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() annotation allows you to inject instances.

Of course, that instance should have an @Injectible() annotation.

Let me show you an example.

@Injectable()
export class PersonService {
  getName() {
    return 'ninezero90hy';
  }
}

@Injectable()
export class AppService {
  constructor(private readonly personService: PersonService) {
    Logger.log(this.personService.getName())
  }
}

print 'ninezero90hy'

--

If there is no AppService or PersonService @Injectible() annotation, an error occurs.

--

Using the @Injectible() annotation

You can define the injection range.

This means that the nestjs container creates an instance.

--

Reference:

export declare enum Scope {
    /**
     * The provider can be shared across multiple classes. The provider lifetime
     * is strictly tied to the application lifecycle. Once the application has
     * bootstrapped, all providers have been instantiated.
     */
    DEFAULT = 0,
    /**
     * A new private instance of the provider is instantiated for every use
     */
    TRANSIENT = 1,
    /**
     * A new instance is instantiated for each request processing pipeline
     */
    REQUEST = 2
}

If you don't use @Injectible() annotations, nestjs doesn't manage instances, so an error will occur in the DI process.