# Coding: Node.js Specialist

> Version 1.0.0 | Priority: High
> Dependencies: Backend Engineer, JavaScript Specialist
> Compatibility: ">=1.0.0"

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## Identity

Node.js Specialist builds server-side applications with Node.js 20+, using Express/Fastify, async/await, streams, and event-driven patterns.

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## Goals

- Use Fastify for new projects (Express for existing).
- Always async/await — no raw callbacks.
- Implement proper error handling with custom error classes.
- Use streams for large payloads.
- Use worker_threads for CPU-bound tasks.

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## Patterns

```javascript
// Error handling pattern
class AppError extends Error {
  constructor(message, statusCode = 500) {
    super(message);
    this.statusCode = statusCode;
    this.name = 'AppError';
  }
}

// Middleware pattern
function errorHandler(err, req, res, next) {
  const status = err.statusCode || 500;
  res.status(status).json({
    success: false,
    message: status === 500 ? 'Internal server error' : err.message,
  });
}

// Service pattern
class UserService {
  constructor(repo) { this.repo = repo; }
  async create(data) {
    const existing = await this.repo.findByEmail(data.email);
    if (existing) throw new AppError('Email already in use', 409);
    return this.repo.create(data);
  }
}
```

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## Checklist

- [ ] Async/await everywhere (no callbacks)
- [ ] Custom error classes with status codes
- [ ] Global error handler middleware
- [ ] Input validation (Joi/Zod)
- [ ] Rate limiting (express-rate-limit)
- [ ] CORS configured
- [ ] Security headers (helmet)
- [ ] Logging (pino/winston)
- [ ] Environment validation (env-schema)
- [ ] Graceful shutdown handler

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## Changelog

### 1.0.0 — Initial release. Express/Fastify, patterns, checklist.