Tuesday 17 October 2023

Visual Testing with Playwright




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If you haven't checked out Playwright yet, I highly recommend giving it a try. It's easy to get started with and there's a growing community of developers sharing their experiences and best practices. Let's make testing web applications easier and more efficient together!

Let's first learn to install Playwright

Here are the steps to install Playwright along with the relevant links:

  1. Install Node.js on your system, if it is not already installed. You can download it from the official website: https://nodejs.org/en/

  2. Open a command prompt (or terminal) and install Playwright by running the following command: npm playwright

    This will install Playwright and its dependencies.

  3. Once the installation is complete, you can start using Playwright in your Node.js projects.

Here are some useful links that you may find helpful:

Visual Comparisons in Playwright Test: Ensuring Pixel-Perfect Quality


When it comes to modern web development and testing, ensuring a seamless and pixel-perfect user experience is of paramount importance. Visual testing plays a crucial role in guaranteeing that your web applications look and behave as intended across different browsers and platforms. Playwright Test, a powerful end-to-end testing framework, equips you with the capability to effortlessly perform visual comparisons, enabling you to catch any visual discrepancies with ease.

In this blog post, we'll explore the world of visual comparisons in the Playwright Test. We'll delve into how you can use Playwright Test to generate and compare screenshots, ensuring that your web application's visual elements remain consistent, even as your code evolves.

Understanding Visual Comparisons

Playwright Test incorporates a feature that enables you to produce and visually compare screenshots of web pages. This is achieved using the await expect(page).toHaveScreenshot() method. On the first execution, Playwright Test generates reference screenshots, and subsequent test runs compare the newly generated screenshots with these reference images.

Let's look at a simple example to understand how this works:


import { test, expect } from '@playwright/test';

test('example test', async ({ page }) => {
  await page.goto('https://playwright.dev');
  await expect(page).toHaveScreenshot();
});


When you run this test for the first time, Playwright Test will generate reference screenshots and save the last screenshot to the file system, ready to be added to your repository. If the test runner can't find a reference screenshot, it will create one.


Snapshot Structure and Naming

The snapshots and reference images are stored in a folder named after your test file. For instance, if your test file is named example.spec.ts, the folder for the reference screenshots will be example.spec.ts-snapshots.

The snapshot files are named with a structure like example-test-1-chromium-darwin.png, which consists of the following elements:

  • example-test-1.png: An auto-generated snapshot name. You can also specify a custom name as the first argument of the toHaveScreenshot() method.

  • chromium-darwin: This part includes the browser name and platform. Snapshots may differ between browsers and platforms due to variations in rendering, fonts, and other factors. Therefore, you need different snapshots for different configurations.

If you're not running tests on the same operating system as your CI/CD system, you can use Docker to generate or update the screenshots. Here's an example:


docker run --rm --network host -v $(pwd):/work/ -w /work/ -it mcr.microsoft.com/playwright:v1.39.0-jammy /bin/bash
npm install
npx playwright test --update-snapshots


Updating Reference Screenshots

Occasionally, you'll need to update the reference screenshots. This might be necessary when the web page's visual appearance has changed due to design updates or other factors. To update the reference screenshots, simply use the --update-snapshots flag when running your tests:

npx playwright test --update-snapshots




Customizing Visual Comparisons

Playwright Test leverages the pixelmatch library for visual comparisons. You can customize the behavior of pixel comparison by providing options. For example, you can set the maxDiffPixels option to specify the maximum allowable pixel difference in the images:


import { test, expect } from '@playwright/test';

test('example test', async ({ page }) => {
  await page.goto('https://playwright.dev');
  await expect(page).toHaveScreenshot({ maxDiffPixels: 100 });
});


To apply a default maxDiffPixels value across all tests in your project, you can set it in your Playwright configuration file (playwright.config.ts):


import { defineConfig } from '@playwright/test';

export default defineConfig({
  expect: {
    toHaveScreenshot: { maxDiffPixels: 100 },
  },
});


Beyond Screenshots: Snapshot for Text and Binary Data

While visual comparisons are essential, Playwright Test offers even more capabilities. You can use the expect(value).toMatchSnapshot(snapshotName) method to compare not only screenshots but also text or arbitrary binary data. Playwright Test automatically detects the content type and applies the appropriate comparison algorithm.

Here's an example of comparing text content against a reference:


import { test, expect } from '@playwright/test';

test('example test', async ({ page }) => {
  await page.goto('https://playwright.dev');
  expect(await page.textContent('.hero__title')).toMatchSnapshot('hero.txt');
});


Snapshots for text or binary data are stored in separate directories next to the test files, making it easy to organize and manage different types of snapshots.

Incorporating visual comparisons into your testing process with Playwright Test ensures that your web applications maintain their pixel-perfect quality, even as your code evolves. By detecting visual discrepancies early in the development cycle, you can deliver a superior user experience.


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