Middleware functions are functions that execute during the request-response lifecycle.

A middleware function receives three important arguments:

(req, res, next)

• req: Contains incoming request information.
• res: Provides methods for sending the response.
• next: Passes control to the next middleware.

Middleware can:

• Read request information.
• Modify request or response objects.
• Perform authentication.
• Validate input.
• Log requests.
• Handle errors.
• Terminate requests by sending responses.

If middleware neither sends a response nor calls next(), the request may remain pending.

Middleware order matters because Express executes middleware in registration order.

function requestLogger(req, res, next) {
  console.log({
    method: req.method,
    path: req.originalUrl,
    timestamp: new Date().toISOString()
  });

  next();
}

app.use(requestLogger);