Express Middleware

Middleware is a function that runs between the incoming request and your final response, with the signature (req, res, next) , letting you log, parse, authenticate, or transform every request in a pipeline.

Learn Express Middleware in our free Node.js course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Node.js course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

By the end of this lesson you'll write your own middleware, understand why order matters, share data down the chain with req , use built-in parsers like express.json() , and add a catch-all error handler.

What You'll Learn in This Lesson

1️⃣ What Middleware Is

A middleware function takes three arguments: req (the request), res (the response), and next (a function that hands control to whatever comes next). Register it with app.use() and it runs for every request. The golden rule: always either call next() or send a response.

The middleware logged the request, then called next() to let the route handler send the actual reply. Drop the next() and the browser would spin forever — the request would never reach the handler.

2️⃣ Order Matters: a Pipeline

Middleware runs in registration order, like a pipeline. Each function can attach data to req so later functions — including your route handler — can read it. This is how things like the logged-in user or a request timer get shared down the chain.

The logger ran first, then auth, then the handler — exactly the order they were registered. Because each stored data on req , the final handler could read both req.startTime and req.user .

3️⃣ Built-in and Error-Handling Middleware

Express bundles useful middleware. express.json() parses a JSON request body onto req.body . And a special error-handling middleware — recognized by its four arguments (err, req, res, next) — catches anything that throws, so you can return a clean error instead of crashing.

Your turn. Fill in the two ___ blanks to complete a logger and a timestamp middleware, then run it.

Mini-Challenge: An API Key Gate

Write a middleware that only lets the request through when ?key=secret is present, and rejects everything else with a 401. Register it before your routes.

Common Errors (and the fix)

• The request hangs forever — A middleware neither called next() nor sent a response. Make sure every path does one or the other.
• req.body is undefined — You forgot app.use(express.json()) before the route, so the JSON body was never parsed.
• Error handler never runs — Error-handling middleware must have all four arguments (err, req, res, next) and be registered after your routes.
• "Cannot set headers after they are sent" — You sent a response and then called next() or sent again. End the cycle once; don't continue after responding.
• Middleware runs in the wrong order — Express respects registration order. Put parsers and auth above the routes that depend on them.

Pro Tips

• 💡 Scope middleware to a path: app.use("/admin", checkAdmin) runs only for URLs starting with /admin .
• 💡 Pass errors forward: in async middleware, use next(err) to route a failure to your error handler.
• 💡 One error handler at the end: keep a single four-argument handler last so it catches everything.

📋 Quick Reference — Express Middleware

Goal

Syntax

Global middleware

app.use((req, res, next) => {})

Pass control on

next()

Parse JSON body

app.use(express.json())

Path-scoped

app.use("/api", mw)

Send an error onward

next(err)

Error handler

(err, req, res, next) => {}

Frequently Asked Questions

🎉 Lesson Complete!

• ✅ Middleware is a function with the signature (req, res, next)
• ✅ Always call next() or send a response — never both, never neither
• ✅ Middleware runs in registration order , forming a pipeline
• ✅ Attach data to req to share it down the chain
• ✅ express.json() parses JSON bodies onto req.body
• ✅ Error handlers take four arguments and go last
• ✅ Next lesson: REST APIs with Express — design clean CRUD endpoints

Practice quiz

Continue this course

• Previous: Express Intro & Routing
• Next: REST APIs with Express