Angular Dependency Injection | Types of dependency injection in Angular

Estimated study time: 9 minutes. The different provider strategies Angular's DI system supports.

Angular's Dependency Injection system isn't limited to just registering a class and letting Angular instantiate it. There are several provider types, each suited to a different scenario.

1. useClass — The Default

Tells Angular which class to instantiate when a dependency is requested. This is what happens implicitly with @Injectable({ providedIn: 'root' }).

providers: [{ provide: LoggerService, useClass: LoggerService }]

// Swap in a different implementation for testing or environments
providers: [{ provide: LoggerService, useClass: MockLoggerService }]

2. useValue — Providing a Fixed Value

Useful for configuration objects, constants, or mock data — anything that isn't a class you want Angular to construct.

export const APP_CONFIG = new InjectionToken('APP_CONFIG');

providers: [
  { provide: APP_CONFIG, useValue: { apiUrl: 'https://api.example.com', retries: 3 } }
]

3. useFactory — Computed at Runtime

Runs a function to produce the dependency, useful when the value depends on other injected services or runtime conditions.

providers: [
  {
    provide: LoggerService,
    useFactory: (env: EnvironmentService) =>
      env.isProduction ? new RemoteLoggerService() : new ConsoleLoggerService(),
    deps: [EnvironmentService]
  }
]

4. useExisting — Aliasing

Points one token to an already-registered provider, useful when you need two different injection tokens to resolve to the same instance.

providers: [
  { provide: NewLoggerService, useExisting: LoggerService }
]

Choosing the Right Type