How dependency injection works in Angular

Introduction

Dependency Injection helps Angular automatically provide the things your components or services need. It saves you from creating or managing those things yourself.

When a component needs to use a service like fetching data from a server.

When you want to share data or logic between different parts of your app.

When you want to make your code easier to test by replacing real services with fake ones.

When you want Angular to manage the creation and lifetime of objects automatically.

Syntax

Angular

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

@Injectable({ providedIn: 'root' })
export class MyService {
  // service code here
}

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

@Component({
  selector: 'app-example',
  template: `<p>Example works!</p>`,
  standalone: true
})
export class ExampleComponent {
  private myService = inject(MyService);
  // use myService in component
}

The @Injectable decorator marks a class as available for injection.

The inject() function is used inside components or services to get the dependency.

Examples

This creates a service called LoggerService that Angular can inject anywhere.

Angular

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

@Injectable({ providedIn: 'root' })
export class LoggerService {
  log(message: string) {
    console.log(message);
  }
}

This component uses dependency injection to get LoggerService and calls its log method when a button is clicked.

Angular

import { Component, inject } from '@angular/core';
import { LoggerService } from './logger.service';

@Component({
  selector: 'app-log-example',
  template: `<button (click)="logMessage()">Log</button>`,
  standalone: true
})
export class LogExampleComponent {
  private logger = inject(LoggerService);

  logMessage() {
    this.logger.log('Button clicked!');
  }
}

Sample Program

This example shows a GreetingService that provides a greeting message. The GreetingComponent injects this service and displays the greeting in its template.

Angular

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

@Injectable({ providedIn: 'root' })
export class GreetingService {
  getGreeting() {
    return 'Hello from GreetingService!';
  }
}

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

@Component({
  selector: 'app-greeting',
  template: `<h1>{{ greeting }}</h1>`,
  standalone: true
})
export class GreetingComponent {
  private greetingService = inject(GreetingService);
  greeting = this.greetingService.getGreeting();
}

Important Notes

Angular creates one instance of a service by default (singleton) when provided in 'root'.

You can inject services in components, other services, or directives.

Using dependency injection makes your app easier to maintain and test.

Summary

Dependency Injection lets Angular provide needed services automatically.

Use @Injectable to make a class injectable and inject() to get it.

This helps keep your code clean, reusable, and easy to test.

Practice

1. What is the main purpose of dependency injection in Angular?

easy

A. To write all code inside a single file

B. To manually create instances of services inside components

C. To provide components with the services they need automatically

D. To prevent components from using any services

Solution

1. Step 1: Understand dependency injection concept

   Dependency injection automatically provides required services to components without manual creation.

2. Step 2: Compare options with this concept

   Only To provide components with the services they need automatically describes automatic provision of services, which is the core of dependency injection.

3. Final Answer:

   To provide components with the services they need automatically -> Option C

4. Quick Check:

   Dependency injection = automatic service provision [OK]

Hint: Dependency injection means Angular gives services automatically [OK]

Common Mistakes:

• Thinking you must create service instances manually
• Believing dependency injection restricts service use
• Confusing dependency injection with code organization

2. Which decorator makes a class injectable in Angular?

easy

A. @Component

B. @Input

C. @NgModule

D. @Injectable

Solution

1. Step 1: Identify Angular decorators for injection

   The decorator @Injectable marks a class as available for dependency injection.

2. Step 2: Eliminate other decorators

   @Component is for components, @NgModule for modules, and @Input for input properties, none make a class injectable.

3. Final Answer:

   @Injectable -> Option D

4. Quick Check:

   @Injectable = makes class injectable [OK]

Hint: Use @Injectable to mark services for injection [OK]

Common Mistakes:

• Confusing @Component with @Injectable
• Using @NgModule to make services injectable
• Forgetting to add @Injectable to services

3. Given this Angular service and component code, what will be logged?

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

@Injectable({ providedIn: 'root' })
export class LoggerService {
  log(message: string) {
    console.log('Log:', message);
  }
}

import { Component } from '@angular/core';
import { LoggerService } from './logger.service';

@Component({ selector: 'app-root', template: '' })
export class AppComponent {
  constructor(private logger: LoggerService) {
    this.logger.log('Hello');
  }
}

medium

A. Log: Hello

B. Error: LoggerService not found

C. No output

D. Log: undefined

Solution

1. Step 1: Understand service injection in constructor

   The LoggerService is injected into AppComponent via constructor and used to log a message.

2. Step 2: Analyze the log call

   The log method prints 'Log:' plus the message 'Hello', so console shows 'Log: Hello'.

3. Final Answer:

   Log: Hello -> Option A

4. Quick Check:

   Injected service logs message correctly [OK]

Hint: Injected services work via constructor parameters [OK]

Common Mistakes:

• Expecting error if service is providedIn root
• Thinking log method won't run without template
• Confusing console output with return value

4. What is wrong with this Angular service injection code?

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

@Injectable()
export class DataService {
  getData() { return [1, 2, 3]; }
}

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

@Component({ selector: 'app-data', template: '' })
export class DataComponent {
  constructor(private dataService: DataService) {}
}

medium

A. DataComponent constructor should not have parameters

B. DataService is not provided in any injector

C. DataService class is missing @Component decorator

D. getData method must be static

Solution

1. Step 1: Check service provider configuration

   The service has @Injectable() but no providedIn or provider registration, so Angular doesn't know where to provide it.

2. Step 2: Understand injection failure cause

   Without being provided, Angular throws an error when injecting DataService into DataComponent.

3. Final Answer:

   DataService is not provided in any injector -> Option B

4. Quick Check:

   Missing provider causes injection error [OK]

Hint: Always provide services via providedIn or providers array [OK]

Common Mistakes:

• Assuming @Injectable alone provides the service
• Adding @Component to services incorrectly
• Thinking constructor parameters are optional for injection

5. You want to inject two different services of the same class but with different configurations in Angular. Which approach correctly achieves this?

hard

A. Use Angular injection tokens to provide different configurations and inject by token

B. Create two different classes extending the service and inject each separately

C. Use @Injectable({ providedIn: 'root' }) on the class and inject twice in constructor

D. Inject the service once and manually change its configuration before each use

Solution

1. Step 1: Understand multiple instances need

   Injecting the same class twice with different configs requires distinguishing them in Angular's injector.

2. Step 2: Use injection tokens for different configs

   Angular injection tokens allow providing multiple versions of the same class with different configurations and injecting them separately.

3. Step 3: Evaluate other options

   Use @Injectable({ providedIn: 'root' }) on the class and inject twice in constructor injects the same instance twice, B duplicates classes unnecessarily, D is error-prone and not recommended.

4. Final Answer:

   Use Angular injection tokens to provide different configurations and inject by token -> Option A

5. Quick Check:

   Injection tokens enable multiple configured instances [OK]

Hint: Use injection tokens for multiple differently configured services [OK]

Common Mistakes:

• Injecting same service twice without tokens
• Creating duplicate service classes unnecessarily
• Changing service config manually at runtime