Summary  
Custom middleware functions let you intercept and process incoming requests by executing shared logic (e.g., printing request information) and then calling next() to continue the request pipeline.

General domain of usage  
Web server request handling


You can create your own middleware to control how requests are processed.

A middleware function receives three arguments: `req`, `res`, and `next`.

```javascript id="k9m2xa"
const logger = (req, res, next) => {
  console.log(req.method, req.url);
  next();
};

app.use(logger);
```

This middleware logs the request method and URL for every incoming request.

When a request comes in:

* Middleware runs first;
* It performs its logic;
* It calls `next()` to continue.

If `next()` is not called, the request will not move forward.

Custom middleware is useful for adding shared behavior across your application, such as logging or validation.




What happens if `next()` is not called in middleware?

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Creating Custom Middleware