1. Introduction

This tutorial aims to guide you on how to utilize JSON Web Tokens (JWT) for user authentication in a Next.js application. By the end of this tutorial, you'll be able to:

• Generate JWTs for authenticated users.
• Validate JWTs on incoming requests.
• Use JWTs to protect sensitive routes in your Next.js application.

Prerequisites: Familiarity with JavaScript, React.js, and Next.js. Basic understanding of JSON Web Tokens and HTTP requests would be beneficial.

2. Step-by-Step Guide

What is JWT?

JSON Web Token (JWT) is an open standard used to securely transmit information between two parties. It's encoded as a JSON object, which is used to securely transfer data over the web.

Why use JWT?

JWTs provide a way of authenticating users on a per-request basis. This is useful for single page applications where the server does not maintain user sessions.

How to use JWT in Next.js?

JWT authentication in Next.js involves generating a token when a user logs in and then validating that token on subsequent requests.

3. Code Examples

Example 1: Generating a JWT

When a user logs in, you'll want to generate a JWT that can be sent back to the client and stored there.

import jwt from 'jsonwebtoken';

// User login data
const userData = {
  id: 1,
  username: 'testUser',
  email: 'testUser@example.com'
};

// Generate JWT
const token = jwt.sign(userData, 'your-secret-key', { expiresIn: '1h' });

// Now you can send back the token to client

In the above code, we are using the jsonwebtoken package to sign a new token. The first argument is the payload data we want to include in the token. The second argument is a secret key used for signing the token. The third argument is an options object where we can specify things like the token's expiry time.

Example 2: Validating a JWT

When you receive a request with a JWT, you'll want to validate it. Here's how you can do this in a Next.js API route:

import jwt from 'jsonwebtoken';

export default function handler(req, res) {
  const token = req.headers.authorization.split(' ')[1]; // Bearer <token>

  try {
    const decoded = jwt.verify(token, 'your-secret-key');
    req.userData = decoded;
    next();
  } catch (err) {
    return res.status(401).json({ message: 'Authentication failed' });
  }
}

In the code above, we are extracting the JWT from the Authorization header and using the jwt.verify() method to validate it. If the token is valid, the decoded data is added to the request object and passed along to the next middleware function. If the token is not valid, a 401 status code and an error message are returned.

4. Summary

This tutorial has provided an introduction to using JWT for authentication in Next.js. We've learned how to generate and validate JWTs, and how to use them to authenticate users.

The next steps could be learning how to handle token expiration and renewal, or how to use refresh tokens. You might also want to explore using third-party authentication providers like Auth0 or Firebase.

5. Practice Exercises

1. Exercise: Create a Next.js application with registration and login functionality. Generate JWTs for authenticated users and return them in the response.
   Hint: Use the jsonwebtoken package to generate tokens.

2. Exercise: Add middleware to your Next.js application that validates incoming JWTs and returns a 401 status code if the token is invalid.
   Hint: Extract the token from the Authorization header and use jwt.verify() to validate it.

3. Exercise: Add a protected route to your application that only returns data if the request includes a valid JWT.
   Hint: Use the middleware from the previous exercise to protect this route.

Solutions and further practice can be found in the official Next.js and jsonwebtoken package documentation.