NestJS vs Fastify

When building Node.js apps, there are two common paths you can take. NestJS gives you structure and lots of built-in tools, while Fastify is all about being as fast and lightweight as possible.

NestJS was made to help teams keep big projects organized. With plain Express, apps could quickly become messy as they grew. NestJS fixes that by offering things like modules, dependency injection, and tools that make it easier for large teams to work together.

Fastify was built with speed in mind. It’s one of the fastest Node.js frameworks and keeps things efficient with features like fast JSON handling and smart routing. Even though it’s lightweight, it still has useful features like async/await and a strong plugin system.

This article compares the two frameworks so you can decide which one fits your project best.

What is NestJS?

NestJS makes building Node.js apps easier by bringing in proven ideas from frameworks like Angular and Spring. Instead of every developer setting up their own way of doing things, NestJS gives you a clear structure that works well for teams and large projects.

At the core of NestJS are a few main concepts:

• Dependency injection – makes it easy to manage how different parts of your app connect and keeps testing simple
• Decorators – let you handle routing, validation, and other common tasks in a clean, readable way
• Modules – help organize your code into small, reusable pieces
• Guards and pipes – handle things like authentication and data transformation in a consistent way

These features solve problems that usually pop up as Node.js apps grow. Instead of every developer coming up with their own solutions for things like validation, error handling, or database access, NestJS provides a standard approach that the whole team can use right away.

What is Fastify?

Fastify is built with one main goal: speed. It’s designed to handle requests as quickly as possible while still giving developers modern tools to work with. It does this by optimizing the things web frameworks do most often, like handling JSON, routing requests, and parsing input.

Some of its core ideas include:

• JSON Schema validation – makes input validation and data handling both fast and reliable
• Efficient routing – uses smart algorithms to quickly match requests to routes
• Plugin system – lets you add features in a modular way without slowing things down
• Async/await first – built with modern JavaScript patterns in mind

If you’ve used Express before, Fastify will feel familiar, but it’s much faster. You still get useful features like automatic JSON validation and a flexible plugin system, but without the performance trade-offs.

Framework comparison

Understanding how these frameworks prioritize different aspects of development helps determine which fits your project needs.

Your choice depends on whether you prioritize architectural consistency or raw performance. NestJS excels when building complex applications with large teams. Fastify shines when performance is critical and you need maximum efficiency.

Getting started

Let's examine how each framework handles initial setup and basic functionality.

NestJS provides comprehensive tooling from the start:

npm install -g @nestjs/cli
nest new api-project
nest generate resource posts

This creates a complete CRUD structure with separation of concerns:

@Injectable()
export class PostsService {
  private posts = [{ id: '1', title: 'First Post' }];

  findAll() {
    return this.posts;
  }

  create(createPostDto: CreatePostDto) {
    const post = { id: Date.now().toString(), ...createPostDto };
    this.posts.push(post);
    return post;
  }
}

@Controller('posts')
export class PostsController {
  constructor(private readonly postsService: PostsService) {}

  @Get()
  findAll() {
    return this.postsService.findAll();
  }

  @Post()
  create(@Body() createPostDto: CreatePostDto) {
    return this.postsService.create(createPostDto);
  }
}

NestJS generates complete application structures with separation of concerns built in. The CLI creates DTOs for validation, services for business logic, and controllers for HTTP handling. This scaffolding ensures consistency but requires understanding the architectural patterns before you can modify the generated code effectively.

Fastify emphasizes simplicity and immediate productivity:

mkdir fastify-api && cd fastify-api
npm init -y
npm install fastify

Create your application with schema-based validation:

const fastify = require('fastify')({ logger: true });

const postSchema = {
  type: 'object',
  required: ['title', 'content'],
  properties: {
    title: { type: 'string', minLength: 1 },
    content: { type: 'string', minLength: 1 }
  }
};

let posts = [{ id: '1', title: 'First Post', content: 'Hello World' }];

fastify.get('/posts', {
  schema: { response: { 200: { type: 'array', items: postSchema } } }
}, async () => posts);

fastify.post('/posts', {
  schema: { body: postSchema, response: { 201: postSchema } }
}, async (request, reply) => {
  const post = { id: Date.now().toString(), ...request.body };
  posts.push(post);
  reply.code(201);
  return post;
});

fastify.listen({ port: 3000 });

Fastify applications start with simple functions and grow through plugins. The JSON Schema approach provides both validation and documentation while enabling fast serialization. You build up functionality incrementally rather than starting with a complete architectural framework.

The schema-first approach makes API contracts explicit and enables Fastify to optimize request/response processing. This delivers excellent performance while maintaining clear validation rules that serve as documentation.

Validation and serialization

Both frameworks handle data validation differently, reflecting their core priorities.

NestJS uses class-based validation with decorators:

import { IsString, IsNotEmpty, MaxLength } from 'class-validator';

export class CreatePostDto {
  @IsString()
  @IsNotEmpty()
  @MaxLength(100)
  title: string;

  @IsString()
  @IsNotEmpty()
  content: string;
}

@Controller('posts')
export class PostsController {
  @Post()
  @UsePipes(new ValidationPipe({ transform: true }))
  create(@Body() createPostDto: CreatePostDto) {
    return this.postsService.create(createPostDto);
  }
}

This approach integrates validation with TypeScript types and provides clear error messages. The decorators make validation rules visible alongside the data structure, and the validation pipe automatically handles error responses. However, this runtime validation adds overhead compared to compiled validation approaches.

Fastify leverages JSON Schema for both validation and performance optimization:

const createPostSchema = {
  type: 'object',
  required: ['title', 'content'],
  properties: {
    title: { type: 'string', minLength: 1, maxLength: 100 },
    content: { type: 'string', minLength: 1 }
  },
  additionalProperties: false
};

fastify.post('/posts', {
  schema: { body: createPostSchema, response: { 201: postSchema } }
}, async (request, reply) => {
  const post = { id: Date.now().toString(), ...request.body };
  reply.code(201);
  return post; // Automatically serialized using schema
});

JSON Schema validation happens at the framework level and provides both input validation and output serialization optimization. Fastify compiles these schemas for maximum performance, making validation extremely fast. The schemas also serve as API documentation and enable automatic serialization that skips unnecessary JSON.stringify operations.

Database integration

Database integration reveals how these frameworks handle data persistence and ORM integration.

NestJS provides comprehensive database integration through modules:

@Entity()
export class Post {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  title: string;

  @Column('text')
  content: string;

  @CreateDateColumn()
  createdAt: Date;
}

@Injectable()
export class PostsService {
  constructor(
    @InjectRepository(Post)
    private postsRepository: Repository<Post>,
  ) {}

  async create(createPostDto: CreatePostDto): Promise<Post> {
    const post = this.postsRepository.create(createPostDto);
    return this.postsRepository.save(post);
  }

  async findAll(): Promise<Post[]> {
    return this.postsRepository.find();
  }
}

NestJS database integration provides complete type safety and relationship management through decorators. The dependency injection system makes testing database operations straightforward by allowing clean mocking. However, the ORM abstraction adds overhead and complexity compared to lighter database approaches.

Fastify stays lightweight with plugin-based database integration:

await fastify.register(require('@fastify/postgres'), {
  connectionString: 'postgres://user:pass@localhost/db'
});

fastify.decorate('createPost', async function(postData) {
  const client = await this.pg.connect();
  try {
    const { rows } = await client.query(
      'INSERT INTO posts (title, content) VALUES ($1, $2) RETURNING *',
      [postData.title, postData.content]
    );
    return rows[0];
  } finally {
    client.release();
  }
});

fastify.post('/posts', {
  schema: { body: createPostSchema }
}, async (request, reply) => {
  const post = await fastify.createPost(request.body);
  reply.code(201);
  return post;
});

Fastify's plugin approach keeps database integration lightweight and flexible. You can use any database library or ORM without framework restrictions. The decorator pattern keeps database methods organized while maintaining excellent performance. This approach requires more manual work but provides complete control over database interactions.

Performance considerations

Performance characteristics differ significantly between these frameworks.

NestJS performance considerations:

// Enable compression
app.use(compression());

// Use caching interceptor
@UseInterceptors(CacheInterceptor)
@CacheTTL(30)
@Get()
findAll() {
  return this.postsService.findAll();
}

NestJS applications require performance optimization techniques because the architectural overhead impacts speed. The dependency injection, decorators, and abstractions create latency that becomes noticeable under load. Caching, query optimization, and careful use of interceptors help mitigate these costs.

Fastify performance optimizations are built-in:

// Automatic JSON serialization optimization
const responseSchema = {
  type: 'object',
  properties: { id: { type: 'string' }, title: { type: 'string' } }
};

fastify.get('/posts/:id', {
  schema: { response: { 200: responseSchema } }
}, async (request, reply) => {
  const post = await findPost(request.params.id);
  return { id: post.id, title: post.title };
});

Fastify achieves excellent performance through careful framework design. JSON Schema validation compiles to native code speeds. The routing algorithm optimizes for common patterns. Serialization skips unnecessary steps when schemas are provided. These optimizations happen automatically without requiring additional configuration.

Testing approaches

Testing strategies reflect each framework's architectural priorities.

NestJS provides comprehensive testing utilities:

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

  beforeEach(async () => {
    const module = await Test.createTestingModule({
      providers: [
        PostsService,
        { provide: getRepositoryToken(Post), useValue: mockRepository }
      ],
    }).compile();

    service = module.get<PostsService>(PostsService);
  });

  it('should create a post', async () => {
    const postData = { title: 'Test', content: 'Content' };
    const result = await service.create(postData);
    expect(result.title).toBe(postData.title);
  });
});

NestJS testing leverages the dependency injection system to make mocking predictable and comprehensive. The testing module system mirrors your application structure, making it easy to test components in isolation while maintaining their dependencies.

Fastify testing focuses on HTTP behavior:

const { test } = require('tap');
const { build } = require('./helper');

test('GET /posts returns list of posts', async (t) => {
  const app = await build(t);

  const res = await app.inject({ url: '/posts' });

  t.equal(res.statusCode, 200);
  t.equal(res.headers['content-type'], 'application/json; charset=utf-8');

  const posts = JSON.parse(res.payload);
  t.ok(Array.isArray(posts));
});

test('POST /posts creates new post', async (t) => {
  const app = await build(t);
  const payload = { title: 'Test Post', content: 'Test content' };

  const res = await app.inject({ method: 'POST', url: '/posts', payload });

  t.equal(res.statusCode, 201);
  const post = JSON.parse(res.payload);
  t.equal(post.title, payload.title);
});

Fastify testing emphasizes HTTP integration testing using the built-in inject method. This approach tests your complete application behavior including routing, validation, and serialization. The testing approach matches the framework's focus on performance and simplicity.

Final thoughts

NestJS and Fastify both have strong advantages for Node.js development, but they focus on different goals. NestJS is the better choice if you are building a complex application with a large team because it gives you structure, patterns, and tools that keep projects organized and easier to maintain. Fastify is best when performance is the top priority since it delivers excellent speed while staying lightweight and simple to use.

For most real-world projects, NestJS is often the safer pick because its structure helps teams avoid problems as applications grow. If maximum speed is more important than architecture, Fastify is the right choice.

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