Shared modules in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does a Shared Module affect service instances in NestJS?

Consider a NestJS Shared Module that exports a service. What happens to the service instance when this Shared Module is imported into multiple feature modules?

NestJS

import { Module } from '@nestjs/common';
import { SharedService } from './shared.service';

@Module({
  providers: [SharedService],
  exports: [SharedService],
})
export class SharedModule {}

// FeatureModuleA and FeatureModuleB both import SharedModule

A SharedService is not instantiated unless explicitly provided in each feature module.

B Each feature module gets its own separate instance of SharedService.

C SharedService instances are created per request in each feature module.

D All feature modules share the same single instance of SharedService.

💡 Hint

Think about how NestJS handles providers exported from a shared module.

Importing a non-global Shared Module into multiple feature modules shares the same instance of the providers because NestJS uses a single injector per application context.

📝 Syntax

intermediate

Correct way to define a Shared Module in NestJS

Which of the following code snippets correctly defines a Shared Module that exports a service for reuse?

A

import { Module } from '@nestjs/common';
import { SharedService } from './shared.service';

@Module({
  providers: [SharedService],
  exports: [SharedService],
})
export class SharedModule {}

B

import { Module } from '@nestjs/common';
import { SharedService } from './shared.service';

@Module({
  imports: [SharedService],
  exports: [SharedService],
})
export class SharedModule {}

C

import { Module } from '@nestjs/common';
import { SharedService } from './shared.service';

@Module({
  providers: [SharedService],
  imports: [SharedService],
})
export class SharedModule {}

D

import { Module } from '@nestjs/common';
import { SharedService } from './shared.service';

@Module({
  exports: [SharedService],
})
export class SharedModule {}

💡 Hint

Remember that services must be listed as providers to be injectable.

Services must be declared in the providers array to be instantiated by NestJS. Exporting them makes them available to other modules.

🔧 Debug

advanced

Why does importing a Shared Module twice cause multiple instances?

Given a Shared Module imported into two different modules, why might you see multiple instances of a service instead of one?

NestJS

import { Module } from '@nestjs/common';
import { SharedService } from './shared.service';

@Module({
  providers: [SharedService],
  exports: [SharedService],
})
export class SharedModule {}

// AppModule imports SharedModule twice accidentally

A Because SharedService is not exported, so each module creates its own instance.

B Because the Shared Module is imported multiple times without being global, NestJS creates separate instances per import.

C Because SharedService is marked as transient, so new instances are created each time.

D Because the Shared Module is lazy loaded, causing multiple instances.

💡 Hint

Consider how NestJS handles module imports and singleton scope.

When a module is imported multiple times without being global, NestJS creates separate injector contexts, leading to multiple instances of providers.

❓ state_output

advanced

What is the output when a Shared Service maintains state across modules?

Consider a SharedService with a counter property incremented by two different feature modules. What will be the final counter value after both modules increment it once?

NestJS

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

@Injectable()
export class SharedService {
  counter = 0;
  increment() {
    this.counter++;
  }
}

// Both FeatureModuleA and FeatureModuleB import SharedModule which exports SharedService
// Each calls sharedService.increment() once

A 2

B 1

C 0

D Error: counter is undefined

💡 Hint

Think about whether the service instance is shared or duplicated.

Since the Shared Module is imported normally, the same instance of SharedService is shared across modules, so the counter increments twice.

🧠 Conceptual

expert

How to make a Shared Module globally available in NestJS?

Which approach correctly makes a Shared Module globally available so it does not need to be imported in every module?

A Declare the Shared Module inside main.ts bootstrap function.

B Use the 'global: true' option inside @Module decorator.

C Add @Global() decorator to the Shared Module and export the providers.

D Import the Shared Module in the root AppModule only.

💡 Hint

Check NestJS documentation about global modules.

Using the @Global() decorator marks the module as global, so its exported providers are available everywhere without explicit imports.

Practice

1. What is the main purpose of a Shared Module in NestJS?

easy

A. To define database entities

B. To create a new controller for handling requests

C. To share providers and services across multiple modules

D. To configure middleware for the application

Solution

1. Step 1: Understand the role of Shared Modules

   Shared modules are designed to hold providers and services that can be reused in different parts of the app.

2. Step 2: Compare with other module roles

   Controllers handle requests, entities define data models, and middleware configures request handling, which are not the main purpose of shared modules.

3. Final Answer:

   To share providers and services across multiple modules -> Option C

4. Quick Check:

   Shared modules = share providers [OK]

Hint: Shared modules share providers, not controllers or entities [OK]

Common Mistakes:

• Confusing shared modules with controllers
• Thinking shared modules define database entities
• Assuming shared modules configure middleware

2. Which of the following is the correct way to export a provider from a shared module in NestJS?

easy

A. Add the provider to the exports array in the module decorator

B. Add the provider to the controllers array

C. Add the provider to the imports array

D. Add the provider to the bootstrap array

Solution

1. Step 1: Recall module decorator properties

   Providers are shared by listing them in the exports array of the module decorator.

2. Step 2: Understand other arrays

   controllers is for controllers, imports is for importing other modules, and bootstrap is not used in modules.

3. Final Answer:

   Add the provider to the exports array in the module decorator -> Option A

4. Quick Check:

   Export providers via exports array [OK]

Hint: Use exports array to share providers [OK]

Common Mistakes:

• Putting providers in controllers array
• Confusing imports with exports
• Using bootstrap array which doesn't exist in modules

3. Given this shared module code, what will be the output when AppModule injects CommonService and calls getMessage()?

import { Module } from '@nestjs/common';
import { CommonService } from './common.service';

@Module({
  providers: [CommonService],
  exports: [CommonService],
})
export class SharedModule {}

// CommonService
export class CommonService {
  getMessage() {
    return 'Hello from SharedModule';
  }
}

// AppModule
import { Module } from '@nestjs/common';
import { SharedModule } from './shared.module';

@Module({
  imports: [SharedModule],
})
export class AppModule {
  constructor(private commonService: CommonService) {}

  printMessage() {
    console.log(this.commonService.getMessage());
  }
}

medium

A. Error: CommonService not found

B. Undefined

C. Hello from SharedModule

D. Hello from AppModule

Solution

1. Step 1: Check provider export and import

   SharedModule exports CommonService, and AppModule imports SharedModule, so CommonService is available in AppModule.

2. Step 2: Understand service method call

   Calling getMessage() returns the string 'Hello from SharedModule'.

3. Final Answer:

   Error: CommonService not found -> Option A

4. Quick Check:

   Service must be injected via constructor and registered as provider [OK]

Hint: Import shared module to access exported services [OK]

Common Mistakes:

• Forgetting to export the provider
• Not importing the shared module in AppModule
• Assuming service returns undefined

4. Identify the error in this shared module setup:

import { Module } from '@nestjs/common';
import { LoggerService } from './logger.service';

@Module({
  providers: [LoggerService],
  imports: [LoggerService],
  exports: [LoggerService],
})
export class LoggerModule {}

medium

A. LoggerService must be declared as a global provider

B. LoggerService should not be in the imports array

C. LoggerService should not be exported

D. LoggerService must be in controllers array

Solution

1. Step 1: Review module decorator arrays

   The imports array is for importing other modules, not providers or services.

2. Step 2: Identify correct placement

   LoggerService should be in providers and exports, but not in imports.

3. Final Answer:

   LoggerService should not be in the imports array -> Option B

4. Quick Check:

   Imports array = modules only [OK]

Hint: Imports array only accepts modules, not providers [OK]

Common Mistakes:

• Putting providers in imports array
• Confusing controllers with providers
• Thinking exports are optional for sharing

5. You have two modules, FeatureModule and SharedModule. SharedModule exports UtilityService. You want FeatureModule to use UtilityService and also export it for other modules. How should you configure FeatureModule to achieve this?

hard

A. Import SharedModule and add UtilityService to FeatureModule controllers

B. Only import SharedModule in FeatureModule, no exports needed

C. Declare UtilityService again in FeatureModule providers and exports

D. Import SharedModule and export UtilityService in FeatureModule's exports array

Solution

1. Step 1: Import SharedModule in FeatureModule

   To use UtilityService, FeatureModule must import SharedModule which exports it.

2. Step 2: Export UtilityService from FeatureModule

   To share UtilityService further, FeatureModule must export it in its exports array.

3. Final Answer:

   Import SharedModule and export UtilityService in FeatureModule's exports array -> Option D

4. Quick Check:

   Import then export to re-share service [OK]

Hint: Import then export shared service to re-share it [OK]

Common Mistakes:

• Not exporting UtilityService from FeatureModule
• Re-declaring UtilityService providers unnecessarily
• Adding services to controllers array