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

Dependency injection (DI) means a class does not create the things it depends on. It asks for them, and Angular's injector creates and supplies them. A class declares what it needs, and the framework decides how to build it.

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

@Component({ selector: 'app-home', template: `<p>Home</p>` })
export class Home {
  private logger = inject(Logger)

  constructor() {
    this.logger.log('Home created')
  }
}

providedIn: 'root' registers one shared instance (a singleton) for the whole app, and unused services can be removed from the bundle (tree-shaking).

DI is used because it:

• Avoids hard-coded new calls, so classes are loosely coupled.
• Makes testing easy, since you can swap a service for a fake.
• Lets you control the scope: app-wide, per route or per component.

Older code injects through constructor parameters, which does the same thing.

Ask for dependencies instead of creating them.