Middleware in Express are functions that run during the request-response cycle. Each middleware receives req, res, and next. Calling next() passes control to the next middleware in the chain.
Middleware can execute code, modify the request/response objects, end the cycle, or call next().
const express = require('express');
const app = express();
// --- Application-level middleware ---
// Request logger:
app.use((req, res, next) => {
console.log(`[${new Date().toISOString()}] ${req.method} ${req.url}`);
next(); // MUST call next() or the request hangs
});
// Attach a request ID:
app.use((req, res, next) => {
req.requestId = Math.random().toString(36).slice(2);
res.setHeader('X-Request-ID', req.requestId);
next();
});
// --- Route-level middleware ---
function requireJson(req, res, next) {
if (req.headers['content-type'] !== 'application/json') {
return res.status(415).json({ error: 'Content-Type must be application/json' });
}
next();
}
app.post('/api/data', requireJson, (req, res) => {
res.json({ message: 'Data received', id: req.requestId });
});
// --- Error-handling middleware (4 parameters) ---
app.use((err, req, res, next) => {
console.error(err.stack);
res.status(500).json({ error: err.message || 'Internal Server Error' });
});
app.listen(3000);