nodejs-expert

---
name: nodejs-expert
description: Expert in Node.js backend development with Express, NestJS, TypeScript, and best practices. Use for building scalable Node.js APIs and services.
allowed-tools: Read, Write, Edit, Grep, Glob, Bash
---

# Node.js Backend Development Expert

## Purpose
Provide expert guidance on Node.js backend development including Express, NestJS, TypeScript, API design, database integration, and production best practices.

## When to Use This Skill
- Building REST APIs with Express or NestJS
- TypeScript backend development
- Database integration (PostgreSQL, MongoDB)
- Authentication and authorization
- Error handling and logging
- Testing Node.js applications
- Performance optimization
- Production deployment

## Express.js API Structure

```
src/
├── app.ts
├── server.ts
├── config/
│   ├── database.ts
│   └── environment.ts
├── middleware/
│   ├── auth.ts
│   ├── errorHandler.ts
│   └── validation.ts
├── routes/
│   ├── index.ts
│   ├── users.ts
│   └── posts.ts
├── controllers/
│   ├── users.controller.ts
│   └── posts.controller.ts
├── services/
│   ├── users.service.ts
│   └── posts.service.ts
├── models/
│   ├── user.model.ts
│   └── post.model.ts
├── utils/
│   ├── logger.ts
│   └── validation.ts
└── types/
    └── express.d.ts
```

## Express + TypeScript Example

### app.ts
```typescript
import express, { Express } from 'express';
import helmet from 'helmet';
import cors from 'cors';
import compression from 'compression';
import { errorHandler } from './middleware/errorHandler';
import { requestLogger } from './middleware/logger';
import routes from './routes';

export function createApp(): Express {
  const app = express();

  // Security
  app.use(helmet());
  app.use(cors({
    origin: process.env.ALLOWED_ORIGINS?.split(','),
    credentials: true
  }));

  // Parsing
  app.use(express.json({ limit: '10mb' }));
  app.use(express.urlencoded({ extended: true }));
  app.use(compression());

  // Logging
  app.use(requestLogger);

  // Routes
  app.use('/api/v1', routes);

  // Health check
  app.get('/health', (req, res) => {
    res.json({ status: 'ok', timestamp: new Date().toISOString() });
  });

  // Error handling
  app.use(errorHandler);

  return app;
}
```

### Controller
```typescript
import { Request, Response, NextFunction } from 'express';
import { UserService } from '../services/users.service';
import { CreateUserDto, UpdateUserDto } from '../dto/user.dto';
import { validate } from '../utils/validation';
import { AppError } from '../utils/errors';

export class UserController {
  constructor(private userService: UserService) {}

  async create(req: Request, res: Response, next: NextFunction) {
    try {
      const dto = await validate(CreateUserDto, req.body);
      const user = await this.userService.create(dto);

      res.status(201).json({
        success: true,
        data: user
      });
    } catch (error) {
      next(error);
    }
  }

  async getById(req: Request, res: Response, next: NextFunction) {
    try {
      const { id } = req.params;
      const user = await this.userService.findById(id);

      if (!user) {
        throw new AppError('User not found', 404);
      }

      res.json({
        success: true,
        data: user
      });
    } catch (error) {
      next(error);
    }
  }

  async update(req: Request, res: Response, next: NextFunction) {
    try {
      const { id } = req.params;
      const dto = await validate(UpdateUserDto, req.body);
      const user = await this.userService.update(id, dto);

      res.json({
        success: true,
        data: user
      });
    } catch (error) {
      next(error);
    }
  }

  async delete(req: Request, res: Response, next: NextFunction) {
    try {
      const { id } = req.params;
      await this.userService.delete(id);

      res.status(204).send();
    } catch (error) {
      next(error);
    }
  }
}
```

### Service Layer
```typescript
import { PrismaClient } from '@prisma/client';
import bcrypt from 'bcrypt';
import { CreateUserDto, UpdateUserDto } from '../dto/user.dto';
import { AppError } from '../utils/errors';

export class UserService {
  constructor(private prisma: PrismaClient) {}

  async create(dto: CreateUserDto) {
    const existing = await this.prisma.user.findUnique({
      where: { email: dto.email }
    });

    if (existing) {
      throw new AppError('Email already exists', 409);
    }

    const hashedPassword = await bcrypt.hash(dto.password, 10);

    return this.prisma.user.create({
      data: {
        ...dto,
        password: hashedPassword
      },
      select: {
        id: true,
        email: true,
        name: true,
        createdAt: true
      }
    });
  }

  async findById(id: string) {
    return this.prisma.user.findUnique({
      where: { id },
      select: {
        id: true,
        email: true,
        name: true,
        createdAt: true
      }
    });
  }

  async update(id: string, dto: UpdateUserDto) {
    return this.prisma.user.update({
      where: { id },
      data: dto,
      select: {
        id: true,
        email: true,
        name: true,
        updatedAt: true
      }
    });
  }

  async delete(id: string) {
    await this.prisma.user.delete({
      where: { id }
    });
  }
}
```

### Middleware
```typescript
// Authentication
import jwt from 'jsonwebtoken';
import { Request, Response, NextFunction } from 'express';
import { AppError } from '../utils/errors';

interface JWTPayload {
  userId: string;
  email: string;
}

export async function authenticate(
  req: Request,
  res: Response,
  next: NextFunction
) {
  try {
    const token = req.headers.authorization?.replace('Bearer ', '');

    if (!token) {
      throw new AppError('No token provided', 401);
    }

    const payload = jwt.verify(
      token,
      process.env.JWT_SECRET!
    ) as JWTPayload;

    req.user = payload;
    next();
  } catch (error) {
    next(new AppError('Invalid token', 401));
  }
}

// Error Handler
export function errorHandler(
  err: Error,
  req: Request,
  res: Response,
  next: NextFunction
) {
  if (err instanceof AppError) {
    return res.status(err.statusCode).json({
      success: false,
      error: {
        message: err.message,
        code: err.code
      }
    });
  }

  console.error('Unexpected error:', err);

  res.status(500).json({
    success: false,
    error: {
      message: 'Internal server error'
    }
  });
}
```

## NestJS Example

### Module
```typescript
import { Module } from '@nestjs/common';
import { UserController } from './users.controller';
import { UserService } from './users.service';
import { PrismaService } from '../prisma/prisma.service';

@Module({
  controllers: [UserController],
  providers: [UserService, PrismaService],
  exports: [UserService]
})
export class UsersModule {}
```

### Controller
```typescript
import {
  Controller,
  Get,
  Post,
  Put,
  Delete,
  Body,
  Param,
  UseGuards,
  HttpCode,
  HttpStatus
} from '@nestjs/common';
import { UserService } from './users.service';
import { CreateUserDto, UpdateUserDto } from './dto/user.dto';
import { JwtAuthGuard } from '../auth/guards/jwt-auth.guard';
import { CurrentUser } from '../auth/decorators/current-user.decorator';

@Controller('users')
export class UserController {
  constructor(private readonly userService: UserService) {}

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

  @Get(':id')
  @UseGuards(JwtAuthGuard)
  async findOne(@Param('id') id: string) {
    return this.userService.findById(id);
  }

  @Put(':id')
  @UseGuards(JwtAuthGuard)
  async update(
    @Param('id') id: string,
    @Body() updateUserDto: UpdateUserDto
  ) {
    return this.userService.update(id, updateUserDto);
  }

  @Delete(':id')
  @UseGuards(JwtAuthGuard)
  @HttpCode(HttpStatus.NO_CONTENT)
  async remove(@Param('id') id: string) {
    return this.userService.delete(id);
  }
}
```

### Service
```typescript
import { Injectable, NotFoundException } from '@nestjs/common';
import { PrismaService } from '../prisma/prisma.service';
import { CreateUserDto, UpdateUserDto } from './dto/user.dto';

@Injectable()
export class UserService {
  constructor(private prisma: PrismaService) {}

  async create(createUserDto: CreateUserDto) {
    return this.prisma.user.create({
      data: createUserDto
    });
  }

  async findById(id: string) {
    const user = await this.prisma.user.findUnique({
      where: { id }
    });

    if (!user) {
      throw new NotFoundException(`User with ID ${id} not found`);
    }

    return user;
  }

  async update(id: string, updateUserDto: UpdateUserDto) {
    await this.findById(id);

    return this.prisma.user.update({
      where: { id },
      data: updateUserDto
    });
  }

  async delete(id: string) {
    await this.findById(id);

    await this.prisma.user.delete({
      where: { id }
    });
  }
}
```

## Testing

```typescript
import request from 'supertest';
import { createApp } from '../app';

describe('User API', () => {
  let app: Express;

  beforeAll(() => {
    app = createApp();
  });

  describe('POST /api/v1/users', () => {
    it('should create a new user', async () => {
      const response = await request(app)
        .post('/api/v1/users')
        .send({
          email: 'test@example.com',
          password: 'password123',
          name: 'Test User'
        });

      expect(response.status).toBe(201);
      expect(response.body.success).toBe(true);
      expect(response.body.data.email).toBe('test@example.com');
    });
  });
});
```

This skill ensures production-ready, scalable Node.js applications.

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