Middleware Functions in Node.js
From the Express.js REST API cheat sheet · Express.js Basic Setup · verified Jul 2026
Middleware Functions
Create custom middleware for authentication, logging, and request processing
javascript
// Custom middleware
const auth = (req, res, next) => {
const token = req.header('Authorization');
if (!token) {
return res.status(401).json({ error: 'Access denied' });
}
next();
};
// Apply middleware
app.use(auth);
app.get('/protected', auth, (req, res) => {
res.json({ message: 'Protected route' });
});🟢 Essential - Middleware is the heart of Express
💡 Order matters - middleware runs in sequence
⚠️ Don't forget next() or response will hang
📌 app.use() applies to all routes, router.use() to specific routes
⚡ Keep middleware focused on single responsibility
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