Angular Interview Questions and Answers (2025)

Question: What is data binding in Angular? Explain different types of data binding.

Answer:

Data binding in Angular is the mechanism that allows synchronization of data between the component class (TypeScript) and the view (HTML template). It establishes a communication channel where the component’s properties and methods are bound to the view, and vice versa, allowing dynamic changes to reflect in the UI and user interactions to be reflected in the component’s state.

Angular supports four types of data binding:

1. Interpolation ({{ }})
2. Property Binding ([ ])
3. Event Binding (( ))
4. Two-Way Binding ([( )])

Each type of data binding serves a specific purpose and enables different types of interactions between the component and the view.

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1. Interpolation ({{ }})

Interpolation is the most common form of data binding, used to display dynamic data from the component class inside the template.

• Purpose: To bind component data to HTML elements as text content.
• Syntax: {{ expression }}
• Example:

  <h1>{{ title }}</h1>

  In this example, the value of the title property from the component is displayed inside the <h1> tag. If the value of title changes in the component, it will automatically update in the view.

How it works: Angular evaluates the expression inside the curly braces ({{}}) and updates the DOM whenever the component’s property changes.

---

2. Property Binding ([ ])

Property binding allows you to bind an element’s properties (e.g., src, href, disabled, etc.) to component properties. It’s useful for setting the values of HTML element properties dynamically.

• Purpose: To bind component properties to DOM element attributes or properties.
• Syntax: [property]="expression"
• Example:

  <img [src]="imageUrl">

  In this example, the src property of the <img> element is dynamically set to the value of the imageUrl property in the component.

How it works: Angular evaluates the expression on the right side of the binding (imageUrl) and updates the property of the element whenever the component’s property changes.

---

3. Event Binding (( ))

Event binding is used to listen to events (e.g., click, change, input) in the template and trigger corresponding methods or expressions in the component class.

• Purpose: To bind user events in the view to component methods (e.g., handle clicks, input changes).
• Syntax: (event)="handler"
• Example:

  <button (click)="onClick()">Click me</button>

  Here, when the <button> is clicked, the onClick() method in the component is triggered.

How it works: Angular listens for the event specified in the parentheses (e.g., click), and when the event occurs, it calls the corresponding method (onClick()) in the component class.

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4. Two-Way Binding ([( )])

Two-way binding is a combination of property binding and event binding. It allows data to flow both from the component to the view and from the view to the component, meaning that the component’s data and the view’s input fields are kept in sync.

• Purpose: To bind an element’s value to a component property and listen for changes, so when the user interacts with the view, the component data updates automatically.
• Syntax: [(ngModel)]="property"
• Example:

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

  In this example, the username property in the component is bound to the <input> element. Any changes made by the user in the input field will update the component’s username, and any changes in the component’s username will update the input field.

How it works: Angular sets up two-way binding by combining property binding for the element’s value ([ngModel]="username") and event binding for the input’s change event ((ngModelChange)="username=$event"). The value in the input field and the component property are kept in sync.

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Summary of Data Binding Types:

Type	Direction of Data Flow	Syntax	Purpose/Usage
Interpolation	Component → View	{{ expression }}	To display data from the component in the view (e.g., text content).
Property Binding	Component → View	[property]="expression"	To set DOM properties or attributes dynamically (e.g., src, href).
Event Binding	View → Component	(event)="handler"	To respond to user events (e.g., click, input, change).
Two-Way Binding	Component ↔ View (Bidirectional)	[(ngModel)]="property"	To synchronize data between the component and the view (e.g., form inputs).

---

Examples of Data Binding in Angular:

1. Interpolation Example:

<p>{{ greetingMessage }}</p>

• Component:

  export class AppComponent {
    greetingMessage = 'Hello, Angular!';
  }

2. Property Binding Example:

<img [src]="imageUrl" alt="Angular Logo">

• Component:

  export class AppComponent {
    imageUrl = 'assets/angular-logo.png';
  }

3. Event Binding Example:

<button (click)="onClick()">Submit</button>

• Component:

  export class AppComponent {
    onClick() {
      console.log('Button clicked!');
    }
  }

4. Two-Way Binding Example:

<input [(ngModel)]="username" placeholder="Enter your username">
<p>Your username: {{ username }}</p>

• Component:

  export class AppComponent {
    username = '';
  }

---

Conclusion:

Data binding in Angular plays a crucial role in connecting the component class with the view, enabling dynamic interactions and updates. The four types of data binding—interpolation, property binding, event binding, and two-way binding—allow for efficient communication between the component and its template, facilitating the development of interactive, data-driven web applications.

Question: What is dependency injection in Angular? How does it work?

Answer:

Dependency Injection (DI) in Angular is a design pattern used to implement Inversion of Control (IoC), allowing a component or service to receive its dependencies from an external source rather than creating them internally. DI is a core concept in Angular and is used to improve modularity, maintainability, and testability by decoupling classes from their dependencies.

In simpler terms, dependency injection is a way to provide a class with its required dependencies (such as services, values, or other components) without the class needing to create or manage them directly.

---

How Dependency Injection Works:

1. Injector: Angular uses an injector to manage and provide the dependencies. The injector is responsible for creating instances of classes (like services) and injecting them where they are needed. It maintains a container of services and their configurations.

2. Providers: A provider is a recipe that tells the Angular injector how to create an instance of a service or dependency. Providers are defined in Angular’s DI system, and they are often configured in a module, component, or service.

3. Constructor Injection: The most common form of dependency injection in Angular is constructor injection. When a class (e.g., a component or service) requires a dependency, it declares that dependency in its constructor. Angular’s DI system then provides the necessary instance of the service when the class is instantiated.

4. Tree of Injectors: Angular has a hierarchical injector system, where each module, component, and service can define its own injector, and child injectors can inherit from parent injectors. This allows for more granular control over the scope and lifetime of dependencies.

---

Steps in Dependency Injection in Angular:

1. Define a Service (Dependency): A service is typically a class that provides certain functionality or data. You decorate the service class with @Injectable() to indicate that Angular can inject this service as a dependency.

   Example:

   @Injectable({
     providedIn: 'root',  // The service is provided at the root level, making it a singleton
   })
   export class UserService {
     getUser() {
       return { name: 'John Doe', age: 30 };
     }
   }

2. Declare the Service as a Dependency in the Component: In the component where you need the service, you inject the service into the constructor. Angular will then automatically provide the instance of the service when the component is created.

   Example:

   import { Component } from '@angular/core';
   import { UserService } from './user.service';
   
   @Component({
     selector: 'app-user-profile',
     templateUrl: './user-profile.component.html',
     styleUrls: ['./user-profile.component.css']
   })
   export class UserProfileComponent {
     user: any;
   
     constructor(private userService: UserService) {
       this.user = this.userService.getUser();
     }
   }

   • private userService: UserService in the constructor is where Angular injects the UserService instance into the UserProfileComponent.

3. Angular Injector Resolves the Dependencies:

   • When the UserProfileComponent is instantiated, Angular’s DI system looks for the UserService in the current injector (and parent injectors, if necessary). It then creates and injects an instance of the UserService into the component’s constructor.

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Key Concepts of Dependency Injection in Angular:

1. @Injectable() Decorator:

   • The @Injectable() decorator is used to define a class as a service that can be injected into other classes. When using DI, the service is typically marked as injectable so that Angular can manage it. The providedIn property inside @Injectable() specifies where the service should be provided (e.g., 'root' for the entire app, or a specific module).

   Example:

   @Injectable({
     providedIn: 'root',  // The service is a singleton, available globally
   })
   export class MyService {
     constructor() {}
   }

2. Providers:

   • A provider is a configuration that defines how Angular creates an instance of a dependency. By default, Angular uses a class provider, but you can also use other types like value providers, factory providers, or existing providers.

   Example:

   providers: [
     { provide: SomeService, useClass: SomeOtherService }
   ]

   This provider configuration tells Angular to inject SomeOtherService wherever SomeService is requested.

3. Hierarchical Injector:

   • Angular uses a hierarchical injector system, meaning that injectors are nested within modules, components, and services. A child injector can inherit from its parent injector, and Angular uses this hierarchy to determine where to find a dependency.

4. Singleton Services:

   • When a service is provided at the root level using providedIn: 'root', Angular creates a single instance of the service that is shared across the entire application. This is called a singleton service.
   • You can also provide services at the component or module level for more localized instances.

5. Scope and Lifetime:

   • The scope of a service determines where and how long its instance is available:
     • Root scope (via providedIn: 'root'): The service is available throughout the entire application and typically has a singleton instance.
     • Module scope (via providers in a module): The service is available only within that specific module and its components.
     • Component scope (via providers in a component): The service is available only within that specific component and its children.

---

Example of Dependency Injection in Angular:

Here is an example where a UserService is injected into a component:

1. Service Definition (user.service.ts):

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

@Injectable({
  providedIn: 'root',  // Provides the service globally
})
export class UserService {
  private user = { name: 'John Doe', age: 30 };

  getUser() {
    return this.user;
  }

  updateUser(name: string, age: number) {
    this.user.name = name;
    this.user.age = age;
  }
}

2. Component Definition (user-profile.component.ts):

import { Component, OnInit } from '@angular/core';
import { UserService } from './user.service';

@Component({
  selector: 'app-user-profile',
  templateUrl: './user-profile.component.html',
  styleUrls: ['./user-profile.component.css']
})
export class UserProfileComponent implements OnInit {
  user: any;

  constructor(private userService: UserService) {}

  ngOnInit(): void {
    this.user = this.userService.getUser();
  }

  updateUser() {
    this.userService.updateUser('Jane Doe', 25);
    this.user = this.userService.getUser();
  }
}

3. Component Template (user-profile.component.html):

<h1>User Profile</h1>
<p>Name: {{ user.name }}</p>
<p>Age: {{ user.age }}</p>
<button (click)="updateUser()">Update User</button>

Explanation:

• UserService is marked with @Injectable({ providedIn: 'root' }), making it globally available as a singleton service.
• UserProfileComponent injects UserService via its constructor (private userService: UserService). Angular automatically resolves the dependency and provides the instance of UserService.
• When the component is initialized, it retrieves the user data using this.userService.getUser().
• When the “Update User” button is clicked, the updateUser() method in the component updates the user data through the service.

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Advantages of Dependency Injection in Angular:

1. Decoupling:

   • DI helps decouple classes from their dependencies, making them easier to maintain and test. Components or services do not need to know how their dependencies are created or configured.

2. Testability:

   • Since dependencies can be injected, it is easier to mock or replace services during testing, ensuring better unit testability.

3. Maintainability:

   • DI promotes the use of modular architecture, which makes the codebase more maintainable. Services can be easily swapped or extended without changing dependent components.

4. Reusability:

   • With DI, services can be reused across different components or modules without having to rewrite the logic or create new instances.

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Conclusion:

Dependency Injection (DI) in Angular simplifies the management of dependencies by providing a way for services, classes, or other dependencies to be injected into components or other services rather than being created manually. By using Angular’s DI system, developers can achieve greater modularity, testability, and maintainability in their applications, allowing for better code reuse and cleaner architecture.

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