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.

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