Angular Dependency Injection

Version: Angular 21 (2025) Tags: DI, Services, Providers, Tokens, inject()

References: DI Guide • API • inject()

API Changes

This section documents recent version-specific API changes.

• NEW: Functional injection with inject() — Preferred over constructor injection in modern Angular source

• NEW: inject() with Optional decorator — inject(Service, { optional: true })

• NEW: inject() with SkipSelf — inject(Service, { skipSelf: true })

• NEW: Tree-shakable InjectionToken — new InjectionToken<T>(desc, { providedIn: 'root' })

Best Practices

• Use providedIn: 'root' for singleton services

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

• Use inject() function for cleaner code

@Component({})
export class MyComponent {
  private service = inject(MyService);
  private router = inject(Router);
}

• Use InjectionToken for non-class dependencies

export const API_URL = new InjectionToken<string>('apiUrl');

providers: [
  { provide: API_URL, useValue: 'https://api.example.com' }
]

constructor(@Inject(API_URL) private apiUrl: string) {}

• Use factory providers for complex instantiation

providers: [
  {
    provide: AuthService,
    useFactory: (http: HttpClient, config: AppConfig) => {
      return new AuthService(http, config.apiUrl);
    },
    deps: [HttpClient, APP_CONFIG]
  }
]

• Use multi providers for multiple implementations

providers: [
  { provide: HTTP_INTERCEPTORS, useClass: AuthInterceptor, multi: true },
  { provide: HTTP_INTERCEPTORS, useClass: LogInterceptor, multi: true }
]

• Use @Optional for optional dependencies

constructor(@Optional() private logger: LoggerService) {
  this.logger?.log('Optional dependency');
}

• Use @SkipSelf to avoid self-injection

constructor(@SkipSelf() @Optional() private parent: ParentService) {}

• Use forwardRef for circular dependencies

constructor(@Inject(forwardRef(() => ParentService)) private parent: ParentService) {}

• Use hierarchical injectors for scoping

// Component-level provider
@Component({
  providers: [MyService]
})
export class MyComponent {}

// Lazy-loaded module provider
@Injectable({ providedIn: MyModule })
export class MyService {}

• Use interface injection pattern

export interface CacheInterface {
  get(key: string): any;
}

export const CACHE_TOKEN = new InjectionToken<CacheInterface>('cache');

• Use providedIn: 'any' for lazy-loaded services

@Injectable({ providedIn: 'any' })
export class LazyService {}