Middleware functions in Express.js are functions that have access to the request, response, and next middleware function in the application’s request-response cycle. They can perform operations like logging, authentication, and parsing. For example:
const express = require('express');
const app = express();
app.use((req, res, next) => {
console.log(`${req.method} ${req.url}`);
next(); // Pass control to the next middleware
});