javascript - Run program on init

Question

I would create a program (script) that launches actions when it's get run, so I'm not using routes in this program

I'm using NestJS framework (requirement).

Actually I'm trying to write my code in main.ts file and importing a service with my methods .

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';
import {AppService} from './app.service'
import { TreeChildren } from 'typeorm';
async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  await app.listen(3000);
}
let appService: AppService; <- can't use appService methods
this.appService.
bootstrap();

My service

@Injectable()
export class AppService {
  constructor(
    @InjectRepository(File) private readonly fileRepository: Repository<File>,
  ) {}

  async getTypes(): Promise<File[]> {
    return await this.fileRepository.find();
  }
}

I would use services to treat my operations so I sould use DI, which is not working in a non class file.

I would know how to run my operations in init time in a proper way

Answer 1

There are two ways to do this:

A) Lifecycle Event

Use a Lifecycle Event (similar to change detection hooks in Angular) to run code and inject the services needed for it, e.g.:

Service

export class AppService implements OnModuleInit {
  onModuleInit() {
    console.log(`Initialization...`);
    this.doStuff();
  }
}

Module

export class ApplicationModule implements OnModuleInit {
  
  constructor(private appService: AppService) {
  }

  onModuleInit() {
    console.log(`Initialization...`);
    this.appService.doStuff();
  }
}
  

B) Execution Context

Use the Execution Context to access any service in your main.ts:

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  await app.listen(3000);
  const appService = app.get(AppService);
}