Middleware functions in Express.js process requests before reaching the final route handler. You can create custom middleware to add functionalities like logging, authentication, etc. Example of custom middleware:
const express = require('express');
const app = express();
const loggerMiddleware = (req, res, next) => {
console.log(`${req.method} ${req.url}`);
next(); // Pass control to the next middleware
};
app.use(loggerMiddleware); // Use custom middleware