14.1 NestJS Introduction — Decorator-based TypeScript Framework

What is NestJS?

NestJS is a Node.js server framework written in TypeScript. Inspired by Angular, it is designed around Decorators and Dependency Injection (DI).

Features:
  - TypeScript-first: full type safety
  - Declarative code with decorators
  - Separation of concerns with module system
  - Runs on Express/Fastify
  - Testability (DI container)

---

Installation and Project Setup

npm install -g @nestjs/cli
nest new my-project
cd my-project
npm run start:dev

Project Structure

src/
├── app.module.ts       # Root module
├── app.controller.ts   # Root controller
├── app.service.ts      # Root service
├── main.ts             # Entry point
└── users/
    ├── users.module.ts
    ├── users.controller.ts
    ├── users.service.ts
    └── dto/
        ├── create-user.dto.ts
        └── update-user.dto.ts

---

Core Components

Module

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

@Module({
  controllers: [UsersController],
  providers: [UsersService],
  exports: [UsersService],  // Make available to other modules
})
export class UsersModule {}

// app.module.ts (root module)
import { Module } from '@nestjs/common'
import { UsersModule } from './users/users.module'
import { AuthModule } from './auth/auth.module'

@Module({
  imports: [UsersModule, AuthModule],
})
export class AppModule {}

Controller

// users/users.controller.ts
import {
  Controller,
  Get,
  Post,
  Put,
  Delete,
  Body,
  Param,
  Query,
  HttpCode,
  HttpStatus,
} from '@nestjs/common'
import { UsersService } from './users.service'
import { CreateUserDto } from './dto/create-user.dto'
import { UpdateUserDto } from './dto/update-user.dto'

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

  @Get()
  findAll(@Query('page') page = '1', @Query('limit') limit = '10') {
    return this.usersService.findAll({
      page: parseInt(page),
      limit: parseInt(limit),
    })
  }

  @Get(':id')
  findOne(@Param('id') id: string) {
    return this.usersService.findOne(id)
  }

  @Post()
  @HttpCode(HttpStatus.CREATED)
  create(@Body() createUserDto: CreateUserDto) {
    return this.usersService.create(createUserDto)
  }

  @Put(':id')
  update(@Param('id') id: string, @Body() updateUserDto: UpdateUserDto) {
    return this.usersService.update(id, updateUserDto)
  }

  @Delete(':id')
  @HttpCode(HttpStatus.NO_CONTENT)
  remove(@Param('id') id: string) {
    return this.usersService.remove(id)
  }
}

Service

// users/users.service.ts
import { Injectable, NotFoundException } from '@nestjs/common'
import { CreateUserDto } from './dto/create-user.dto'
import { UpdateUserDto } from './dto/update-user.dto'

interface User {
  id: string
  name: string
  email: string
  createdAt: Date
}

interface PaginationParams {
  page: number
  limit: number
}

@Injectable()
export class UsersService {
  private users: User[] = []

  findAll({ page, limit }: PaginationParams) {
    const start = (page - 1) * limit
    return {
      data: this.users.slice(start, start + limit),
      total: this.users.length,
      page,
      limit,
    }
  }

  findOne(id: string): User {
    const user = this.users.find(u => u.id === id)
    if (!user) {
      throw new NotFoundException(`User ID ${id} not found.`)
    }
    return user
  }

  create(createUserDto: CreateUserDto): User {
    const user: User = {
      id: Date.now().toString(),
      ...createUserDto,
      createdAt: new Date(),
    }
    this.users.push(user)
    return user
  }

  update(id: string, updateUserDto: UpdateUserDto): User {
    const user = this.findOne(id)
    Object.assign(user, updateUserDto)
    return user
  }

  remove(id: string): void {
    const index = this.users.findIndex(u => u.id === id)
    if (index === -1) {
      throw new NotFoundException(`User ID ${id} not found.`)
    }
    this.users.splice(index, 1)
  }
}

---

main.ts Configuration

// main.ts
import { NestFactory } from '@nestjs/core'
import { ValidationPipe } from '@nestjs/common'
import { AppModule } from './app.module'

async function bootstrap() {
  const app = await NestFactory.create(AppModule)

  // Global validation pipe
  app.useGlobalPipes(
    new ValidationPipe({
      whitelist: true,             // Strip fields not in DTO
      forbidNonWhitelisted: true,  // Error if extra fields present
      transform: true,             // Auto type conversion
    })
  )

  // CORS configuration
  app.enableCors({
    origin: process.env.ALLOWED_ORIGIN ?? '*',
  })

  // API prefix
  app.setGlobalPrefix('api')

  await app.listen(process.env.PORT ?? 3000)
  console.log(`Server running: http://localhost:${process.env.PORT ?? 3000}`)
}

bootstrap()

---

Pro Tips

NestJS CLI Commands

# Generate module
nest generate module users
nest g mo users  # shorthand

# Generate controller
nest generate controller users
nest g co users

# Generate service
nest generate service users
nest g s users

# Generate full CRUD resource
nest generate resource users
nest g res users  # module + controller + service + DTO at once