Top 20 angular interview questions with answer and example for 2023.

Key Differences between Angular and AngularJS:

1. Architecture:

   • Angular: Uses a component-based architecture, where the application is composed of reusable components.
   • AngularJS: Uses a controller-based architecture and relies heavily on two-way data binding.

2. Language:

   • Angular: Written in TypeScript, a statically typed superset of JavaScript.
   • AngularJS: Written in JavaScript.

3. Dependency Injection:

   • Angular: Provides a powerful dependency injection system to manage the application's components and services.
   • AngularJS: Dependency injection is less sophisticated and relies on function arguments.

4. Performance:

   • Angular: Offers better performance due to ahead-of-time (AOT) compilation and a more efficient change detection mechanism.
   • AngularJS: Slower performance compared to Angular, especially for large applications.

Creating a New Angular Component:

To create a new Angular component, you can use the Angular CLI (Command Line Interface) or manually follow these steps:

1. Using the Angular CLI: Open your terminal and run the following command:

   ng generate component component-name
   

   This will generate a new component with the specified name, including files for the component class, HTML template, CSS styles, and a test file.

2. Manual Creation:

   • Create a new folder for your component within the appropriate directory (e.g., 'app' for a top-level component).
   • Inside the folder, create a TypeScript file for your component class (e.g., component-name.component.ts).
   • Create an HTML file for your component template (e.g., component-name.component.html).
   • Optionally, create a CSS file for styling (e.g., component-name.component.css).

One-Way Data Binding in Angular:

One-way data binding in Angular refers to the flow of data from the component class to the template. Data is bound in a unidirectional manner, meaning that changes in the component class are reflected in the template, but not vice versa. You use interpolation ({{ data }}) and property binding ([property]="data") for one-way data binding.

Example:

html
<!-- Template -->
<p>{{ message }}</p>

<!-- Component Class -->
export class MyComponent {
  message = 'Hello, Angular!';
}

Two-Way Data Binding in Angular:

Two-way data binding in Angular allows for bidirectional data flow between the component class and the template. It's commonly used for forms and uses the [(ngModel)] directive to bind properties in both directions.

Example:

html
<!-- Template -->
<input [(ngModel)]="name">

<!-- Component Class -->
export class MyComponent {
  name: string = 'John';
}

Dependency Injection in Angular:

Dependency injection (DI) in Angular is a design pattern that allows you to inject dependencies (e.g., services) into components, services, or other classes. DI helps manage the application's dependencies and promotes reusability and testability. It is important because it enables loose coupling between components and their dependencies.

Angular Lifecycle Hooks:

Angular provides lifecycle hooks that allow you to tap into various stages of a component's lifecycle. Some commonly used hooks include:

• ngOnInit: Called once when the component is initialized.
• ngOnChanges: Called when input properties change.
• ngOnDestroy: Called before the component is destroyed.

These hooks enable you to perform actions at specific moments in a component's life, such as initializing data or cleaning up resources.

Angular CLI (Command Line Interface):

The Angular CLI is a powerful command-line tool that simplifies Angular development tasks, such as creating components, services, modules, and generating production-ready builds. It provides project scaffolding, development servers, and a convenient way to manage your Angular applications.

Lazy Loading in Angular:

Lazy loading is a technique in Angular that loads parts of your application on-demand, rather than loading the entire application upfront. This can significantly improve initial page load times and reduce the size of the initial bundle. Lazy loading is typically used for feature modules to improve performance.

Angular Services:

Angular services are singleton objects used to organize and share code, data, or functionality across different parts of an Angular application. They are a way to encapsulate and abstract logic that doesn't belong in components, such as data retrieval, business logic, and communication with external APIs.

Angular Directives:

Angular directives are instructions in the HTML that tell Angular how to modify or manipulate the DOM. There are three types of directives in Angular: structural directives, attribute directives, and component directives. Examples of built-in directives include:

• ngFor: Used for iterating over arrays or lists.
• ngIf: Used for conditionally rendering elements.
• ngClass: Used for dynamically adding or removing CSS classes.
• ngStyle: Used for dynamically applying inline styles.

Handling Routing in Angular:

Angular provides a built-in router for managing navigation and routing in a single-page application. You can define routes, associate components with those routes, and navigate between views using the RouterModule. Configuration is typically done in the app's routing module (app-routing.module.ts).

Comparing Angular Template-Driven Forms and Reactive Forms:

Template-Driven Forms:

• Simpler to set up and use.
• Form controls and validation are defined in the HTML template.
• Limited flexibility for complex forms.
• Suitable for simple forms with minimal logic.

Reactive Forms:

• More complex setup but more powerful and flexible.
• Form controls and validation are defined programmatically in the component class.
• Ideal for complex forms with dynamic behavior.
• Better for unit testing.

Making HTTP Requests in Angular using HttpClient:

To make HTTP requests in Angular, you use the HttpClient module. First, import it and inject it into your component or service. Then, use it to send HTTP requests (GET, POST, PUT, DELETE) to a server and handle the responses using observables. Here's an example of making a GET request:

typescript
import { HttpClient } from '@angular/common/http';

@Injectable()
export class MyService {
  constructor(private http: HttpClient) {}

  fetchData() {
    return this.http.get('https://api.example.com/data');
  }
}

Angular Pipes:

Angular pipes are used for data transformation and formatting in templates. They allow you to manipulate data before rendering it in the view. Commonly used pipes include date, uppercase, lowercase, currency, and decimal.

Example:

html
<!-- Template -->
<p>{{ today | date: 'dd/MM/yyyy' }}</p>

<!-- Component Class -->
export class MyComponent {
  today: Date = new Date();
}

Angular Modules:

Angular applications are organized into modules, which group related components, services, and other parts of the application. Modules are defined using the @NgModule decorator and provide a way to encapsulate and manage different parts of an application.

Creating a Custom Directive in Angular:

To create a custom directive in Angular, you define a TypeScript class and decorate it with @Directive. You specify the selector used to apply the directive in the HTML, and you can define behavior using the HostListener and HostBinding decorators.

Example:

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

@Directive({
  selector: '[appHighlight]'
})
export class HighlightDirective {
  constructor(private el: ElementRef, private renderer: Renderer2) {}

  @HostListener('mouseenter') onMouseEnter() {
    this.renderer.setStyle(this.el.nativeElement, 'background-color', 'yellow');
  }

  @HostListener('mouseleave') onMouseLeave() {
    this.renderer.removeStyle(this.el.nativeElement, 'background-color');
  }
}

Using Angular ngFor:

ngFor is a structural directive used for iterating over arrays or lists in the template. It allows you to generate HTML elements dynamically based on the data in the array.

Example:

html
<!-- Template -->
<ul>
  <li *ngFor="let item of items">{{ item }}</li>
</ul>

<!-- Component Class -->
export class MyComponent {
  items: string[] = ['Item 1', 'Item 2', 'Item 3'];
}

Using Angular ngIf:

ngIf is a structural directive used for conditionally rendering elements in the template. It evaluates an expression, and if it's truthy, the element is displayed; otherwise, it's removed from the DOM.

Example:

html
<!-- Template -->
<div *ngIf="showElement">This element is shown if showElement is true.</div>

<!-- Component Class -->
export class MyComponent {
  showElement: boolean = true;
}

Using Angular ngClass and ngStyle Directives:

ngClass and ngStyle are attribute directives used to dynamically manipulate CSS classes and styles based on conditions in the template.

Example for ngClass:

html
<!-- Template -->
<div [ngClass]="{'active': isActive, 'error': hasError}">Dynamic CSS Classes</div>

<!-- Component Class -->
export class MyComponent {
  isActive: boolean = true;
  hasError: boolean = false;
}

Example for ngStyle:

html
<!-- Template -->
<div [ngStyle]="{'color': textColor, 'font-size.px': fontSize}">Dynamic Styles</div>

<!-- Component Class -->
export class MyComponent {
  textColor: string = 'blue';
  fontSize: number = 16;
}

These are fundamental aspects of Angular, and understanding them will help you build robust web applications using the framework.