Angular Interview Questions

Angular is an open-source front-end web framework developed and maintained by Google. It is primarily used for building dynamic, single-page web applications (SPAs) using TypeScript as its main programming language. Angular provides a robust toolset for building scalable, maintainable, and high-performance applications.

Main Features of Angular:

• Two-Way Data Binding: Automatically synchronizes data between the model and the view, reducing boilerplate code.
• Dependency Injection: Built-in dependency injection system improves code reusability and modularity.
• Modularization: Breaks down the application into reusable and manageable feature modules.
• Templating: Uses declarative templates with directives to dynamically render and update UI.
• RESTful API Handling: Simplifies HTTP communication using Angular’s HttpClient module for consuming RESTful services.

Angular is ideal for building enterprise-grade applications, offering advanced tooling, rich ecosystem, and strong support for testing and development workflows.

Angular was introduced to address the complexities of building modern, dynamic, and interactive web applications—especially Single Page Applications (SPAs).

Traditional JavaScript development made it difficult to manage application state, user interactions, and UI updates efficiently. Angular was designed to solve these problems by providing a structured, scalable, and maintainable framework.

Angular provides two types of compilation methods that determine when and how the application code is converted into executable JavaScript:

Single Page Applications (SPAs) are web applications that dynamically update the content of a single HTML page without reloading the entire page from the server. Instead of navigating to a new page for each interaction, SPAs use JavaScript to modify the current page’s content, providing a smoother and faster user experience.

Example: Gmail, Facebook, Twitter, and GitHub are popular examples of SPAs.

A component in Angular is a core building block of the application. It represents a portion of the user interface (UI) and includes the logic and data needed to render and control that part of the UI.

Each component in Angular consists of three main parts:

• HTML template – defines the view or layout of the component.
• TypeScript class – contains the logic and data for the component.
• CSS/SCSS styles – defines the appearance of the component.

Components help in breaking the application into smaller, manageable, and reusable UI pieces.

Example of a component:

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

@Component({
    selector: 'app-header',
    templateUrl: './header.component.html',
    styleUrls: ['./header.component.css']
})
export class HeaderComponent {
    @Input() title: string;
    @Input() links: { name: string, url: string }[];

    constructor() { }
}

In this example:

• selector defines the custom HTML tag <app-header> used in other components.
• templateUrl and styleUrls point to the HTML and CSS files.
• @Input() allows the component to receive data from a parent component.

AngularJS and Angular are both web application frameworks developed by Google, but they differ in architecture, language, performance, and usage. Below is a comparison table outlining their key differences:

The @Component decorator in Angular is a core building block that turns a TypeScript class into a component. It provides metadata that tells Angular how to create and use the component in your application.

Purposes of @Component:

• Defining the Component: It marks a class as an Angular component and enables Angular to instantiate it properly.
• Template Association: Specifies the HTML file or inline HTML that defines the component’s view using the templateUrl or template property.
• Style Binding: Connects the component to one or more style files using the styleUrls or inline styles via styles.
• Selector Definition: Defines a custom HTML tag name that can be used to insert the component into templates.
• Dependency Injection Configuration: Allows you to specify providers that are only available to that component and its children.

Example:

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

@Component({
  selector: 'app-header',
  templateUrl: './header.component.html',
  styleUrls: ['./header.component.css'],
  providers: []
})
export class HeaderComponent {
  title = 'Welcome to My App';
}

In this example:

• selector: Defines the HTML tag <app-header>
• templateUrl: Points to the HTML layout for the view
• styleUrls: Links to the component’s styles
• providers: Can include services to be injected into the component

This decorator allows Angular to fully configure, render, and interact with your component.

Angular CLI (Command Line Interface) is a powerful development tool that simplifies the process of creating, configuring, and managing Angular applications. It automates many development tasks and ensures best practices are followed from the start.

Common Angular CLI Commands:

ng new my-app

Creates a new Angular project named my-app.

ng serve

Runs the application locally with live-reload at http://localhost:4200.

ng generate component header

Generates a new component named header in your project.

ng build --prod

Builds the application for production with optimized output.

Directives in Angular are classes that add behavior to elements in your Angular applications. They are declared using the @Directive decorator and can be reused across multiple components to encapsulate and apply common functionalities.

When to use a directive?

If multiple components need similar functionality (e.g., styling or conditional rendering), you can create a directive to encapsulate this behavior and apply it wherever needed. This avoids code duplication and improves maintainability.

Types of Directives:

1. Component Directives

These are the most common directives in Angular. Technically, every Angular component is a directive with a template. Declared using @Component instead of @Directive, they include:

• A selector (custom HTML tag)
• A template or template URL
• Styles (optional)

2. Structural Directives

These directives change the structure of the DOM. They typically begin with an asterisk (*), such as *ngIf and *ngFor.

<div *ngIf="isReady" class="display_name">
    {{ name }}
</div>

<div class="details" *ngFor="let x of details">
    <p>{{ x.name }}</p>
    <p>{{ x.address }}</p>
    <p>{{ x.age }}</p>
</div>

Explanation:

• *ngIf renders the element only if the condition is true.
• *ngFor iterates through a list and creates a DOM node for each item.

3. Attribute Directives

These directives are used to modify the appearance or behavior of DOM elements (e.g., change background color, apply styles). You can also create custom attribute directives.

Creating a custom attribute directive:

Generate the directive using Angular CLI:

Modify the generated directive as follows:

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

@Directive({
  selector: '[appBlueBackground]'
})
export class BlueBackgroundDirective {
  constructor(el: ElementRef) {
    el.nativeElement.style.backgroundColor = 'blue';
  }
}

Usage in HTML:

This directive applies a blue background to any element it is attached to.

In Angular, Components, Modules, and Services are core concepts that allow developers to build scalable and maintainable applications.

Step 1: Creating an Angular Application

To begin, create a new Angular application by running the following command in the terminal:

This command generates the directory structure and necessary files for your Angular application.

Components

Components are the building blocks of an Angular app. Each component controls a specific part of the UI and includes:

• A template (HTML)
• A stylesheet (CSS/SCSS)
• A class (TypeScript logic)

Generate a component:

Example: test.component.ts

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

@Component({
  selector: 'app-test',
  templateUrl: './test.component.html',
  styleUrls: ['./test.component.css']
})
export class TestComponent implements OnInit {
  constructor() { }
  ngOnInit() { }
}

Modules

A Module groups together components, directives, pipes, and services. Every Angular app has at least one module (the root module), defined with @NgModule.

Types of Modules:

• Root Module: Loads the app (uses BrowserModule)
• Feature Module: Adds features (uses CommonModule)

Example: Root Module (app.module.ts)

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppComponent } from './app.component';
import { TestComponent } from './test/test.component';

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

Generate a feature module:

Example: Feature Module

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

@NgModule({
  declarations: [],
  imports: [
    CommonModule
  ]
})
export class TestModuleModule { }

Services

Services are singleton objects used to share logic or data across multiple components. They are decorated with @Injectable and injected using Angular’s Dependency Injection system.

Generate a service:

Example: Service Definition

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

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

  getMessage() {
    return 'Hello from the service!';
  }
}

You can use this service in any component by importing it and injecting it through the constructor.

Data binding in Angular is a powerful feature that allows seamless communication between the component class (TypeScript code) and the view (HTML template).

It simplifies the process of building interactive applications by automatically synchronizing data between the DOM and the component logic.

Types of Data Binding in Angular:

1. Property Binding
2. Event Binding
3. String Interpolation
4. Two-Way Data Binding

1️. Property Binding

Connects a DOM element property to a component class property.

This sets the src attribute of the <img> tag to the value of imageUrl in the component.

2️. Event Binding

Listens to DOM events and calls a method in the component when an event occurs.

This calls the onClick() method in the component when the button is clicked.

3️. String Interpolation

Displays component data in the view using double curly braces {{ }}.

This outputs the value of userName into the HTML.

4️. Two-Way Data Binding

Combines property and event binding so that changes in the input field reflect in the component and vice versa. It uses [(ngModel)].

This keeps the name property in sync with the input field.

In Angular, data binding is a powerful feature that enables communication between the component class and the DOM. Angular supports both one-way and two-way data binding based on the application needs.

Dependency Injection (DI) in Angular is a design pattern used to supply components and services with their dependencies instead of having them create those dependencies themselves. Rather than instantiating services or objects manually using the new keyword inside a component, Angular’s built-in DI system handles the creation and delivery of those objects automatically.

The primary purpose of DI is to promote better modularity, code reusability, and testability. When components or services receive their dependencies externally, it becomes easier to swap or mock them during unit testing, and the overall architecture becomes more loosely coupled and maintainable.

Angular uses decorators like @Injectable() to mark a class as available for injection, and it registers services with a provider mechanism. Services marked with providedIn: 'root' are registered globally and shared across the entire application.

// my-service.service.ts
import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root'
})
export class MyService {
  getMessage() {
    return "Hello from MyService!";
  }
}

// example.component.ts
import { Component, OnInit } from '@angular/core';
import { MyService } from './my-service.service';

@Component({
  selector: 'app-example',
  template: '<p>{{ message }}</p>'
})
export class ExampleComponent implements OnInit {
  message: string;

  constructor(private myService: MyService) {}

  ngOnInit() {
    this.message = this.myService.getMessage();
  }
}

In this example, Angular automatically provides an instance of MyService to the component via the constructor. This is Dependency Injection in action — simplifying code while maintaining separation of concerns.

Lazy loading in Angular is a powerful performance optimization technique where specific feature modules are loaded on demand, rather than during the initial application load. This means that when the application starts, only the core or most essential modules are loaded first. The other modules, typically those associated with different routes or rarely used features, are fetched and loaded into the browser only when the user navigates to them.

The purpose of lazy loading is to reduce the initial bundle size of the application. By not loading unnecessary modules upfront, the browser has fewer resources to download, parse, and render during startup. This significantly improves the load time and responsiveness of the app, especially in large-scale enterprise applications or SPAs (Single Page Applications) with many features.

Lazy loading is commonly implemented in Angular by defining routes with the loadChildren property inside the RouterModule. This tells Angular to load the module only when that particular route is visited.

// app-routing.module.ts
const routes: Routes = [
  {
    path: 'admin',
    loadChildren: () => import('./admin/admin.module').then(m => m.AdminModule)
  }
];

In the example above, the AdminModule will not be loaded until the user navigates to /admin. This keeps the initial application lightweight and enhances performance.

Lazy loading is particularly effective when used with Angular's routing system and modular architecture. It allows developers to structure applications in a way that improves maintainability and scalability while providing a better user experience by decreasing load times and bandwidth usage.

In Angular, data can be passed between components in several ways. The most common method is by using the @Input and @Output decorators to pass data between a parent and a child component.

1. Parent to Child using @Input:

Use the @Input decorator in the child component to receive data from the parent.

// child.component.ts
import { Component, Input } from '@angular/core';

@Component({
  selector: 'app-child',
  templateUrl: './child.component.html',
  styleUrls: ['./child.component.css']
})
export class ChildComponent {
  @Input() childData: string;
}

// child.component.html

  
Data from parent: {{ childData }}

// parent.component.ts
import { Component } from '@angular/core';

@Component({
  selector: 'app-parent',
  templateUrl: './parent.component.html',
  styleUrls: ['./parent.component.css']
})
export class ParentComponent {
  parentData: string = 'Hello from Parent Component!';
}

// parent.component.html

  

App Module Setup:

Declare both components in your module.

// app.module.ts
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { AppComponent } from './app.component';
import { ParentComponent } from './parent/parent.component';
import { ChildComponent } from './child/child.component';

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

Angular interceptors are special services that sit between the application and the backend server, allowing you to intercept and manipulate HTTP requests and responses globally. They are part of Angular’s HttpClient module and implement the HttpInterceptor interface.

Interceptors are particularly useful for:

• Adding authentication tokens (e.g., JWT) to HTTP headers
• Logging or debugging HTTP requests/responses
• Handling global errors or retry mechanisms
• Modifying request or response bodies

Here’s a simple example of an Angular interceptor that adds an Authorization header to outgoing requests:

To use an interceptor, register it in the providers array of your module:

This configuration ensures the interceptor is used throughout the application to enhance HTTP request handling consistently.

Angular lifecycle hooks are special methods that allow developers to execute custom logic at specific moments during the lifespan of a component or directive. These hooks provide insight into the creation, update, and destruction phases of a component, giving precise control over how and when to run code.

Angular calls these lifecycle methods automatically as a component moves through different phases, such as initialization, change detection, rendering, and destruction. By implementing these hooks, developers can respond to input changes, manipulate the DOM, or clean up resources.

By implementing these lifecycle hooks in your component class, you can optimize resource management, control rendering behavior, and enhance performance.

Authentication in Angular is typically implemented using JWT (JSON Web Token), Angular Guards, and interceptors. This setup ensures that only authenticated users can access certain routes and that secure tokens are included with each request to protected APIs.

Here’s how you can secure a route using an AuthGuard that checks if a user is authenticated:

Now protect routes using this guard:

With this setup, unauthorized users will be redirected to the login page if they try to access protected routes. Additional layers such as token expiration handling and route role-based access can also be added for better security.

The Signal API is a key tool in simplifying state management, making Angular applications more responsive and efficient. Signal API has evolved to provide more efficient tracking of reactive state changes and dependencies.

It helps in managing data flow by introducing reactive signals, which automatically detect changes in state and update the view without requiring manual change detection. This reduces the complexity of handling reactivity and optimizes performance by minimizing unnecessary updates.

By using signals, Angular can perform fine-grained change detection, enabling applications to scale better and respond faster to state changes.

NgZone in Angular is a service that helps Angular know when to update the view by tracking asynchronous operations. It acts as a wrapper around browser APIs like setTimeout, Promise, or HTTP calls, and ensures Angular’s change detection is triggered after these operations complete.

Without NgZone, Angular might not automatically recognize changes made outside its context. NgZone bridges this gap by informing Angular whenever such asynchronous tasks complete, ensuring the UI stays in sync with the application state.

Ivy is Angular's next-generation rendering engine introduced starting from Angular 9. It was designed to significantly improve performance, reduce bundle sizes, and enhance development experience. Ivy makes Angular applications faster and more efficient both at runtime and during development.

NgModule in Angular serves as a fundamental organizational unit for structuring and managing an application. It allows developers to group related components, directives, pipes, and services into a cohesive block of functionality.

The primary purpose of @NgModule is to define a compilation context. It tells Angular how to compile and launch the application or a specific feature module. Every Angular app has at least one root module (usually AppModule) and can have many feature modules.

Here is a simple example of an NgModule:

This modular architecture promotes better organization, reuse of code, lazy loading of features, and maintainability of large Angular applications.

Web Workers in Angular provide a way to run computationally intensive tasks in the background, separate from the main UI thread. This approach enhances the performance and responsiveness of Angular applications, particularly those that perform heavy data processing.

Here are the key benefits of using Web Workers:

• Improved Performance: By moving intensive computations to background threads, Web Workers free up the main thread, ensuring smoother and faster rendering of the user interface.
• Enhanced User Experience: Complex or long-running operations don't freeze the UI, allowing users to interact with the application without delays.
• Parallel Processing: Tasks can be processed in parallel, which helps in handling large datasets or complex calculations more efficiently.
• Better Scalability: With distributed processing, applications can scale more effectively and perform consistently even under heavy load or data processing scenarios.

Using Web Workers in Angular is especially beneficial for apps involving real-time data analysis, image processing, machine learning tasks, or large JSON processing.

In Angular, pipes are used to transform data in templates. They can be categorized into pure and impure pipes based on how and when they execute.

Use pure pipes for performance-critical areas, and reserve impure pipes for cases where the data is dynamic and needs constant transformation.

To validate both the format and uniqueness of an email input in Angular, we use Reactive Forms with a combination of built-in validators and a custom asynchronous validator.

Here's how to implement it:

import { FormBuilder, FormGroup, Validators, AbstractControl, ValidationErrors } from '@angular/forms';
import { of } from 'rxjs';
import { map, delay } from 'rxjs/operators';
import { Component } from '@angular/core';

@Component({
  selector: 'app-email-form',
  template: \`
    <form [formGroup]="emailForm">
      <input type="email" formControlName="email" placeholder="Enter email">
      <div *ngIf="emailForm.get('email')?.errors?.['emailInUse']">Email already in use</div>
      <div *ngIf="emailForm.get('email')?.errors?.['email']">Invalid email format</div>
    </form>
  \`
})
export class EmailFormComponent {
  emailForm: FormGroup;

  constructor(private fb: FormBuilder) {
    this.emailForm = this.fb.group({
      email: ['', 
        [Validators.required, Validators.email], 
        [this.uniqueEmailValidator.bind(this)]
      ]
    });
  }

  uniqueEmailValidator(control: AbstractControl) {
    const emailsInUse = ['test@example.com', 'user@example.com'];
    return of(emailsInUse.includes(control.value)).pipe(
      delay(500), // simulate API delay
      map(isInUse => isInUse ? { emailInUse: true } : null)
    );
  }
}

Explanation:

• Validators.email ensures the email is in a proper format.
• uniqueEmailValidator is an async validator that simulates a server check for already-used emails.
• The validation error emailInUse is shown if the email already exists.

This approach improves user experience by providing real-time feedback and ensures valid, unique email submission.

To handle multiple asynchronous API calls and ensure the data is displayed only after all responses are received, we can use the RxJS forkJoin operator in Angular. forkJoin is ideal for making concurrent API requests and receiving the results together once all requests are completed.

Here’s how to implement it:

import { Component, OnInit } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { forkJoin } from 'rxjs';

@Component({
  selector: 'app-data-fetcher',
  template: \`
    <div *ngIf="combinedData">
      <pre>{{ combinedData | json }}</pre>
    </div>
  \`
})
export class DataFetcherComponent implements OnInit {
  combinedData: any;

  constructor(private http: HttpClient) {}

  ngOnInit() {
    this.getData();
  }

  getData() {
    const api1$ = this.http.get('https://api1.example.com/data');
    const api2$ = this.http.get('https://api2.example.com/data');

    forkJoin([api1$, api2$]).subscribe(
      ([api1Response, api2Response]) => {
        // Combine or process responses here
        this.combinedData = {
          api1: api1Response,
          api2: api2Response
        };
      },
      error => {
        console.error('Error fetching data:', error);
      }
    );
  }
}

Explanation:

• forkJoin waits for all provided observables (API calls) to complete.
• Once complete, it emits an array of results in the same order as passed.
• This is useful when you want to load multiple sets of data before rendering the view.

This approach ensures efficient parallel API calls and consistent UI rendering only after all data is available.

To protect specific routes in an Angular application and ensure that only authenticated users can access them, Route Guards are used—specifically the CanActivate interface. A route guard checks whether navigation to a route should be allowed or not based on the user's authentication status.

Here is an example of how to implement an authentication guard:

In this code:

• AuthGuard implements the CanActivate interface to determine if the route can be activated.
• authService.isAuthenticated() checks if the user is logged in.
• If not authenticated, the user is redirected to the login page.

To apply this guard to a route, configure your routes as follows:

This approach ensures that only authorized users can access certain routes in your Angular application.

When an Angular application needs to fetch data from multiple APIs and display them together on the same page, it’s important to ensure that all asynchronous API calls are completed before processing or displaying the data. This can be effectively achieved using the forkJoin operator from RxJS.

forkJoin is a utility that allows concurrent execution of multiple observables and emits the final values as an array once all observables complete. It is particularly useful when you need all API results before continuing further processing.

Here’s an example of how to use forkJoin in an Angular service or component:

By using forkJoin, the application ensures that both API calls complete successfully before proceeding with rendering or manipulating the data, thus improving reliability and user experience.

In Angular, if a component is not updating as expected, especially when using the OnPush change detection strategy, the problem is often due to Angular not detecting object mutations. Angular with OnPush only checks for changes when:

• The component receives new inputs (with different references).
• An event or observable triggers change detection.

Step 1: Check if OnPush is enabled:

@Component({
    selector: 'app-sample',
    templateUrl: './sample.component.html',
    changeDetection: ChangeDetectionStrategy.OnPush
})

Step 2: Ensure immutability: If an object is mutated directly (e.g., this.data.name = 'new'), Angular won't detect the change. Instead, create a new object reference like this:

updateData() {
    this.data = { ...this.data, newValue: 'updated' };
}

Step 3: Manually trigger change detection: If reference change isn't an option or change needs to be forced, use ChangeDetectorRef:

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

constructor(private cd: ChangeDetectorRef) {}

someMethod() {
    // Some logic here...
    this.cd.markForCheck();
}

Conclusion: When using ChangeDetectionStrategy.OnPush, always ensure you're either replacing the object (immutability) or manually telling Angular to check for changes using markForCheck(). This ensures updates are recognized and the view reflects the latest data.

One effective way to optimize a large Angular application is by implementing lazy loading. Lazy loading allows Angular to load feature modules only when they are needed, rather than including them in the initial bundle. This reduces the initial load time and improves overall performance, especially in large-scale applications.

To implement lazy loading, define routes using the loadChildren syntax in the routing module. Here's an example:

In this example, Angular loads FeatureModule only when the user navigates to the /feature route. This approach minimizes the initial bundle size and helps applications scale efficiently by loading only the necessary resources on demand.