Express.js Middleware Patterns

const jwt = require('jsonwebtoken')

const authenticateToken = (req, res, next) => {
  const authHeader = req.headers['authorization']
  const token = authHeader && authHeader.split(' ')[1]

  if (!token) {
    return res.status(401).json({ error: 'Access token required' })
  }

  jwt.verify(token, process.env.JWT_SECRET, (err, user) => {
    if (err) {
      return res.status(403).json({ error: 'Invalid or expired token' })
    }
    req.user = user
    next()
  })
}

module.exports = { authenticateToken }

const errorHandler = (err, req, res, next) => {
  const statusCode = err.statusCode || 500
  const message = err.message || 'Internal Server Error'

  res.status(statusCode).json({
    success: false,
    error: message,
    ...(process.env.NODE_ENV === 'development' && { stack: err.stack }),
  })
}

module.exports = errorHandler

const asyncHandler = (fn) => (req, res, next) => {
  Promise.resolve(fn(req, res, next)).catch(next)
}

// Usage
const getUsers = asyncHandler(async (req, res) => {
  const users = await User.find()
  res.json(users)
})

const validateRequest = (schema) => {
  return (req, res, next) => {
    const { error } = schema.validate(req.body)
    if (error) {
      return res.status(400).json({
        error: error.details[0].message,
      })
    }
    next()
  }
}

// Usage with Joi
const userSchema = Joi.object({
  email: Joi.string().email().required(),
  password: Joi.string().min(6).required(),
})

router.post('/register', validateRequest(userSchema), registerUser)