// ✅ ALWAYS: Use class-validator decorators
export class CreateUserDto {
  @IsString()
  @MinLength(2)
  name: string;

  @IsEmail()
  email: string;

  @IsString()
  @MinLength(8)
  password: string;
}

Exception Filters (REQUIRED)

// ✅ ALWAYS: Custom exceptions for domain errors
export class UserNotFoundException extends NotFoundException {
  constructor(userId: string) {
    super(`User with ID ${userId} not found`);
  }
}

// Global exception filter
@Catch()
export class AllExceptionsFilter implements ExceptionFilter {
  catch(exception: unknown, host: ArgumentsHost) {
    // Handle all exceptions
  }
}

Decision Tree

Need CRUD?                 → Use @nestjs/crud
Need auth?                 → Use @nestjs/passport + JWT
Need caching?              → Use @nestjs/cache-manager
Need queue processing?     → Use @nestjs/bull
Need GraphQL?              → Use @nestjs/graphql
Need WebSockets?           → Use @nestjs/websockets

Code Examples

Controller with Guards

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

  @Get(':id')
  async findOne(@Param('id') id: string): Promise<User> {
    return this.usersService.findById(id);
  }

  @Post()
  @UsePipes(new ValidationPipe({ transform: true }))
  async create(@Body() dto: CreateUserDto): Promise<User> {
    return this.usersService.create(dto);
  }
}

Service with Repository

@Injectable()
export class UsersService {
  constructor(
    @InjectRepository(User)
    private usersRepository: Repository<User>,
  ) {}

  async findById(id: string): Promise<User> {
    const user = await this.usersRepository.findOne({ where: { id } });
    if (!user) throw new UserNotFoundException(id);
    return user;
  }
}

Commands

nest new myapp
nest generate module users
nest generate controller users
nest generate service users
npm run start:dev
npm run test

Resources

Backend patterns: backend.md

nestjs

Critical Patterns

Module Organization (REQUIRED)

DTOs with Validation (REQUIRED)