Lifecycle Events

There are two application lifecycle events, OnModuleInit and OnModuleDestroy hooks. It's a good practice to use them for all the initialization stuff and avoid putting anything directly in the constructor. The constructor should be used only to initialize the class members and for injecting the required dependencies.

import { Injectable, OnModuleInit, OnModuleDestroy } from '@nestjs/common';

@Injectable()
export class UsersService implements OnModuleInit, OnModuleDestroy {
  onModuleInit() {
    console.log(`Initialization...`);
  }
  
  onModuleDestroy() {
    console.log(`Cleanup...`);
  }
}

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

@Injectable()
export class UsersService {
  onModuleInit() {
    console.log(`Initialization...`);
  }

  onModuleDestroy() {
    console.log(`Cleanup...`);
  }
}

In order to defer initialization of the application, you can either use await keyword or return a Promise.

async onModuleInit(): Promise<any> {
  await this.fetch();
}

async onModuleInit() {
  await this.fetch();
}