1. Introduction

In this tutorial, we will be creating our first Middleware in Nuxt.js. Middleware functions are those that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle. These functions are used to modify req and res objects for tasks like parsing request bodies, adding response headers, etc.

By the end of this tutorial, you will be able to:
- Understand what middleware is in Nuxt.js
- Create your own middleware
- Use your middleware in a Nuxt.js application

Prerequisites:
- Basic knowledge of JavaScript
- Basic understanding of Vue.js and Nuxt.js

2. Step-by-Step Guide

In Nuxt.js, middleware is used for functions that run before rendering a page or a group of pages. Middleware can be applied at the application level, at the layout level, or at the page level.

To create a middleware, you should:
- Create a .js file inside the middleware/ directory in your Nuxt.js project.
- Export a function from this file which receives three arguments: context, req, and next.

The context provides additional objects/params from Nuxt to middleware, req is the request object from Node.js, and next is a function to end the middleware and run the next middleware if any.

3. Code Examples

Let's create a simple middleware that checks if a user is authenticated.

1. Create the Middleware File

Create a new file in the middleware directory and name it auth.js.

// middleware/auth.js

export default function ({ store, redirect }) {
  // If the user is not authenticated
  if (!store.state.authenticated) {
    return redirect('/login')
  }
}

This middleware checks if the user state authenticated is false. If it is, it redirects the user to the login page. The state would typically be changed upon user login.

1. Using Middleware in a Page

To use this middleware in a page, we add a middleware property to our page component.

// pages/secure.vue

export default {
  middleware: 'auth'
}

In this setup, if a user tries to access the secure page and they are not authenticated, they will be redirected to the login page.

4. Summary

In this tutorial, we've learned what middleware is in Nuxt.js, how to create a middleware, and how to use it in a Nuxt.js application. The next steps would be to explore different use cases for middleware such as validation, authorization, and more.

5. Practice Exercises

1. Exercise 1: Create a middleware that redirects the user to the homepage if they are already authenticated and they visit the login page.

2. Exercise 2: Create a middleware that checks if a user has a specific role before accessing a page.

Tips for further practice:
- Try creating middleware that interacts with Vuex store
- Create middleware for error handling
- Explore how to use external libraries in your middleware

Remember, the best way to learn is by doing, so don't hesitate to write your own middleware and use them in your Nuxt.js applications. Happy coding!