Best Practices for Building Scalable NestJS Applications

Introduction

With the highly accelerated pace of development in modern times, it becomes highly important to have server-side applications scalable, maintainable, and efficient. NestJS comes out as a progressive framework for Node.js that provides a modular structure to achieve robust applications. Inspired from Angular, it brings forth significant features, such as Dependency Injection (DI), modules, and pipes, making it highly popular among the developers.

Below are five best practices that will help you maximize the full potential of NestJS, ensure your applications are production-ready, scalable, and easy to maintain.

History and Evolution

NestJS was specifically developed to help address the increasing complexity problem server-side applications made of Node.js had been facing. When developing with Node.js, initially, the codebases were unstructured and monolithic, making the applications complex to scale and maintain.

Actually, in order to overcome these challenges, NestJS borrowed from Angular such concepts of modular architecture, DI, and even service-based design patterns. From the moment of its inception, the system evolved to include great middleware, interceptors, and guards for use by developers when building performance-driven applications with clean separation of concerns.

Problem Statement

As the size of server-side applications grows, so does the problem of managing complexity. Without the emphasis on structured approach, the code base can grow too large to be maintained, extended, or tested. Developers commonly face problems with:

1. Tight coupling between components: Changes to one part of the system spawn unpredictable problems elsewhere.
2. Poor error handling: Contributing to undiagnosed problems that can prove very challenging to debug in production.
3. Inconsistent coding practices: Will make it a bit challenging to collaborate across the teams, but will further decrease code quality as a whole.
4. No proper testing Let the application be prone to bugs and regressions that go unnoticed.

All these issues are natively handled by NestJS through its module design, DI system support, consistent coding practices, and full support for testing tools.

Technology Overview

NestJS simplifies server-side development while bringing structure to the application. At its core, the framework is based on three pillars:

• Modularity : The applications are subdivided into smaller, reusable modules.
• Dependency injection: Dependency injection decouples dependencies to allow better adaptability and testability.
• Uniform Structure: Just like Angular, it comes with uniform structure throughout all the layers, which makes it much easier for the developer to maintain.

Let's now see some best practice on application development using NestJS.

Best Practices

1. Modularize Your Application

Modularization-This is one of the core practices in NestJS. You can separate your application into modules with specific responsibilities, making it easy to handle, maintain, and scale. You can allow teams to work on different features or functionalities that do not affect other aspects of your application.

Benefits of Modularization:

• Easier Maintenance: You can update individual modules independently.
• Improved Reusability: Modules can be reused across different projects or features.
• Faster Development: Parallel development becomes possible with distinct modules.
• Simplified Testing: Testing is easier as each module can be tested in isolation.

Code Example: Monolithic vs Modular Approach

Monolithic Example:

@Module({
  imports: [UsersModule, ProductsModule],
  controllers: [AppController],
  providers: [AppService],
})
export class AppModule {}

Modular Example:

@Module({
  controllers: [AppController],
  providers: [AppService],
})
export class AppModule {}

@Module({
  controllers: [UsersController],
  providers: [UsersService],
})
export class UsersModule {}

@Module({
  controllers: [ProductsController],
  providers: [ProductsService],
})
export class ProductsModule {}

With this approach, each module becomes a self-contained unit, allowing for better maintainability and scalability.

2. Use Dependency Injection

Dependency injection lets you decouple components and services for making your application more flexible and testable. That's exactly what NestJS provides: a provider-based system for embracing dependency injection, which means that now it is possible to inject dependencies into any class.

Benefits of DI:

• Loose Coupling: Dependencies can be swapped out without changing the rest of the application.
• Enhanced Testability: Components can be tested in isolation by mocking dependencies.
• Reusability: Services and components are more reusable since they aren’t bound to specific implementations.

Code Example: Tight Coupling vs. DI

Without DI:

@Injectable()
export class UsersService {
  constructor(private readonly usersRepository: UsersRepository) {}
}

With DI:

@Injectable()
export class UsersService {
  constructor(private readonly usersRepository: UsersRepositoryInterface) {}
}

In the second example, the UsersService relies on an interface rather than a specific implementation, allowing you to inject different repository implementations without changing the service code.

3. Implement Error Handling and Logging

Error handling and logging are imperative to the robustness and security of applications. NestJS has in-built logging mechanisms, exception filters, among others, to help you catch and track errors throughout the application lifecycle.

Benefits:

• Improved Reliability: Graceful error handling prevents application crashes.
• Faster Debugging: Detailed logs provide insights into where and why errors occurred.
• Enhanced Security: Logs can alert you to potential vulnerabilities and threats.

Code Example: Basic vs Advanced Error Handling

Basic Error Handling:

@Controller('users')
export class UsersController {
  @Get()
  async findAll() {
    try {
      // some code
    } catch (error) {
      throw new HttpException('Error fetching users', 500);
    }
  }
}

Advanced Logging:

@Controller('users')
export class UsersController {
  constructor(private readonly logger: Logger) {}

  @Get()
  async findAll() {
    try {
      // some code
    } catch (error) {
      this.logger.error('Error fetching users', error);
      throw new HttpException('Error fetching users', 500);
    }
  }
}

By injecting a logger into the controller, you gain better insights into the application’s behavior and can troubleshoot more effectively.

4. Follow a Consistent Coding Style

Consistency is one factor toward maintainability. NestJS encourages developers to have best practices on coding style, including ESLint and Prettier, when enforcing guidelines. This type of consistency would make your code readable and prevent possible errors and really help the members of your team collaborate.

Benefits:

• Readability: Developers can easily navigate and understand the codebase.
• Reduced Errors: Following consistent rules minimizes the chances of introducing bugs.
• Easier Collaboration: Teams can work more efficiently when everyone adheres to the same style guide.

Example of Coding Consistency:

@Controller('users')
export class UsersController {
  @Get()
  async findAll(): Promise<User[]> {
    return this.usersService.findAll();
  }
}

By adhering to conventions such as indentation and naming rules, the code becomes easier to read and maintain.

5. Write Comprehensive Tests

Testing is the most important part in application development. With NestJS, testing comes naturally out of the box with tools such as Jest for unit, integration, and end-to-end tests. Thorough testing ensures that your application remains reliable and free of regressions.

Benefits:

• Increased Reliability: Tests catch bugs before they hit production.
• Better Maintainability: Well-tested code is easier to refactor and maintain.
• Enhanced Confidence: Developers can make changes without fear of breaking the application.

Code Example: Unit and Integration Tests

Unit Test Example:

describe('UsersService', () => {
  let service: UsersService;

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      providers: [UsersService],
    }).compile();
    service = module.get<UsersService>(UsersService);
  });

  it('should return an array of users', async () => {
    const result = await service.findAll();
    expect(result).toBeArray();
  });
});

Integration Test Example:

describe('UsersController', () => {
  let controller: UsersController;

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      controllers: [UsersController],
      providers: [UsersService],
    }).compile();
    controller = module.get<UsersController>(UsersController);
  });

  it('should return an array of users', async () => {
    const result = await controller.findAll();
    expect(result).toBeArray();
  });
});

Tests provide the confidence needed to ensure your application functions correctly and is resistant to regressions.

Conclusion

Following these best practices on application building in NestJS will ensure that the code is maintainable, scalable, and reliable. This includes breaking down an application into modules and using Dependency Injection, effective error handling, maintaining a uniform style of coding, or going as far as writing tests exhaustively, which give the foundation to a good-architected application.

Coming in the right approach can let you build robust applications that are easier to maintain and scale as they grow.