If you’ve done any serious development in your career you’ve likely reached the point when you realized the importance of automated testing during development. Depending on your experience, this realization might hit you in one big burst or it may gently come to you over time, but it will eventually become second nature. Automatic testing comes in many forms, from unit testing, when you test isolated pieces of code, to integration and functional testing, when you test how different parts of your system behave together. This article is not about an overview on automatic testing in general. It is about a particular and a relatively new niche referred to as visual regression testing.
Visual regression testing takes an alternative approach to testing web pages. Instead of just making sure that some element or a text value is present in the DOM, the test actually opens the page and checks if this specific block looks exactly like you want it to. Just to make sure that you picked up the difference, let me give you an example. Imagine, that you want your website to greet your visitors with a friendly message:
To make sure it works, you can (and should) unit test the piece of code that produces the message, checking that it inserts the correct name. You can also write a functional test using Selenium or Protractor to see if the element is actually present on the page with the correct text. But this is not enough. We want to test not just that the text is generated correctly or appears in the DOM but to make sure that the whole element looks correct, i.e., making sure that the element is not hidden by display: none or that someone hasn’t accidentally overridden the color of the text. There are a number of tools to do that, but today we will be looking at one option in particular — PhantomCSS.
What is PhantomCSS?
PhantomCSS is a Node.js tool to perform visual regression testing. It is open source and developed by the guys at Huddle. PhantomCSS allows you to run a headless browser, open a page and take a screenshot of the whole page or a particular element on the page. This screenshot will be stored as a baseline image for future reference. Whenever you change anything on the website, you can run PhantomCSS again. It will take another screenshot and compare it to the original image. If there are no differences found, the test will pass. If, however, the screenshots don’t match, the test will fail and a new image showing the difference will be created for you to review. This approach makes this tool perfect for testing changes in CSS.
PhantomCSS is built on top of several key components:
CasperJS – a tool for interacting with a PhantomCSS or SlimerJS browser. It allows you to open a page and perform user interactions, such as clicking on buttons or inputting values. Additionally, CasperJS provides its own testing framework and the ability to capture screenshots of a page.
PhantomJS 2 or SlimerJS – two different headless browsers, either of which can be used with PhantomCSS. A headless browser is just like a normal browser without a user interface.
Resemble.js – a library for comparing images.
PhantomCSS can be used together with both PhantomJS and SlimerJS, but in this article, we’ll be using PhantomJS.
Let’s Take It for a Spin
Let’s set up a tiny test project to see how we can use this tool in practice. For that, we’ll need a web page to test and a simple Node.js web server for CasperJS to be able to open the page.
Setting up a Test Project
Create an index.html file with some sample content:
<div class=”tag tag-first”>The moving finger writes, and having written moves on.</div>
<div class=”tag tag-second”>Nor all thy piety nor all thy wit, can cancel half a line of it.</div>
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via Reme Le Hane