Node.js Middleware Complete Guide

What is Middleware?

Middleware are functions that run between receiving a request and sending a response. Each has access to req, res, and next. They power logging, auth, parsing, and error handling in Express.

The Middleware Signature

function middleware(req, res, next) {
  // do something
  next(); // pass control to the next middleware
}
app.use(middleware);

Example: Request Logger

app.use((req, res, next) => {
  console.log(`${req.method} ${req.url} - ${new Date().toISOString()}`);
  next();
});

Example: Authentication Guard

function requireAuth(req, res, next) {
  const token = req.headers.authorization;
  if (!token) return res.status(401).json({ error: "Unauthorized" });
  req.user = verifyToken(token);
  next();
}

app.get("/profile", requireAuth, (req, res) => {
  res.json(req.user);
});

Built-in & Popular Middleware

Middleware	Purpose
express.json()	Parse JSON bodies
cors	Enable cross-origin requests
helmet	Security headers
morgan	HTTP request logging

Error-Handling Middleware (4 args)

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).json({ error: "Something broke" });
});

FAQs

Does middleware order matter?

Yes — middleware runs top to bottom. Put logging and body parsing early, error handlers last.

What if I forget next()?

The request hangs forever. Always call next() (or send a response). More in our Node.js guides.