07 — Middleware

Core Concepts

• Middleware — functions that run between request and response
• Signature — (req, res, next) => {} — call next() to pass to next middleware
• Order matters — middleware runs in the order it's registered
• app.use() — apply middleware to all routes
• Built-in — express.json(), express.urlencoded(), express.static()
• Error middleware — (err, req, res, next) — 4 parameters
• Third-party — cors, helmet, morgan, compression, rate-limit

Middleware Flow

Request → [cors] → [helmet] → [morgan] → [json parser] → [auth] → [route handler] → Response
                                                                         ↓ (error)
                                                               [error handler] → Response