Hello readers 👋, welcome to the 10th blog in our Node.js series! In the last post, we built a clean REST API using Express.js for a users resource. We defined routes and handled different HTTP methods. Today, we are going to explore a concept that sits quietly at the heart of every Express application: middleware . If you have ever wanted to log every request, check authentication tokens, or validate incoming data before it hits your route handler, you have needed middleware. Let's understand what middleware is, how it fits into the request lifecycle, the different types, and how to chain multiple middleware to build powerful, reusable pipelines. Let's get started. What middleware is in Express Middleware, in Express, are functions that have access to the request object ( req ), the response object ( res ), and the next middleware function in the application's request-response cycle. The next middleware function is conventionally denoted by a variable named next .…