Dependency Injection

Dependency provider types:

• useClass : Instantiate with given class when the service is injected

providers: [
  { provide: Logger, useClass: ConsoleLogger }
]

• useValue: Binds the service with a specific value. Suitable for mocking data in unit tests

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

• useFactory: Provide service with it's own dependency through a function.

providers: [
  {
    provide: Logger,
    useFactory: (config: ConfigService) => 
      new AdvancedLogger(config.logLevel),
    deps: [ConfigService]
  }
]

• useExisting: Set multiple token/service name points to a single service (Aliasing)

providers: [
  { provide: Logger, useClass: FileLogger },
  { provide: FileLoggerAlias, useExisting: Logger }
]

Hierarchical injectors:

![[Services#Injectable() Decorator]]