Application lifecycle in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to implement the OnModuleInit lifecycle hook in a NestJS service.

NestJS

import { Injectable, [1] } from '@nestjs/common';

@Injectable()
export class AppService implements [1] {
  onModuleInit() {
    console.log('Module initialized');
  }
}

Drag options to blanks, or click blank then click option'

AModuleInit

BOnInit

COnModuleInit

DInitModule

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-existent interface like OnInit or InitModule.

Forgetting to import the interface from '@nestjs/common'.

The OnModuleInit interface is used to hook into the module initialization lifecycle event in NestJS.

2fill in blank

medium

Complete the code to implement the OnApplicationBootstrap lifecycle hook in a NestJS service.

NestJS

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

@Injectable()
export class AppService implements [1] {
  onApplicationBootstrap() {
    console.log('Application bootstrap completed');
  }
}

Drag options to blanks, or click blank then click option'

AOnModuleDestroy

BOnApplicationBootstrap

COnBootstrap

DOnAppStart

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing with OnModuleDestroy or other lifecycle hooks.

Using incorrect interface names like OnBootstrap.

The OnApplicationBootstrap interface allows running code after all modules have been initialized and the application is bootstrapped.

3fill in blank

hard

Fix the error in the code by completing the lifecycle hook interface for graceful shutdown.

NestJS

import { Injectable, [1] } from '@nestjs/common';

@Injectable()
export class AppService implements [1] {
  async onModuleDestroy() {
    console.log('Module is being destroyed');
  }
}

Drag options to blanks, or click blank then click option'

AOnModuleDestroy

BOnDestroy

COnAppDestroy

DOnDestroyModule

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect interface names like OnDestroy or OnAppDestroy.

Not implementing the interface but defining the method.

The OnModuleDestroy interface is the correct lifecycle hook for cleanup when a module is destroyed.

4fill in blank

hard

Fill both blanks to implement the OnApplicationShutdown lifecycle hook and handle shutdown logic.

NestJS

import { Injectable, [1] } from '@nestjs/common';

@Injectable()
export class AppService implements [2] {
  onApplicationShutdown(signal?: string) {
    console.log(`Application shutting down due to: ${signal}`);
  }
}

Drag options to blanks, or click blank then click option'

AOnApplicationShutdown

BOnShutdown

COnAppShutdown

DOnShutdownApplication

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for import and implementation.

Using incorrect interface names like OnShutdown or OnAppShutdown.

The OnApplicationShutdown interface is used to run code during application shutdown, such as cleanup tasks.

5fill in blank

hard

Fill all three blanks to implement OnModuleInit, OnApplicationBootstrap, and OnModuleDestroy lifecycle hooks in a NestJS service.

NestJS

import { Injectable, [1], [2], [3] } from '@nestjs/common';

@Injectable()
export class AppService implements [1], [2], [3] {
  onModuleInit() {
    console.log('Module initialized');
  }

  onApplicationBootstrap() {
    console.log('Application bootstrap completed');
  }

  onModuleDestroy() {
    console.log('Module destroyed');
  }
}

Drag options to blanks, or click blank then click option'

AOnModuleInit

BOnApplicationBootstrap

COnModuleDestroy

DOnApplicationShutdown

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up interface names or missing one of them.

Using OnApplicationShutdown instead of OnModuleDestroy for module destruction.

These three interfaces correspond to key lifecycle events: module initialization, application bootstrap, and module destruction.

Practice

1. Which lifecycle hook in NestJS is used to run code right after a module has been initialized?

easy

A. OnModuleInit

B. OnApplicationBootstrap

C. OnApplicationShutdown

D. OnModuleDestroy

Solution

1. Step 1: Understand module initialization timing

   The OnModuleInit hook runs immediately after the module's dependencies are resolved and the module is set up.

2. Step 2: Match the hook to the description

   OnApplicationBootstrap runs after the entire app starts, OnApplicationShutdown runs before app stops, and OnModuleDestroy runs before module is destroyed.

3. Final Answer:

   OnModuleInit -> Option A

4. Quick Check:

   Run code after module setup = OnModuleInit [OK]

Hint: Module init code runs in OnModuleInit hook [OK]

Common Mistakes:

• Confusing OnModuleInit with OnApplicationBootstrap
• Using OnApplicationShutdown for startup code
• Thinking OnModuleDestroy runs after module init

2. Which of the following is the correct way to implement the OnApplicationShutdown lifecycle hook in a NestJS service?

easy

A. export class MyService implements OnApplicationShutdown { onApplicationShutdown(signal?: string) { /* code */ } }

B. export class MyService implements OnModuleInit { onApplicationShutdown() { /* code */ } }

C. export class MyService implements OnApplicationBootstrap { onApplicationShutdown(signal: number) { /* code */ } }

D. export class MyService implements OnModuleDestroy { onApplicationShutdown(signal?: string) { /* code */ } }

Solution

1. Step 1: Identify correct interface and method signature

   The OnApplicationShutdown interface requires implementing onApplicationShutdown(signal?: string) method.

2. Step 2: Check method name and parameter types

   export class MyService implements OnApplicationShutdown { onApplicationShutdown(signal?: string) { /* code */ } } correctly implements OnApplicationShutdown with optional string parameter signal. Others mismatch interface or method name.

3. Final Answer:

   export class MyService implements OnApplicationShutdown { onApplicationShutdown(signal?: string) { /* code */ } } -> Option A

4. Quick Check:

   Correct interface and method signature = export class MyService implements OnApplicationShutdown { onApplicationShutdown(signal?: string) { /* code */ } } [OK]

Hint: Match interface name with method signature exactly [OK]

Common Mistakes:

• Implementing wrong interface for shutdown hook
• Using wrong method name or parameters
• Confusing OnApplicationShutdown with OnModuleInit

3. Consider this NestJS service code snippet:

export class AppService implements OnModuleInit, OnApplicationShutdown {
  onModuleInit() {
    console.log('Module initialized');
  }
  onApplicationShutdown(signal?: string) {
    console.log('App shutting down:', signal);
  }
}

What will be printed if the application starts and then receives a SIGINT signal to stop?

medium

A. Module initialized only

B. App shutting down: SIGINT\nModule initialized

C. App shutting down: SIGINT only

D. Module initialized\nApp shutting down: SIGINT

Solution

1. Step 1: Understand lifecycle hook order on start

   onModuleInit runs after module setup, so 'Module initialized' prints on app start.

2. Step 2: Understand shutdown hook on SIGINT

   When SIGINT signal is received, onApplicationShutdown runs and logs 'App shutting down: SIGINT'.

3. Final Answer:

   Module initialized\nApp shutting down: SIGINT -> Option D

4. Quick Check:

   Start logs init, stop logs shutdown with signal [OK]

Hint: Startup logs OnModuleInit, shutdown logs OnApplicationShutdown [OK]

Common Mistakes:

• Assuming shutdown logs before startup
• Missing signal parameter in shutdown log
• Thinking only one hook runs

4. This NestJS service code has an error:

export class MyService implements OnApplicationShutdown {
  onApplicationShutdown() {
    console.log('Cleaning up');
  }
}

What is the error and how to fix it?

medium

A. Should implement OnModuleDestroy instead of OnApplicationShutdown

B. Wrong method name; should be onShutdown instead

C. Missing optional signal parameter in onApplicationShutdown method

D. No error; code is correct

Solution

1. Step 1: Check method signature for OnApplicationShutdown

   The interface requires onApplicationShutdown(signal?: string) with an optional string parameter.

2. Step 2: Identify missing parameter in method

   The method lacks the optional signal parameter, causing a signature mismatch error.

3. Final Answer:

   Missing optional signal parameter in onApplicationShutdown method -> Option C

4. Quick Check:

   Method signature must match interface including optional parameters [OK]

Hint: Include optional signal parameter in onApplicationShutdown method [OK]

Common Mistakes:

• Omitting optional signal parameter
• Using wrong method name
• Confusing OnApplicationShutdown with OnModuleDestroy

5. You want to perform cleanup tasks only when the app shuts down due to a SIGTERM signal, ignoring other signals. How should you implement this in a NestJS service?

hard

A. Override the main bootstrap function to handle SIGTERM manually

B. Implement OnApplicationShutdown and check if signal === 'SIGTERM' inside onApplicationShutdown method

C. Use OnApplicationBootstrap to detect shutdown signals

D. Implement OnModuleDestroy and check for SIGTERM signal there

Solution

1. Step 1: Choose correct lifecycle hook for app shutdown

   OnApplicationShutdown runs when the app is stopping and receives the shutdown signal.

2. Step 2: Filter signal inside onApplicationShutdown method

   Inside onApplicationShutdown(signal?: string), check if signal === 'SIGTERM' to run cleanup only for that signal.

3. Final Answer:

   Implement OnApplicationShutdown and check if signal === 'SIGTERM' inside onApplicationShutdown method -> Option B

4. Quick Check:

   Filter shutdown signal inside OnApplicationShutdown hook [OK]

Hint: Check signal inside OnApplicationShutdown to filter SIGTERM [OK]

Common Mistakes:

• Using OnModuleDestroy which runs on module destroy, not app shutdown
• Trying to detect shutdown in OnApplicationBootstrap
• Manually overriding bootstrap instead of lifecycle hooks