Try It Yourself - SyntaxStudy Editor

HTML / CSS / JS

const express = require('express');
const app     = express();
app.use(express.json());

// Middleware 1 – rate limiter (simplified)
const hits = {};
function rateLimit(req, res, next) {
    const ip = req.ip;
    hits[ip] = (hits[ip] || 0) + 1;
    if (hits[ip] > 100) {
        return res.status(429).json({ error: 'Too Many Requests' });
    }
    next(); // pass to middleware 2
}

// Middleware 2 – auth check
function authenticate(req, res, next) {
    const token = req.headers.authorization?.split(' ')[1];
    if (!token) return next(new Error('No token provided')); // → error handler
    req.userId = 'user-123'; // decoded from token in real code
    next(); // pass to route handler
}

// Route with two middleware in sequence
app.get('/profile', rateLimit, authenticate, (req, res) => {
    res.json({ userId: req.userId });
});

// Error-handling middleware (4 args, registered last)
app.use((err, req, res, next) => {
    console.error(err.message);
    res.status(err.status || 500).json({ error: err.message });
});

app.listen(3000);