Middleware

Middleware functions are the backbone of Express.js. They run between receiving a request and sending a response — allowing you to add functionality like logging, authentication, parsing, and error handling.

How Middleware Works

Middleware functions have access to the req, res, and next objects:

app.use((req, res, next) => {
    console.log('New request:');
    console.log('Path:', req.path);
    console.log('Method:', req.method);
    next(); // Pass control to the next middleware/route
});

Important: Always call next() unless you're ending the request. Without it, the request hangs forever.

Types of Middleware

• Application-level — app.use() runs on every request
• Route-level — Runs only on specific routes
• Built-in — Comes with Express (e.g., express.static())
• Third-party — Installed via NPM (e.g., morgan, cors)

Static Files Middleware

Serve CSS, images, and JavaScript files from a public/ folder:

app.use(express.static('public'));

Now any file in public/ is accessible. For example, public/styles.css can be loaded as /styles.css in your HTML.

Morgan — Logging Middleware

const morgan = require('morgan');
app.use(morgan('dev'));

Morgan logs details about every request — method, URL, status code, and response time. Essential for debugging.

Custom Middleware Example

// Authentication check middleware
const requireAuth = (req, res, next) => {
    if (req.headers.authorization) {
        next(); // User is authenticated
    } else {
        res.status(401).send('Unauthorized');
    }
};

// Use on specific routes
app.get('/dashboard', requireAuth, (req, res) => {
    res.render('dashboard');
});