Express Middleware
📌 Covered in this chapter:
Application vs Router Middleware · Custom Middleware · next() · Error Middleware
Understand Express middleware — functions that run between the request and response — including logging, auth, and error middleware.
1Express Middleware — What You'll Learn
Understand Express middleware — functions that run between the request and response — including logging, auth, and error middleware.
Here's everything this chapter covers, in the order you'll learn it:
- What middleware is
- Application-level middleware
- Router-level middleware
- Built-in middleware (express.json())
- Custom middleware
- Request logging middleware
- Authentication middleware
- Validation middleware
- Error-handling middleware
- Middleware execution order
- The next() function
- Async middleware
2Working Example
💻 Example: Express Middleware
JavaScript
▶ Run in Compiler
function requestLogger(request, response, next) {
console.log(request.method, request.url);
next();
}
app.use(requestLogger);3Best Practices & Common Pitfalls
💡 Key things to remember:
- Middleware runs in the exact order you register it with app.use() — order matters, especially for auth and error handlers.
- Forgetting to call next() (when appropriate) will leave the request hanging forever, just like forgetting response.end() in raw Node.js.
❓ Frequently Asked Questions (FAQ)
Q What's the most important thing to understand about express middleware?
Focus on: Application vs Router Middleware · Custom Middleware · next() · Error Middleware. These are the core building blocks this chapter's examples are built around, and they show up repeatedly in later chapters of this course.
Q Do I need external npm packages for express middleware?
Only where explicitly shown in the code examples above (like Express, Zod, or Socket.IO) — otherwise, this chapter relies entirely on Node.js's own built-in capabilities.