How do you inject one service into another?

A service declares what it needs, and Angular's injector supplies it. With constructor injection, you list the other service as a constructor parameter.

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

@Injectable({
  providedIn: 'root'
})
export class DataService {
  constructor(private logger: LoggerService) {}

  getData(): any[] {
    this.logger.log('Getting data...');
    return [];
  }
}

Angular reads the parameter type, finds the provider for LoggerService, and passes the instance in when it creates DataService. Both services are root-provided, so the whole app shares one logger.

The style guide now prefers the inject() function, which reads better when a class has many dependencies. You write private logger = inject(LoggerService); in a field initializer or call it in the constructor.

Avoid circular dependencies, where service A depends on service B and service B depends on A as well. Angular cannot create either one and throws an error.