Middleware are functions that run in sequence during the request-response cycle in Express. Each receives (req, res, next) and can inspect/modify the request and response, end the response, or call next() to pass control to the next middleware. They're the core of how Express apps are structured.
app.( {
.();
();
});
The critical rule: call next() to continue the chain. Forget it (and don't send a response), and the request hangs forever.
Request → [logger] → [auth] → [body parser] → [route handler] → Response
next() next() next() res.send()
Each middleware runs in order; any can short-circuit by sending a response.
// 1. Application-level — runs for all (or path-matched) requests
app.use(express.json()); // built-in: parse JSON bodies
app.use(cors()); // third-party
// 2. Custom — e.g. authentication that can SHORT-CIRCUIT
function requireAuth(req, res, next) {
if (!req.headers.authorization) {
return res.status(401).json({ error: "Unauthorized" }); // STOP — don't call next()
}
req.user = decodeToken(req.headers.authorization); // attach data for later handlers
next(); // authorized → continue
}
// 3. Route-specific
app.get("/profile", requireAuth, (req, res) => res.json(req.user));
Middleware can short-circuit (send a response and not call next(), e.g. blocking unauthorized requests) or enrich the request (attaching req.user for downstream handlers).
// special signature with 4 params → Express treats it as an error handler
app.use((err, req, res, next) => {
console.error(err);
res.status(500).json({ error: "Server error" });
});
// triggered by next(err) or a thrown error; place it LAST
An error handler has four parameters (err first). You reach it by calling next(err); it should be registered last.
Middleware runs in the ORDER you register it. Body parser before routes that
read the body; auth before protected routes; error handler last.
Middleware is the architectural backbone of Express (and similar frameworks) — authentication, logging, body parsing, CORS, validation, and error handling are all middleware.
Understanding the (req, res, next) contract (especially that you must call next() or send a response), how middleware can short-circuit or enrich requests, the four-argument error handler, and that order matters, is essential for building, structuring, and debugging any Express application.