Shared modules in NestJS

Introduction

Shared modules help you reuse code easily across different parts of your NestJS app. They keep your code clean and organized.

When you have services or components used in many places in your app.

When you want to avoid repeating the same providers in multiple modules.

When you want to share utility functions or constants across modules.

When you want to keep your app modular and easy to maintain.

Syntax

NestJS

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

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

The exports array makes providers available to other modules that import this shared module.

Only providers listed in exports can be used outside the shared module.

Examples

This shared module provides and exports a LoggerService to be used in other modules.

NestJS

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

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

UserModule imports SharedModule to use its exported services like LoggerService.

NestJS

import { Module } from '@nestjs/common';
import { SharedModule } from './shared.module';
import { UserService } from './user.service';

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

Sample Program

This example shows a SharedModule that exports LoggerService. UserModule imports SharedModule and uses LoggerService inside UserService. When running, it logs a message and returns a user object.

NestJS

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

@Injectable()
export class LoggerService {
  log(message: string) {
    console.log(`Log: ${message}`);
  }
}

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

@Injectable()
export class UserService {
  constructor(private logger: LoggerService) {}

  createUser(name: string) {
    this.logger.log(`Creating user: ${name}`);
    return { name };
  }
}

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

// Simulate usage
async function bootstrap() {
  const logger = new LoggerService();
  const userService = new UserService(logger);

  const user = userService.createUser('Alice');
  console.log(user);
}

bootstrap();

Important Notes

Always export only what other modules need to use from your shared module.

Import the shared module in any module that needs its exported providers.

Shared modules help avoid duplicate instances of services across your app.

Summary

Shared modules let you reuse services and providers across your NestJS app.

Use exports to share providers with other modules.

Import shared modules wherever you need their services.

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