and styles. For example:

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  title = 'My Angular App';
}

Components are modular and reusable, making them essential for building scalable applications.

4. What is data binding in Angular, and what are its types?

Data binding in Angular is a mechanism that connects the application’s data (model) with the user interface (view). There are four types of data binding:

  1. Interpolation ({{ }}): Displays component data in the view.
  2. Property Binding ([ ]): Binds component data to an HTML element’s property.
  3. Event Binding (( )): Listens for user events and triggers component methods.
  4. Two-Way Binding ([( )]): Combines property and event binding to synchronize the model and view.
    For example:
<input [(ngModel)]="name">
<p>Hello, {{ name }}!</p>

Data binding simplifies the development process by automating updates between the model and view.

5. What is dependency injection in Angular?

Dependency injection (DI) in Angular is a design pattern that allows a class to receive its dependencies from an external source rather than creating them itself. Angular’s DI system provides services, components, and other dependencies to classes through constructor parameters. For example:

constructor(private service: MyService) {}

This approach promotes modularity, testability, and reusability. Angular’s injector maintains a hierarchy of injectors, ensuring that dependencies are provided efficiently. DI is a core feature of Angular, enabling developers to build loosely coupled and maintainable applications.

6. What is Angular CLI, and why is it used?

Angular CLI (Command Line Interface) is a powerful tool for scaffolding, building, and managing Angular applications. It simplifies tasks like creating components, services, modules, and more through commands like ng generate. For example:

ng generate component my-component

Angular CLI also supports building the application for production (ng build), running tests (ng test), and serving the application locally (ng serve). It ensures consistency and best practices across projects, making it an essential tool for Angular developers.

7. What is a module in Angular, and how is it structured?

A module in Angular is a container for a cohesive block of functionality, defined using the @NgModule decorator. It organizes components, directives, pipes, and services into a single unit. The @NgModule decorator includes metadata like declarations, imports, exports, and providers. For example:

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

Modules promote modularity and reusability, allowing developers to break the application into smaller, manageable parts.

8. What is a service in Angular, and how is it created?

A service in Angular is a class that provides reusable functionality, such as data fetching or business logic, across components. Services are created using the @Injectable decorator and registered with the dependency injection system. For example:

@Injectable({
  providedIn: 'root'
})
export class MyService {
  getData() {
    return 'Data from service';
  }
}

Services promote code reusability and separation of concerns, making them a key part of Angular applications.

9. What is routing in Angular, and how is it configured?

Routing in Angular enables navigation between different views or components in a single-page application. It is configured using the RouterModule and a set of routes defined in an array. For example:

const routes: Routes = [
  { path: '', component: HomeComponent },
  { path: 'about', component: AboutComponent }
];
@NgModule({
  imports: [RouterModule.forRoot(routes)],
  exports: [RouterModule]
})
export class AppRoutingModule {}

Routing enhances user experience by allowing seamless navigation without page reloads.

10. What is lazy loading in Angular?

Lazy loading in Angular is a technique that loads feature modules on demand, rather than at the initial load. This improves application performance by reducing the initial bundle size. Lazy loading is configured in the routing module using the loadChildren property. For example:

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

Lazy loading is essential for optimizing large applications with multiple modules.

11. What are Angular directives, and what are their types?

Directives in Angular are instructions that modify the DOM’s behavior or structure. There are three types of directives:

  1. Component Directives: Define components with templates.
  2. Structural Directives: Modify the DOM layout (e.g., *ngIf, *ngFor).
  3. Attribute Directives: Change the appearance or behavior of elements (e.g., ngClass, ngStyle).
    Directives are a powerful feature for creating dynamic and interactive UIs.

12. What is the difference between ngIf and ngSwitch directives?

The ngIf directive conditionally adds or removes an element from the DOM based on a boolean expression. For example:

<div *ngIf="isVisible">Content</div>

The ngSwitch directive is used for conditional rendering with multiple cases. For example:

<div [ngSwitch]="value">
  <p *ngSwitchCase="'A'">Case A</p>
  <p *ngSwitchCase="'B'">Case B</p>
</div>

While ngIf is simpler for single conditions, ngSwitch is better for multiple conditions.

13. What is the purpose of the ngFor directive?

The ngFor directive is used to iterate over a collection and render a template for each item. For example:

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

It is commonly used for displaying lists or tables. The ngFor directive supports additional features like tracking items by index or a unique identifier using trackBy.

14. What is Angular’s change detection mechanism?

Angular’s change detection mechanism automatically updates the view when the model changes. It uses a tree of change detectors to track changes in component properties. Angular employs two strategies:

  1. Default: Checks all components on every change.
  2. OnPush: Only checks components when their input properties change.
    Change detection ensures that the UI remains in sync with the application state.

15. What is the purpose of the async pipe in Angular?

The async pipe in Angular is used to subscribe to observables or promises directly in the template. It automatically manages subscriptions and updates the view when new data is emitted. For example:

<div>{{ data$ | async }}</div>

The async pipe simplifies working with asynchronous data and avoids manual subscription management.

Certainly! Here are the next set of Angular interview questions and detailed answers:

16. What is the purpose of the ngClass directive in Angular?

The ngClass directive in Angular is used to dynamically add or remove CSS classes to an HTML element based on an expression. It allows you to apply multiple classes conditionally. For example:

<div [ngClass]="{'active': isActive, 'error': hasError}">Content</div>

In this example, the active class is applied if isActive is true, and the error class is applied if hasError is true. The ngClass directive is particularly useful for creating dynamic and responsive user interfaces, where the appearance of elements changes based on the application’s state.

17. What is the difference between ngModel and formControl in Angular?

ngModel is a directive used for two-way data binding in template-driven forms, allowing you to bind an input element to a property in the component. For example:

<input [(ngModel)]="name">

formControl, on the other hand, is used in reactive forms to manage the state of individual form controls programmatically. For example:

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

While ngModel is simpler and suited for small forms, formControl provides more control and is ideal for complex forms with dynamic behavior.

18. What is the purpose of the ViewChild decorator in Angular?

The ViewChild decorator in Angular is used to access a child component, directive, or DOM element from the parent component. It allows you to interact with the child’s properties and methods. For example:

@ViewChild('myElement') myElement: ElementRef;
ngAfterViewInit() {
  console.log(this.myElement.nativeElement);
}

In this example, myElement refers to a template reference variable (#myElement) in the parent component’s template. The ViewChild decorator is commonly used for accessing DOM elements or interacting with child components.

19. What is the purpose of the ngOnInit lifecycle hook in Angular?

The ngOnInit lifecycle hook in Angular is called after the component has been initialized and its inputs have been set. It is commonly used for initialization logic, such as fetching data from a service or setting up subscriptions. For example:

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

Unlike the constructor, which is used for dependency injection, ngOnInit is the ideal place for component initialization tasks. It ensures that the component is fully set up before performing any operations.

20. What is the difference between ngOnInit and ngAfterViewInit?

ngOnInit is called after the component has been initialized and its inputs have been set, making it suitable for initialization logic. ngAfterViewInit, on the other hand, is called after the component’s view and child views have been fully initialized. It is used for tasks that require access to the DOM or child components. For example:

ngAfterViewInit() {
  console.log('View initialized');
}

While ngOnInit is used for general setup, ngAfterViewInit is used for view-specific tasks.

21. What is the purpose of the ngOnDestroy lifecycle hook in Angular?

The ngOnDestroy lifecycle hook in Angular is called just before the component is destroyed. It is used for cleanup tasks, such as unsubscribing from observables or releasing resources. For example:

ngOnDestroy() {
  this.subscription.unsubscribe();
}

This hook ensures that the component does not leave behind any memory leaks or unused resources, making it essential for maintaining application performance.

22. What is the purpose of the RouterOutlet directive in Angular?

The RouterOutlet directive in Angular is used as a placeholder in the template where the router should render the components for the current route. For example:

<router-outlet></router-outlet>

When the user navigates to a route, the corresponding component is rendered inside the RouterOutlet. This directive is a key part of Angular’s routing system, enabling seamless navigation between views in a single-page application.

23. What is the purpose of the ActivatedRoute service in Angular?

The ActivatedRoute service in Angular provides access to information about the current route, such as route parameters, query parameters, and data. It is commonly used to retrieve dynamic values from the URL. For example:

constructor(private route: ActivatedRoute) {}
ngOnInit() {
  this.route.params.subscribe(params => {
    this.id = params['id'];
  });
}

The ActivatedRoute service is essential for building dynamic and data-driven applications that rely on route parameters.

24. What is the purpose of the HttpClient service in Angular?

The HttpClient service in Angular is used to make HTTP requests to a server, such as GET, POST, PUT, and DELETE requests. It provides a simple and powerful API for handling HTTP communication. For example:

constructor(private http: HttpClient) {}
getData() {
  return this.http.get('https://api.example.com/data');
}

The HttpClient service supports features like interceptors, error handling, and response typing, making it a robust tool for working with APIs.

25. What is the purpose of interceptors in Angular?

Interceptors in Angular are used to intercept and modify HTTP requests and responses globally. They are commonly used for tasks like adding headers, logging, or handling errors. For example:

@Injectable()
export class AuthInterceptor implements HttpInterceptor {
  intercept(req: HttpRequest<any>, next: HttpHandler) {
    const authReq = req.clone({
      headers: req.headers.set('Authorization', 'Bearer token')
    });
    return next.handle(authReq);
  }
}

Interceptors are registered in the providers array of the AppModule and provide a centralized way to manage HTTP communication.