Services

Services are a core concept in NestJS, used to organize and manage business logic within an application. A service in NestJS is annotated with the @Injectable() decorator, allowing it to be easily injected into other components (like controller, other services, use-cases) to perform specific tasks. Services help in keeping the code modular, testable and easier to maintain by seperating concerns.

Services vs Use Cases

In general, services and use cases are very similar. The difference is in symantics and how they are used. Here is a general guideline:

• A Service is used to share functionality across multiple modules or use cases.
• A Use Case is a service that is used for a very specific task which is not shared with other use cases.

Usage

So when functionality is shared across multiple use cases, or even outside of the module, we create a service. Services provide a way to reuse code and keep the business logic in one place.

Services are injected into controllers, use cases or other services.

@Injectable()
export class UserService {
  constructor(
    private readonly userRepository: UserRepository
  ) {}

  async createUser (user: User): Promise<User> {
    return await this.userRepository.create(user)
  }
}

A service is always added in the providers array in the module. When a service is used outside of the module, it should be listed in the exports array of the module.

See Injectables for more information on how to use services in NestJS.

constructor(private todoService: TodoService) {}