Express middleware and next()

Model answer. Middleware is a function (req, res, next) => {} that runs during the request–response cycle. Each one can read or change req/res, end the response, or call `next()` to pass control to the next middleware.

const app = express();
app.use(express.json());                       // application-level: every request
app.use((req, res, next) => {                  // logging
  const start = Date.now();
  res.on('finish', () => console.log(req.method, req.url, res.statusCode, Date.now() - start, 'ms'));
  next();
});
const requireLogin = (req, res, next) => (req.user ? next() : res.status(401).json({ error: 'Sign in' }));
app.get('/account', requireLogin, (req, res) => res.json(req.user));   // route-level

• Order matters: middleware runs in the order it is registered. A body parser must come before the routes that read req.body; a 404 handler goes after all routes.
• If a middleware neither responds nor calls next(), the request hangs.
• Application-level middleware (app.use(fn)) applies to all routes, or to a path prefix (app.use('/api', fn)). Route-level middleware is passed to specific routes, or to an express.Router().
• Calling next(err) skips to the error-handling middleware.