Application lifecycle in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ lifecycle

intermediate

What is the output when using OnModuleInit in NestJS?

Consider this NestJS service implementing OnModuleInit. What will be logged when the module initializes?

NestJS

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

@Injectable()
export class MyService implements OnModuleInit {
  onModuleInit() {
    console.log('Module initialized');
  }
}

A 'Module initialized' is logged every time a method in MyService is called

B An error is thrown because OnModuleInit requires async method

C 'Module initialized' is logged once when the module starts

D Nothing is logged because onModuleInit is not called automatically

💡 Hint

OnModuleInit runs once when the module is ready.

The onModuleInit method runs once automatically when the module finishes loading. It is used for setup tasks.

❓ component_behavior

intermediate

What happens if you implement OnApplicationShutdown in a provider?

Given this provider implementing OnApplicationShutdown, what will happen when the app shuts down?

NestJS

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

@Injectable()
export class ShutdownService implements OnApplicationShutdown {
  onApplicationShutdown(signal?: string) {
    console.log(`App shutting down due to: ${signal}`);
  }
}

A The method is never called because shutdown hooks are disabled by default

B The message logs with the shutdown signal when the app closes gracefully

C The method runs repeatedly during app runtime causing performance issues

D An error occurs because the signal parameter is required but not passed

💡 Hint

Shutdown hooks run when the app closes.

OnApplicationShutdown runs once when the app is closing. The optional signal shows the reason.

🔧 Debug

advanced

Why does this NestJS lifecycle hook not run?

This service implements OnModuleDestroy but the method never runs. Why?

NestJS

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

@Injectable()
export class MyService implements OnModuleDestroy {
  onModuleDestroy() {
    console.log('Module is being destroyed');
  }
}

// The app never calls app.close()

A Because app.close() is not called, OnModuleDestroy is never triggered

B Because OnModuleDestroy requires async method signature

C Because the service is not injected anywhere, so lifecycle hooks don't run

D Because onModuleDestroy method name is case sensitive and should be onModuleDestroyHook

💡 Hint

Lifecycle destroy hooks run only when app closes.

OnModuleDestroy runs only when the app is closed via app.close(). Without calling close, it won't run.

📝 Syntax

advanced

Which option correctly implements OnApplicationBootstrap?

Choose the correct syntax for implementing OnApplicationBootstrap in a NestJS service.

A

export class AppService implements OnApplicationBootstrap {
  onApplicationBootstrap(): void {
    console.log('App bootstrapped');
  }
}

B

export class AppService implements OnApplicationBootstrap {
  onApplicationBootstrap() {
    console.log('App bootstrapped');
  }
}

C

export class AppService implements OnApplicationBootstrap {
  onApplicationBootstrap(async) {
    console.log('App bootstrapped');
  }
}

D

export class AppService implements OnApplicationBootstrap {
  onApplicationBootstrap() => {
    console.log('App bootstrapped');
  }
}

💡 Hint

Method signature must be valid TypeScript.

The correct method signature includes the return type void and no arrow function syntax.

🧠 Conceptual

expert

What is the order of NestJS lifecycle hooks during app startup and shutdown?

Arrange the lifecycle hooks in the order they are called during app startup and shutdown.

A 2,1,4,3

B 2,1,3,4

C 1,2,4,3

D 1,2,3,4

💡 Hint

Module init happens before app bootstrap; shutdown hooks run before destroy hooks.

During startup, onModuleInit runs first, then onApplicationBootstrap. During shutdown, onApplicationShutdown runs before onModuleDestroy.