express-middleware

# Express Middleware Skill

Patterns for creating and using middleware in Express.js.

## Middleware Basics

### Middleware Structure

```typescript
import { Request, Response, NextFunction } from 'express'

// Basic middleware
function middleware(req: Request, res: Response, next: NextFunction) {
  // Do something
  next()  // Pass to next middleware
}

// Middleware with error
function middlewareWithError(req: Request, res: Response, next: NextFunction) {
  if (!req.headers.authorization) {
    return next(new Error('No authorization header'))
  }
  next()
}

// Async middleware
async function asyncMiddleware(req: Request, res: Response, next: NextFunction) {
  try {
    const data = await fetchSomething()
    req.data = data
    next()
  } catch (error) {
    next(error)
  }
}
```

### Middleware Order

```typescript
import express from 'express'

const app = express()

// Order matters! Middleware runs top to bottom

// 1. Body parsing (run early)
app.use(express.json())
app.use(express.urlencoded({ extended: true }))

// 2. Logging (run early)
app.use(morgan('dev'))

// 3. Security headers
app.use(helmet())

// 4. CORS
app.use(cors())

// 5. Authentication (before routes)
app.use(authenticate)

// 6. Routes
app.use('/api', routes)

// 7. 404 handler (after routes)
app.use(notFoundHandler)

// 8. Error handler (last)
app.use(errorHandler)
```

## Request Processing

### Request Logging

```typescript
function requestLogger(req: Request, res: Response, next: NextFunction) {
  const start = Date.now()

  // Log on response finish
  res.on('finish', () => {
    const duration = Date.now() - start
    console.log({
      method: req.method,
      path: req.path,
      status: res.statusCode,
      duration: `${duration}ms`,
      ip: req.ip,
    })
  })

  next()
}
```

### Request ID

```typescript
import { v4 as uuid } from 'uuid'

function requestId(req: Request, res: Response, next: NextFunction) {
  const id = req.headers['x-request-id'] || uuid()
  req.id = id