Using Express Middleware

Middleware refers to functions in the Express.js web application framework that have access to the request object (req), the response object (res), and the next function in the application’s request-response cycle. Middleware functions can execute any code, make changes to the request and response objects, end the request-response cycle, and call the next middleware function in the stack. 
The whole idea of middleware is to execute some code before the controller action that sends the response and after the server gets the request from the client. Essentially it is code that executes in the middle of your request, hence the name middleware. 

Middleware functions can be used for a variety of purposes, such as logging, authentication, handling errors, parsing request bodies, and more. They provide a modular way to handle common tasks in web applications and allow the structuring of code in a more organized and reusable manner.

Middleware functions are added to the Express application using the `app.use()` method or specific HTTP verb methods like `app.get()`, `app.post()`, etc., before defining the routes. Middleware can be added globally to be executed for every request or locally to be executed only for specific routes.

Here's a simple example of a middleware function in Express:

const express = require('express');
const app = express();

// Middleware function
app.use((req, res, next) => {
  console.log('Time:', Date.now());
  next(); // Pass control to the next middleware function
});

// Route handler
app.get('/', (req, res) => {
  res.send('Hello World!');
});

app.listen(3000, () => {
  console.log('Server is running on port 3000');
});

In this example, the middleware function logs the current time for every incoming request. It then calls the next() function to pass control to the next middleware function in the stack, which in this case is the route handler for the '/' route.

Middleware functions can be chained together, allowing the creation of complex processing pipelines for incoming requests. They provide a powerful mechanism for building flexible and extensible web applications with Express.js.

Middleware is incredibly powerful for cleaning up code and making things like user authorization and authentication much easier, but it can be used for so much more than just that because of the incredible flexibility of middleware.