Middleware refers to a mechanism that allows you to add extra functionality or processing to the request-response cycle of an application. It sits between the server receiving a request and the final route handler, enabling you to perform tasks such as request preprocessing, authentication, logging, error handling, and more.

Middleware functions have access to the request (req) and response (res) objects, as well as a next function that is used to pass control to the next middleware in the chain. The next function can be called multiple times within a middleware to trigger subsequent middleware or the final route handler.

Here’s an example of how to create and use middleware in Node.js:

// Import required modules
const express = require('express');

// Create an instance of the Express application
const app = express();

// Define a custom middleware function
const loggerMiddleware = (req, res, next) => {
  console.log('Request received:', req.method, req.url);
  next(); // Pass control to the next middleware

// Use the middleware in the application

// Define a route handler
app.get('/', (req, res) => {
  res.send('Hello, World!');

// Start the server
app.listen(3000, () => {
  console.log('Server started on port 3000');

In the above example, the loggerMiddleware function logs the HTTP method and URL of each incoming request. It then calls next() to pass control to the next middleware or the final route handler. The app.use() method is used to register the middleware with the Express application.

Note that the order of middleware registration matters. Middleware functions are executed in the order they are registered, so place them appropriately to achieve the desired behavior.

Multiple middleware functions can be chained together by calling app.use() multiple times. They will be executed in the order they are registered, and each middleware can modify the request or response objects before passing control to the next middleware.