Implementing Visual Regression Testing with Puppeteer and Jest
To satisfy all users’ needs we must create complex apps. But complex inside, these apps must be user-friendly. Otherwise people won’t use them, even if they are useful.
Visual regression testing is a technique to identify and prevent UI inconsistencies by comparing the visual appearance of a web page or application over time. Let’s check how to implement visual regression testing using Puppeteer, a headless browser automation tool, and Jest, a popular testing framework. We'll cover the basics of setting up your environment, writing your first visual regression test, advanced testing techniques, and best practices for maintaining a reliable test suite. By following this guide, you'll learn how to catch UI bugs early in the development cycle, deliver pixel-perfect UIs, and ultimately enhance the user experience of your applications.
1. Setting Up the Environment
Before diving into writing visual regression tests with Puppeteer and Jest, you'll need to set up your development environment. In this section, we'll walk through the process of installing Puppeteer and Jest, as well as creating a sample project for testing.
To work with Puppeteer and Jest, you'll first need Node.js and npm (Node Package Manager) installed on your machine. Download and install the latest version of Node.js from the official website.
Once Node.js and npm are installed, open your terminal and run the following commands to install Puppeteer and Jest as development dependencies in your project:
Bash
npm init -y # Create a new project with default settings
npm install --save-dev jest puppeteer
For this guide, let's assume you have a simple web application with HTML, CSS, and JavaScript files. Create a directory structure like the following:
your-project/
├── public/
│ ├── index.html
│ ├── styles.css
│ └── app.js
└── src/
└── your-test-file.js
public/ directory contains your web application files, while src/ will hold the test files.
Create a jest.config.js file at the root of your project with the following configuration:
Javascript
module.exports = {
clearMocks: true,
testEnvironment: "node",
};
This configuration ensures that mocks are cleared between tests and sets the test environment to Node.js.
In your project's package.json file, update the "test" script to use Jest with Puppeteer:
Json
{
"scripts": {
"test": "jest --env=puppeteer"
}
}
Now your environment is set up and ready for writing visual regression tests using Puppeteer and Jest.
2. Writing Your First Visual Regression Test
With the environment set up, let's write our first visual regression test using Puppeteer and Jest. We'll create a simple test script to capture screenshots and compare them using Jest's image snapshot feature.
Inside the src/ directory, create a new file named your-test-file.js. This file will hold our test cases.
At the top of your-test-file.js, import Puppeteer and Jest:
Javascript
const { expect } = require('@jest/globals');
const puppeteer = require('puppeteer');
Write a describe block for your test suite, and create a beforeAll hook to launch a new browser instance:
领英推荐
Javascript
describe('Visual Regression Test', () => {
let browser, page;
beforeAll(async () => {
browser = await puppeteer.launch({ headless: false });
page = await browser.newPage();
});
// Add your tests here
afterAll(async () => {
await page.close();
await browser.close();
});
});
Setting headless to false allows you to see the browser window during testing. You can set it to true for headless testing in a CI environment.
Inside the describe block, add a test case that navigates to your web application, captures a screenshot, and saves it as an image snapshot:
Javascript
test('Homepage screenshot', async () => {
await page.goto('https://localhost:3000');
await page.screenshot({ path: 'homepage.png' });
expect(await page.screenshot()).toMatchImageSnapshot();
});
Replace 'https://localhost:3000' with the URL of your web application.
To run the visual regression test, simply execute the test script in your terminal:
Bash
npm run test
Jest will generate an image snapshot of the captured screenshot. Subsequent test runs will compare the captured screenshots to the existing snapshots and highlight any differences.
With this first test, you have successfully implemented visual regression testing using Puppeteer and Jest. Next, we'll explore advanced testing techniques to make your test suite even more robust.
3. Advanced Visual Regression Testing Techniques
Now that you have a basic visual regression test set up, let's explore some advanced techniques to make your test suite more comprehensive and robust.
To ensure pixel-perfect UIs at the component level, consider using a library like puppeteer-screenshot-tester. It integrates with Puppeteer and Jest, allowing you to capture and compare screenshots of individual components.
Javascript
const { expect } = require('@jest/globals');
const screenshotTester = require('puppeteer-screenshot-tester');
test('Component Screenshot Test', async () => {
const tester = new screenshotTester.PuppeteerTester({
page,
baselinesDirectory: './__image_snapshots__/baseline',
resultsDirectory: './__image_snapshots__/result',
});
await tester.testSelector('my-component', {
// Additional options, like cropping and threshold
});
});
For broader visual regression testing, you can use BrowserStack or SauceLabs with Puppeteer and Jest. These cloud-based platforms enable you to run tests across various browsers, operating systems, and devices.
To ensure continuous testing, integrate Puppeteer and Jest with your CI/CD pipeline. For example, you can set up a GitHub Actions workflow to run your visual regression tests on every push or pull request.
By incorporating these techniques into your visual regression testing process, you can achieve more comprehensive test coverage and maintain pixel-perfect UIs across various components, browsers, and devices.
4. Best Practices and Tips for Visual Regression Testing
To get the most out of your visual regression testing efforts and minimize the chances of encountering flaky tests or other issues, follow these best practices and tips:
Conclusion
Want to maintain consistent and pixel-perfect user interfaces. Then visual regression testing is your way out. By leveraging the power of Puppeteer and Jest, we've demonstrated how to set up an environment for visual regression testing, write your first test, apply advanced techniques such as component-level testing, and ensure cross-browser and cross-device compatibility.?
By following this guide and incorporating visual regression testing into your development workflow, you'll catch UI bugs early in the development cycle, improve collaboration between designers and developers, and ultimately enhance the user experience of your web applications. With the knowledge and tools provided in this guide, you're well-equipped to tackle visual regression testing and deliver exceptional, consistent UIs to your users.
If you need some help in visual regression testing, feel free to contact Cherish DEV!