Summary  
This chapter explains how to define middleware functions that execute between receiving a request and sending a response, allowing access to request and response objects and using a next callback to control the processing flow.

General domain of usage  
Web applications


Middleware is a function that runs between receiving a request and sending a response.

It has access to the request, the response, and a special function called `next`.

```javascript
app.use((req, res, next) => {
  console.log('Request received');
  next();
});
```

When a request comes in, middleware runs first. After it finishes, it calls `next()` to pass control to the next step.

The flow looks like this:

Request → Middleware → Route → Response

Middleware is used to process requests before they reach your route logic.

Common use cases include:

* Logging Requests;
* Parsing Data;
* Checking Authentication.



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Introduction to Middleware