Functions that run between request and response.
✍️ 2 exercises in this lessonMiddleware functions receive (req, res, next). They can read or modify the request, end the response, or call next() to continue.
Uses: logging, authentication, parsing bodies, serving static files (express.static("public")), CORS and error handling.
An error-handling middleware has four arguments: (err, req, res, next).
Mastering Node.js Express Middleware helps you write clean and reliable Node.js code. Follow standard conventions, test edge cases, and keep logic simple. Try running the code example below and complete the exercises to build confidence.
import express from "express";
const app = express();
// Logger for every request
app.use((req, res, next) => {
console.log(`${req.method} ${req.url}`);
next();
});
// Simple auth for /admin routes
function requireKey(req, res, next) {
if (req.query.key !== "secret") return res.status(401).send("Unauthorized");
next();
}
app.get("/admin", requireKey, (req, res) => res.send("Admin area"));
app.get("/boom", () => { throw new Error("Something broke"); });
// Error handler
app.use((err, req, res, next) => {
res.status(500).json({ error: err.message });
});
app.listen(3000);GET /admin
→ 401 Unauthorized
GET /admin?key=secret
→ Admin area
GET /boom
→ 500 {"error":"Something broke"}
Check what you learned in “Node.js Express Middleware”.
Pass control to the next middleware.
app.use((req, res, next) => {
console.log(req.url);
();
});
How many arguments does an error-handling middleware have?
Stuck on something?
Ask the community in the CodesInTech forum.
Need a developer?
Learning on your own is great — and when you need professional help, our team builds custom software, websites and mobile apps for clients worldwide.
Explore Our Services