What is Injectable in NestJS?

Question

I am studying NestJS, here is my simple service:

import { Injectable } from '@nestjs/common';

const userMock = [{ account: 'dung', password: '12345678' }];

@Injectable()
export class UserService {
  getUser() {
    return userMock
  }
}

I not really understand @Injectable in NestJS. Some tutorial tell @Injectable tell the @Controller know it's an install and can use it as a Dependency Injection. But when I remove it, it's still working.

Please give an example about difference between @Injectable and without @Injectable

Answer 1

@Injectable() is how you tell Nest this is a class that can have dependencies that should be instantiated by Nest and its DI system. The code you posted works because there are no injected dependencies. If, instead, you had

const userMock = [{ account: 'dung', password: '12345678' }];

export class UserService {
  constructor(private readonly otherService: OtherService) {}
  getUser() {
    return userMock
  }
}

OtherService would come back undefined due to UserService not being @Injectable()