5 best practices for NestJS applications

NestJS is a popular framework for building server-side applications in Node.js. It provides a modular, flexible, and scalable architecture for building efficient and maintainable applications. NestJS is heavily inspired by Angular and adopts many of its concepts, making it a great choice for developers already familiar with the Angular ecosystem. With its built-in support for Dependency Injection, Modules, and Pipes, NestJS enables developers to write robust, testable, and loosely coupled code.

When building production-grade applications with NestJS, it’s essential to follow best practices that ensure maintainability, scalability, and reliability. Here’s a summary of key best practices to keep in mind for NestJS beginners:

• Modularize your application: Organize your code into separate modules, each with a specific responsibility. This makes it easier to maintain, test, and reuse code.
• Use Dependency Injection: Leverage NestJS’s built-in Dependency Injection system to manage dependencies between modules and services, making your code more flexible and testable.
• Implement Error Handling and Logging: Use exception filters and pipes to handle errors and exceptions, and integrate logging mechanisms to monitor and debug your application.
• Follow a Consistent Coding Style: Enforce a consistent coding style throughout your application using tools like ESLint and Prettier.
• Write Comprehensive Tests: Use Jest or Mocha to write unit tests, integration tests, and end-to-end tests for your application, ensuring your code is reliable, stable, and works as expected.

By following these best practices, you’ll be able to build scalable, maintainable, and efficient applications with NestJS. Let’s go through each best practice in detail.

Best practice 1 — Modularize your application

Modularizing your NestJS application is a crucial best practice that helps keep your code organized, maintainable, and scalable. This involves breaking down your application into smaller, independent modules, each with a specific responsibility. This approach enables you to develop, test, and deploy individual modules without affecting the entire application.

Benefits of Modularization

Modularization offers several benefits, including:

• Easier Maintenance: With a modular architecture, you can update or fix individual modules without impacting the entire application.
• Improved Reusability: Modules can be reused across multiple applications, reducing code duplication.
• Faster Development: Developers can work on individual modules independently, speeding up the development process.
• Better Testing: Modularization makes it easier to write unit tests and integration tests for individual modules.

Code Sample 1: Monolithic Approach

Consider a simple NestJS application with a single module, app.module.ts, that handles everything:

// app.module.ts
import { Module } from '@nestjs/common';
import { AppController } from './app.controller';
import { AppService } from './app.service';
import { UsersModule } from './users/users.module';
import { ProductsModule } from './products/products.module';

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

In this example, the AppModule imports and manages everything, making it a monolithic module.

Code Sample 2: Modular Approach

Now, let’s refactor the application to use a modular approach:

// app.module.ts
import { Module } from '@nestjs/common';
import { AppController } from './app.controller';
import { AppService } from './app.service';

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

// users.module.ts
import { Module } from '@nestjs/common';
import { UsersController } from './users.controller';
import { UsersService } from './users.service';

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

// products.module.ts
import { Module } from '@nestjs/common';
import { ProductsController } from './products.controller';
import { ProductsService } from './products.service';

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

In this refactored example, we’ve broken down the application into separate modules (AppModule, UsersModule, and ProductsModule), each with its own specific responsibility. This modular approach makes it easier to maintain, test, and reuse individual modules.

Best practice 2 — Use Dependency Injection

Dependency Injection (DI) is a design pattern that allows components to be loosely coupled, making it easier to test, maintain, and extend your application. In NestJS, DI is built-in and encouraged through the use of modules, providers, and injectors. By using DI, you can decouple your components from specific implementations, making your code more flexible and modular.

Benefits of Dependency Injection

DI offers several benefits, such as:

• Loose Coupling: Components are no longer tightly coupled to specific implementations, making it easier to change or replace dependencies.
• Testability: Components are easier to test, as dependencies can be mocked or stubbed.
• Reusability: Components can be reused across multiple applications, as they’re not tied to specific implementations.
• Flexibility: Components can be easily extended or modified, as dependencies can be swapped or updated.

Code Sample 1: Tight Coupling

Consider a simple NestJS service that uses a tight coupling approach:

// users.service.ts
import { Injectable } from '@nestjs/common';
import { UsersRepository } from './users.repository';

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

  async findAll() {
    return this.usersRepository.findAll();
  }
}

// users.repository.ts
import { Injectable } from '@nestjs/common';

@Injectable()
export class UsersRepository {
  async findAll() {
    // implementation
  }
}

In this example, the UsersService is tightly coupled to the UsersRepository implementation.

Code Sample 2: Dependency Injection

Now, let’s refactor the service to use Dependency Injection:

// users.service.ts
import { Injectable } from '@nestjs/common';
import { UsersRepositoryInterface } from './users.repository.interface';

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

  async findAll() {
    return this.usersRepository.findAll();
  }
}

// users.repository.interface.ts
import { Injectable } from '@nestjs/common';

@Injectable()
export abstract class UsersRepositoryInterface {
  abstract findAll();
}

// users.repository.ts
import { Injectable } from '@nestjs/common';
import { UsersRepositoryInterface } from './users.repository.interface';

@Injectable()
export class UsersRepository extends UsersRepositoryInterface {
  async findAll() {
    // implementation
  }
}

In this refactored example, we’ve introduced an interface UsersRepositoryInterface and made the UsersService dependent on this interface, rather than a specific implementation. This allows us to swap out different implementations of the UsersRepository without affecting the UsersService.

Best practice 3 — Implement Error Handling and Logging

Error handling and logging are crucial aspects of building robust and maintainable applications. NestJS provides a built-in mechanism for handling errors and logging, making it easier to detect, diagnose, and resolve issues. By implementing error handling and logging, you can ensure that your application remains stable, secure, and performs optimally.

Benefits of Error Handling and Logging

Error handling and logging offer several benefits:

• Improved Reliability: Errors are caught and handled gracefully, preventing application crashes and downtime.
• Faster Debugging: Logs provide valuable insights into application behavior, making it easier to identify and fix issues.
• Enhanced Security: Errors and logs can be used to detect and respond to security threats and vulnerabilities.
• Better User Experience: Errors are handled transparently, ensuring a seamless user experience.

Code Sample 1: Basic Error Handling

Consider a simple NestJS controller that handles errors using the built-in HttpException mechanism:

// users.controller.ts
import { Controller, Get, HttpException } from '@nestjs/common';

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

In this example, the UsersController catches errors and throws an HttpException with a descriptive message and status code.

Code Sample 2: Advanced Error Handling and Logging

Now, let’s refactor the controller to use a more advanced error handling and logging approach:

// users.controller.ts
import { Controller, Get, Injectable } from '@nestjs/common';
import { Logger } from '@nestjs/logger';

@Injectable()
export class UsersController {
  constructor(private readonly logger: Logger) {}

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

In this refactored example, we’ve injected the Logger service into the UsersController and used it to log errors with descriptive messages and error details. We've also thrown an HttpException to handle the error gracefully.

Best practice 4 — Follow a Consistent Coding Style

Following a consistent coding style is essential for building maintainable, readable, and scalable applications. Consistent coding style ensures that code is uniform, making it easier for developers to understand and work with. NestJS provides a set of guidelines for coding style, and by following these guidelines, you can ensure that your code is consistent, readable, and maintainable.

Benefits of Consistent Coding Style

Consistent coding style offers several benefits, including:

• Improved Readability: Code is easier to read and understand, reducing the time spent on debugging and maintenance.
• Faster Development: Developers can work more efficiently, as they’re familiar with the coding style and conventions.
• Better Maintainability: Code is more maintainable, as consistent style makes it easier to identify and fix issues.
• Reduced Errors: Consistent coding style reduces errors, as developers are less likely to introduce inconsistencies and bugs.

Code Sample 1: Inconsistent Coding Style

Consider a simple NestJS controller with inconsistent coding style:

// users.controller.ts
import { Controller, Get } from '@nestjs/common';

@Controller('users')
export class UsersController {
  @Get()
  async findAll() {
    // code with inconsistent indentation, spacing, and naming conventions
  }
}

In this example, the UsersController has inconsistent indentation, spacing, and naming conventions, making it harder to read and maintain.

Code Sample 2: Consistent Coding Style

Now, let’s refactor the controller to follow a consistent coding style:

// users.controller.ts
import { Controller, Get } from '@nestjs/common';

@Controller('users')
export class UsersController {
  @Get()
  async findAll(): Promise<User[]> {
    // code with consistent indentation, spacing, and naming conventions
  }
}

In this refactored example, we’ve applied consistent coding style guidelines, including:

• Consistent indentation (4 spaces)
• Consistent spacing (around operators and keywords)
• Consistent naming conventions ( PascalCase for classes and camelCase for variables)

Best practice 5 — Write Comprehensive Tests

Writing comprehensive tests is crucial for ensuring the reliability, stability, and maintainability of your NestJS application. Tests help you catch bugs, validate functionality, and prevent regressions. By writing comprehensive tests, you can ensure that your application works as expected and meets the required standards.

Benefits of Comprehensive Tests

Comprehensive tests offer several benefits, including:

• Improved Reliability: Tests ensure that your application works correctly and consistently.
• Faster Debugging: Tests help you identify and fix issues quickly, reducing debugging time.
• Better Maintainability: Tests ensure that changes don’t break existing functionality.
• Enhanced Confidence: Tests give you confidence in your code, allowing you to refactor and improve it.

Code Sample 1: Unit Test

Consider a simple NestJS service with a unit test:

// users.service.ts
import { Injectable } from '@nestjs/common';

@Injectable()
export class UsersService {
  async findAll(): Promise<User[]> {
    // implementation
  }
}

// users.service.spec.ts
import { Test, TestingModule } from '@nestjs/testing';
import { UsersService } from './users.service';

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();
  });
});

In this example, we’ve written a unit test for the UsersService using Jest. The test ensures that the findAll method returns an array of users.

Code Sample 2: Integration Test

Now, let’s consider an integration test for a NestJS controller:

// users.controller.ts
import { Controller, Get } from '@nestjs/common';
import { UsersService } from './users.service';

@Controller('users')
export class UsersController {
  constructor(private readonly usersService: UsersService) {}

  @Get()
  async findAll(): Promise<User[]> {
    return this.usersService.findAll();
  }
}

// users.controller.spec.ts
import { Test, TestingModule } from '@nestjs/testing';
import { UsersController } from './users.controller';
import { UsersService } from './users.service';

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();
  });
});

In this example, we’ve written an integration test for the UsersController using Jest. The test ensures that the findAll method returns an array of users, and also tests the interaction between the controller and the service.

Thanks for reading!