Creating Reusable Libraries and Shared Modules in NestJS

In large-scale applications, code reuse and modularization are key factors in ensuring maintainability and scalability. NestJS, being built on top of TypeScript and leveraging the modular programming paradigm, makes it easy to structure your application in a way that allows for the creation of reusable libraries and shared modules. This can greatly improve the organization of your code and reduce duplication.

In this module, we will explore how to create reusable libraries and shared modules in NestJS, allowing you to create functionality that can be easily shared across different parts of your application or even across different projects.

Table of Contents

1. Introduction to Reusable Libraries and Shared Modules
2. What is a Shared Module in NestJS?
3. Creating a Shared Module
4. What is a Reusable Library in NestJS?
5. Creating a Reusable Library
6. Exporting and Importing Shared Modules and Libraries
7. Best Practices for Creating Reusable Libraries and Shared Modules
8. Conclusion

Introduction to Reusable Libraries and Shared Modules

In NestJS, a module is a fundamental building block of the application. It helps organize code into cohesive units of functionality, encapsulating related providers, controllers, and services.

• Shared Modules: These are modules that contain functionality which is used across multiple parts of the application. For example, if you have logging, authentication, or utility functions that are used across different modules, you can create a shared module to centralize and reuse that logic.
• Reusable Libraries: These are standalone libraries or packages that encapsulate specific functionality or services, which can be shared across multiple NestJS applications or microservices.

By creating these shared modules and reusable libraries, you reduce redundancy, keep the codebase DRY (Don’t Repeat Yourself), and enable scalability as the project grows.

What is a Shared Module in NestJS?

A shared module is simply a module that you can import into other modules to provide common functionality across your application. Typically, a shared module would contain services, utility functions, or common providers that are reused by other modules.

Benefits of Shared Modules:

• Code Reusability: Shared modules allow you to centralize common logic in one place and import them into multiple modules.
• Separation of Concerns: By keeping shared functionality in a dedicated module, it helps separate concerns and keeps your code organized.
• Easier Maintenance: When a shared service or provider needs to be updated, you only need to update it in the shared module, and all dependent modules will automatically benefit from the update.

Creating a Shared Module

Let’s start by creating a simple shared module that provides logging functionality across the application.

Step 1: Create a Logging Service

typescriptCopyEdit// src/common/logging/logging.service.ts
import { Injectable } from '@nestjs/common';

@Injectable()
export class LoggingService {
  log(message: string) {
    console.log(message);
  }

  error(message: string) {
    console.error(message);
  }

  warn(message: string) {
    console.warn(message);
  }
}

Step 2: Create the Shared Module

Now, we can create the shared module that exports the LoggingService so it can be used across the application.

typescriptCopyEdit// src/common/logging/logging.module.ts
import { Module } from '@nestjs/common';
import { LoggingService } from './logging.service';

@Module({
  providers: [LoggingService],
  exports: [LoggingService],  // Exposing the LoggingService for use in other modules
})
export class LoggingModule {}

Step 3: Import the Shared Module

Finally, import the LoggingModule in the modules where you need to use the LoggingService.

typescriptCopyEdit// src/app/app.module.ts
import { Module } from '@nestjs/common';
import { LoggingModule } from './common/logging/logging.module';
import { SomeService } from './some.service';

@Module({
  imports: [LoggingModule],
  providers: [SomeService],
})
export class AppModule {}

In this example, the LoggingModule is imported into the AppModule, which makes the LoggingService available for injection into any of the services or controllers in AppModule.

What is a Reusable Library in NestJS?

A reusable library is a self-contained, reusable piece of functionality that can be packaged and distributed across multiple projects or applications. A library can provide a set of services, utilities, or modules that can be imported and used across your application without modification.

Libraries are typically used for:

• Shared utilities: Code that is used in multiple places, such as date formatting or encryption.
• Third-party integrations: Wrapping third-party services (e.g., sending emails, payment gateways) into reusable modules.
• Microservices: When creating microservices in NestJS, common libraries can be used to keep business logic consistent across multiple services.

Reusable libraries help make your code more modular, which is beneficial for larger applications or when sharing functionality across different projects.

Creating a Reusable Library

Let’s create a reusable library for generating unique IDs using UUID.

Step 1: Create the Library

typescriptCopyEdit// libs/uuid-lib/src/uuid.service.ts
import { Injectable } from '@nestjs/common';
import { v4 as uuidv4 } from 'uuid';

@Injectable()
export class UuidService {
  generateUuid(): string {
    return uuidv4();
  }
}

Step 2: Create the Module for the Library

typescriptCopyEdit// libs/uuid-lib/src/uuid.module.ts
import { Module } from '@nestjs/common';
import { UuidService } from './uuid.service';

@Module({
  providers: [UuidService],
  exports: [UuidService],  // Expose the UuidService for use in other modules
})
export class UuidModule {}

Step 3: Create the Library Package

In this case, we are placing the uuid-lib in the libs folder of the project. Make sure you have the necessary configurations to treat it as a separate module in your NestJS project. You can publish this library as a private package using npm, or keep it inside your project’s libs folder for easier use.

Step 4: Using the Reusable Library

Now, you can import and use the UuidModule in any other module where you need to generate unique IDs.

typescriptCopyEdit// src/app/app.module.ts
import { Module } from '@nestjs/common';
import { UuidModule } from '@libs/uuid-lib';
import { MyService } from './my.service';

@Module({
  imports: [UuidModule],
  providers: [MyService],
})
export class AppModule {}

Step 5: Using the Service in Your Application

typescriptCopyEdit// src/my.service.ts
import { Injectable } from '@nestjs/common';
import { UuidService } from '@libs/uuid-lib';

@Injectable()
export class MyService {
  constructor(private readonly uuidService: UuidService) {}

  generateNewUuid(): string {
    return this.uuidService.generateUuid();
  }
}

Exporting and Importing Shared Modules and Libraries

Once you have created your shared modules or reusable libraries, you can export them from their respective module files, and import them into the modules that need to use their functionality.

For example, in the case of a shared module:

• Exporting is done by adding the module to the exports array of the module.
• Importing is done by adding the module to the imports array of the consuming module.

For reusable libraries:

• You can package and export them as npm packages or treat them as local modules in your NestJS workspace, which are then imported into other applications or services.

Best Practices for Creating Reusable Libraries and Shared Modules

1. Decouple Logic: Make sure that your shared modules and libraries are independent and do not tightly couple with specific business logic.
2. Follow the Single Responsibility Principle (SRP): Ensure that each shared module or reusable library focuses on a single responsibility (e.g., logging, validation, etc.).
3. Write Tests: Reusable libraries should come with their own tests, ensuring that they work independently.
4. Document the Modules: Proper documentation for the usage of shared modules and reusable libraries is important for easier maintenance and understanding.
5. Version Control: If you’re creating reusable libraries as npm packages, use proper versioning to maintain compatibility across applications.

Conclusion

In this module, we learned how to create reusable libraries and shared modules in NestJS. These concepts are essential for maintaining a clean, organized, and modular codebase, especially in large applications. By following the principles outlined here, you can make your code more reusable, maintainable, and scalable, leading to improved productivity and reduced redundancy.

With shared modules for common functionality and reusable libraries for encapsulating specific features, you can streamline development and create a more efficient workflow in your NestJS applications.