Dependency Injection in Angular

1. Introduction

Dependency Injection (DI) is a design pattern used in Angular to allow a class to receive its dependencies from external sources rather than creating them itself. This promotes loose coupling, enhances testability, and improves code organization.

2. Core Concepts

Key Definitions

Dependency: An object that another object requires to function.
Injector: A service that is responsible for instantiating and providing dependencies.
Provider: A recipe for creating a dependency.

3. How Dependency Injection Works

Angular's DI framework allows you to define how dependencies are created and provided. When a class is instantiated, Angular's injector looks up the dependencies defined in the class constructor and provides them automatically.


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

@Injectable({
  providedIn: 'root',
})
export class ApiService {
  getData() {
    return 'Data from API';
  }
}

@Component({
  selector: 'app-example',
  template: `{{data}}`,
})
export class ExampleComponent {
  data: string;

  constructor(private apiService: ApiService) {
    this.data = this.apiService.getData();
  }
}

4. Implementing Dependency Injection

Follow these steps to implement DI in your Angular application:

Define your service using the @Injectable decorator.
Register the service with Angular's DI system.
Inject the service into your component or another service.

5. Best Practices

Tip: Always keep your services stateless when possible, as this will make them easier to test and reuse.

Use providedIn: 'root' to register services globally.
Keep your services focused and single-purpose.
Avoid circular dependencies by carefully managing your service relationships.

6. FAQ

What is the purpose of Dependency Injection?

DI allows for better code organization, makes testing easier, and creates more maintainable applications by decoupling classes from their dependencies.

Can I use multiple instances of a service?

Yes, you can provide different instances of a service at various levels (component, module, etc.) using the providers array.

How do I test components with injected dependencies?

You can use Angular's TestBed to create a testing module and inject the necessary services for your tests.