TOP 20 Angular INTERVIEW QUESTIONS AND ANSWERS YOU NEED TO KNOW

Angular is a popular JavaScript framework used for building web applications. If you are applying for a job that involves Angular development, you can expect to be asked questions about your experience and knowledge of the framework. To help you prepare, here are some top Angular interview questions to expect:

If you want a complete and informative introduction to frontend JavaScript libraries and frameworks, I recommend reading Angular vs React vs Vue: Which Framework is Better?

What is Angular and what is it used for?

Angular is a JavaScript framework used for building web applications. It allows you to create reusable components, manage data, and build client-side applications using HTML, CSS, and TypeScript. Here is a simple example of an Angular component:

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

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

What is a component in Angular?

A component in Angular is a building block of an Angular application, consisting of a template, a class, and metadata. The template defines the component’s user interface, the class contains the data and logic, and the metadata provides additional information about the component. Here is a simple example of an Angular component:

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

@Component({
  selector: 'app-hello',
  template: `<h1>Hello, {{name}}!</h1>`
})
export class HelloComponent {
  name = 'world';
}

What are the differences between AngularJS and Angular?

AngularJS is an older version of Angular, while Angular is a newer, improved version. AngularJS is based on JavaScript and uses the Model-View-Controller (MVC) architecture, while Angular is based on TypeScript and uses the component-based architecture. Here is a simple example of an AngularJS component:

angular.module('myApp', [])
  .component('hello', {
    template: '<h1>Hello, {{$ctrl.name}}!</h1>',
    controller: function() {
      this.name = 'world';
    }
  });

How does Angular differ from React and Vue.js?

Angular is a full-featured framework, while React and Vue.js are libraries for building user interfaces. Angular provides a complete set of tools for building client-side applications, including a router, a dependency injection system, and a templating engine. React and Vue.js only provide the view layer, and you need to use additional libraries for other features. Here is a simple example of a React component:

import React from 'react';

class Hello extends React.Component {
  render() {
    return <h1>Hello, {this.props.name}!</h1>;
  }
}

export default Hello;

And here is a simple example of a Vue.js component:

<template>
  <h1>Hello, {{name}}!</h1>
</template>

<script>
export default {
  data() {
    return {
      name: 'world'
    };
  }
}
</script>

What is a service in Angular?

A service in Angular is a class that provides a specific functionality, such as accessing a database or calling an API. Services are a way to share information and functionality across components, and they are typically injected into components using dependency injection. Here is a simple example of an Angular service:

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

@Injectable()
export class HelloService {
  sayHello() {
    return 'Hello, world!';
  }
}

What is Angular Material and why is it used?

Angular Material is a collection of UI components built with Angular and Material Design principles. It is used for implementing the visual and layout design of an Angular application, and provides a consistent look and feel across all platforms. To use Angular Material, you need to import the components you want to use, and add them to the imports array of the Angular module. Here is a simple example of using Angular Material:

import { NgModule } from '@angular/core';
import { MatButtonModule } from '@angular/material/button';

@NgModule({
  imports: [MatButtonModule]
})
export class MyModule {}

What is a template in Angular?

A template in Angular is a form of HTML that tells Angular how to render the component’s view. The template can contain bindings, directives, and pipes, and it can also reference component methods and properties. Here is a simple example of an Angular template:

<h1>{{title}}</h1>
<p>{{description}}</p>

What is data binding in Angular?

Data binding in Angular is the automatic synchronization of data between the component class and the view. It allows you to update the view whenever the data in the component changes, and vice versa. There are different types of data binding, such as interpolation, property binding, and event binding. Here is a simple example of data binding using interpolation:

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

@Component({
  selector: 'app-hello',
  template: `<h1>Hello, {{name}}!</h1>`
})
export class HelloComponent {
  name = 'world';
}

What are the different types of data binding in Angular?

The different types of data binding in Angular are interpolation, property binding, event binding, and two-way binding. Interpolation is a one-way binding, where the data in the component is displayed in the view. Property binding is a one-way binding, where the data in the component is passed to a property of an HTML element. Event binding is a one-way binding, where the data in the component is updated based on an HTML element event. Two-way binding is a two-way binding, where the data in the component and the view are synchronized. Here is a simple example of data binding using interpolation, property binding, and event binding:

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

@Component({
  selector: 'app-hello',
  template: `
    <h1>Hello, {{name}}!</h1>
    <input [value]="name" (input)="name = $event.target.value">
  `
})
export class HelloComponent {
  name = 'world';
}

What is a directive in Angular and how is it used?

A directive in Angular is a class with a @Directive decorator. It is used to add behavior to an existing element or component, and it can be used to create custom HTML elements and attributes. There are two types of directives in Angular: structural and attribute. Structural directives change the structure of the DOM by adding or removing elements, and attribute directives change the appearance or behavior of an element. Here is a simple example of a directive:

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

@Directive({
  selector: '[myHighlight]'
})
export class HighlightDirective {
  // Directive logic goes here
}

What is dependency injection in Angular and why is it important?

Dependency injection in Angular is a design pattern used to provide a component with its dependencies. It is a way to decouple the component from the dependencies, and it allows the component to be more modular, reusable, and testable. Dependency injection is an important concept in Angular, and it is used throughout the framework to inject services, components, and other dependencies into components. Here is a simple example of dependency injection:

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

@Component({
  selector: 'app-my-component',
  template: '<p>{{message}}</p>'
})
export class MyComponent {
  message: string;

  constructor(@Inject(MyService) private service: MyService) {
    this.message = service.getMessage();
  }
}

What are pipes in Angular and how are they used?

Pipes in Angular are a way to transform and format data in a template. They are similar to functions, but they are used in templates, and they are denoted by the “pipe” character (|). Pipes are a powerful feature of Angular, and they provide a declarative and easy-to-use syntax for formatting and transforming data. They can be used for formatting dates, numbers, currencies, and other values, and they can also be used for filtering and sorting data. Here is a simple example of a pipe:

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

@Pipe({
  name: 'myUpperCase'
})
export class UpperCasePipe implements PipeTransform {
  transform(value: string): string {
    return value.toUpperCase();
  }
}

To use the pipe, you need to import it into the module where it is declared and add it to the declarations array of the module. Then you can use the pipe in a template by adding the pipe name after the “pipe” character (|), like this:

<p>{{name | myUpperCase}}</p>

What is the difference between a structural and attribute directive?

The difference between a structural and attribute directive is the way they change the DOM. Structural directives change the structure of the DOM by adding or removing elements, and they are denoted by an asterisk (*) in the template. Attribute directives change the appearance or behavior of an existing element, and they are denoted by the attribute name in the template. Structural directives are used to control the flow of the template, and they can add or remove elements based on the conditions. Attribute directives are used to change the appearance or behavior of an element, and they can add or remove CSS classes, styles, or event listeners to the element.

What is the Angular CLI and how is it used?

The Angular CLI is a command-line interface tool that helps you create, build, test, and deploy Angular applications. It is a powerful tool that simplifies many of the tasks involved in developing Angular applications, such as creating components and modules, running tests, and building and deploying the application. The Angular CLI uses the conventions and standards of Angular, so it is easy to use and learn, and it provides a consistent development experience. Here is an example of using the Angular CLI to create a new Angular application:

ng new my-app

This command creates a new Angular application called my-app in the current directory, and it generates the project files and dependencies for the application.

What is lazy loading and how is it used in Angular?

Lazy loading in Angular is a technique used to improve the performance of large applications. It allows you to load parts of the application on demand, instead of loading everything at once. This reduces the initial load time of the application, and it makes the application more efficient and scalable. To use lazy loading in Angular, you need to define the routes of the application using the loadChildren property, and specify the module and component to be loaded lazily. Here is a simple example of lazy loading in Angular

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

What is a router outlet in Angular?

A router outlet in Angular is a placeholder that the router uses to display the component for the current route. The router outlet is a directive provided by the RouterModule, and it is used in the template of the component that contains the routing logic. The router outlet allows the router to insert the component for the current route dynamically, and it updates the component when the route changes. Here is a simple example of a router outlet:

<router-outlet></router-outlet>

What is a route in Angular?

A route in Angular is a configuration object that defines how the router should navigate to a component, based on the URL. The route specifies the path and the component to be displayed for the path, and it can also define additional parameters, such as query parameters and fragments. The routes of an Angular application are defined in a routing module, and they are used by the router to navigate to the corresponding component when the URL changes. Here is a simple example of a route:

const routes: Routes = [
  {
    path: 'customers',
    component: CustomersComponent
  }
];

In this example, the routes array contains a single route that maps the /customers path to the CustomersComponent. When the user navigates to the /customers URL, the router will display the CustomersComponent in the router outlet.

What is an Angular module and how is it used?

An Angular module is a class with a @NgModule decorator. It is used to organize the components, directives, and pipes of an Angular application, and it provides a way to import, export, and configure the dependencies of the application. An Angular application can have multiple modules, and each module can have its own components, directives, and pipes. The root module of the application is the entry point of the application, and it defines the components, directives, and pipes that are available to the entire application. Here is a simple example of an Angular module:

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';

@NgModule({
  declarations: [MyComponent, MyPipe, MyDirective],
  imports: [CommonModule],
  exports: [MyComponent, MyPipe, MyDirective],
  providers: [MyService]
})
export class MyModule {}

In this example, the MyModule class is decorated with the @NgModule decorator, which provides metadata about the module. The decorator defines the components, directives, and pipes that belong to the module, and it also specifies the dependencies of the module, such as the CommonModule and the MyService.

What is the difference between Angular 2, 4, and 5?

The difference between Angular 2, 4, and 5 is the version number and the features and improvements introduced in each version. Angular 2 was released in 2016, and it introduced many improvements and new features, such as a more powerful templating engine, a more modular structure, and better support for mobile devices. Angular 4 was released in 2017, and it introduced smaller bundles, faster compilation, and improved type checking. Angular 5 was released in 2018, and it introduced a build optimizer, improved internationalization, and support for progressive web apps.

What are observables in Angular and how are they used?

Observables in Angular are a way to handle asynchronous data, such as data from a server or user input. They are a powerful feature of Angular, and they provide a declarative and reactive approach to handling asynchronous data. Observables are implemented using the Observable pattern, and they are part of the RxJS library. Observables are used extensively in Angular, and they are used for handling HTTP requests, user input, and other asynchronous events. Here is a simple example of an observable:

import { of } from 'rxjs';

const observable = of('Hello', 'world');

observable.subscribe(value => console.log(value));

In this example, the of operator is used to create an observable that emits the values 'Hello' and 'world'. The subscribe method is used to subscribe to the observable, and it specifies the callback function that will be called for each value emitted by the observable. This callback function logs the emitted value to the console.