What is Middleware in Express and How It Works

"Middleware is the invisible assembly line your request travels through before it ever reaches your route."

Introduction

You've set up an Express server. You have routes. Things work. But then real-world requirements show up — you need to log every incoming request, verify that users are authenticated, validate request bodies, handle errors gracefully. Where does all that logic go?

The answer is middleware — one of the most powerful and fundamental concepts in Express.js. Once you understand middleware, you stop writing tangled route handlers and start building clean, composable server applications.

This guide walks through what middleware is, how it fits into the request lifecycle, the different types available, and how to use it in practice.