1. Introduction

In this tutorial, we will delve into the process of conducting end-to-end tests in a Next.js application. End-to-end tests involve testing a workflow from start to finish. They aim to identify system dependencies and ensure that the right information is passed between various system components to deliver the desired output.

By the end of this tutorial, you will learn how to:

• Understand the importance of end-to-end tests
• Write end-to-end tests for Next.js application
• Simulate user interactions in your tests

Prerequisites: Basic understanding of JavaScript and familiarity with Next.js and Jest testing framework.

2. Step-by-Step Guide

End-to-end testing involves simulating a user's behavior in your application and ensuring that it behaves as expected. We will use Jest and Puppeteer for testing our Next.js application.

Installation

First, we need to install Jest and Puppeteer in our project:

npm install --save-dev jest puppeteer

Setting Up

Create a new file jest.config.js in your project root directory and add the following code:

module.exports = {
    testEnvironment: "jest-environment-node",
    testTimeout: 10000
};

3. Code Examples

Let's create a simple end-to-end test for a login page.

Login Page (pages/login.js)

// This page has two input fields (username and password) and a submit button.
function LoginPage() {
    return (
        <div>
            <input id="username" type="text" />
            <input id="password" type="password" />
            <button id="submit">Login</button>
        </div>
    );
}
export default LoginPage;

Test Case (tests/login.test.js)

const puppeteer = require('puppeteer');

let browser;
let page;

beforeAll(async () => {
    // launch the browser
    browser = await puppeteer.launch();
    // create a new page
    page = await browser.newPage();
});

afterAll(() => {
    // close the browser
    browser.close();
});

test('user can login', async () => {
    // navigate to the login page
    await page.goto('http://localhost:3000/login');

    // simulate user typing into the input field
    await page.type('#username', 'myusername');
    await page.type('#password', 'mypassword');

    // simulate user clicking the submit button
    await page.click('#submit');

    // wait for navigation
    await page.waitForNavigation();

    // get the page title
    const title = await page.title();

    // check if the title is correct
    expect(title).toBe('Dashboard');
});

4. Summary

In this tutorial, you learned how to conduct end-to-end testing in a Next.js application. We demonstrated this by creating an end-to-end test for a login page using Jest and Puppeteer.

For further learning, consider exploring how to test more complex user interactions and behaviors. You could also learn how to automate these tests as part of your CI/CD pipeline.

5. Practice Exercises

1. Write an end-to-end test for a signup page. The page should include fields for username, email, and password, and a signup button that redirects to a welcome page upon successful signup.

2. Write an end-to-end test for a user adding an item to a shopping cart and checking out.

3. Write an end-to-end test for a user leaving a comment on a blog post.

Remember to simulate user interactions as accurately as possible in your tests. Think about the different edge cases that could occur and write tests to cover them. Happy testing!