In this tutorial, we will learn about Middleware functions, which have access to the request object (req), the response object (res), and the next middleware function in the application's request-response cycle.

The next middleware function is commonly denoted by a variable named next.

In Express, we can use two types of middleware: application-level middleware and route-level middleware.

Application-Level Middleware

Let's start with application-level middleware. Open the index.js file and add the middleware to the app:

App.use(function(req,res,next){ 
  console.log('Time: ' , Date.now()) 
}); 

Save the changes and run the application. Switch to the command prompt and run the index.js file:

node index.js

Now, go to the browser and navigate to the URL localhost:3000. You should see the current date and time displayed, indicating that the application-level middleware is applied.

If you change the route to /about or /contact, you will still see the date and time displayed, demonstrating that the middleware is applied to the entire application.

Route-Level Middleware

Next, let's explore route-level middleware. Open the index.js file and add the middleware to the about route:

app.get('/about', function (req, res, next) { 
  console.log('Time: ' , Date.now()) 
  res.render('about'); 
}); 

Run the application again. Switch to the command prompt and re-run the index.js file:

node index.js

Now, go to the browser and navigate to the URL /about. You should see the date and time displayed. However, if you change the URL to /contact, the date and time will not be displayed, demonstrating that the middleware is only applied to the about route.

This is how you can use middleware in Node.js.