Shared modules in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a shared module in NestJS?

A shared module is a module that exports providers, components, or services so other modules can use them without redefining. It helps reuse code across the app.

beginner

How do you make a module shared in NestJS?

You export the providers or components inside the module's @Module decorator using the 'exports' array. Then import this module in other modules to use the shared parts.

intermediate

Does importing a shared module multiple times create multiple instances of its providers in NestJS?

No, NestJS automatically shares a single instance of providers across all modules that import it.

beginner

What is the difference between 'imports' and 'exports' in a NestJS module?

'imports' bring in other modules to use their exported providers. 'exports' make providers from the current module available to other modules that import it.

intermediate

Can a shared module import other modules? What happens then?

Yes, a shared module can import other modules. Those imported modules' exported providers become available inside the shared module, but are not automatically exported to modules importing the shared module.

What must you do to make a service available to other modules in NestJS?

A Add the service to the module's exports array

B Add the service to the module's imports array

C Declare the service in the main.ts file

D Use the service without importing the module

You export the service in the module's exports array so other modules importing this module can use it.

If you import a shared module in multiple modules, what happens to its providers?

A They are duplicated for each import

B They are removed from the app

C They are shared as single instances

D They cause an error

Providers in a shared module are singletons and shared across imports unless explicitly scoped differently.

Which decorator property lists modules that this module depends on?

A exports

B imports

C providers

D controllers

'imports' lists modules whose exported providers are needed inside this module.

What is the main benefit of using shared modules?

A To reduce code duplication and reuse services

B To increase app size

C To create multiple instances of services

D To avoid using providers

Shared modules help reuse code and services across different parts of the app, reducing duplication.

If a shared module imports another module, do the imported module's exports become available to modules importing the shared module?

A Yes, automatically

B No, only inside the shared module

C Only if the imported module is global

D Only if the shared module re-exports them

The shared module must re-export the imported module's exports to make them available to modules importing the shared module.

Explain how to create and use a shared module in NestJS.

Think about how to share services across modules without rewriting them.

Describe the difference between 'imports' and 'exports' in a NestJS module and why both are important.

Consider how modules connect and share code.

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