Screenshot Testing: A Detailed Guide
By Mohit Joshi, Community Contributor - November 15, 2022
The testing industry is always discovering and experimenting with the best testing methods to bring out the most impactful ones. When testing the app on different devices, you may notice that there is an anomaly. How can one stop the visual regressions in margins or paddings? User interface iteration is a cumbersome and difficult process. This is where screenshot testing comes into the picture. Snapshot testing is the other word often used interchangeably with screenshot testing.
Let’s dive deeper and thoroughly understand Screenshot testing.
What is Screenshot Testing?
Screenshot testing is the process where you ensure that the output continues to behave as expected. In this, a UI component is first rendered, and then its Screenshot is captured in isolation. This process helps compare the current state of a component with a previously recorded reference screenshot captured under the same preconditions. The failed assertion will reflect in the test if something has changed between the screenshots.
For instance, consider these two screenshots.
If you perform a website screenshot test for the above example, it will reflect a failed test based on changes in the navbars’ styling. The test will highlight if something has changed without stating which Screenshot is correct.
Also Read: How to perform Android screenshot testing?
How Does Screenshot Testing Work?
When you start a test, check for existing screenshots. If it is the first time running the test, the snapshot is generated automatically, and hence the test gets completed. However, if it is the second test, then some screenshot is already created and stored in the files. A new screenshot is then created and compared with the already stored Screenshot.
The test is failed when the current state does not match the previous Screenshot; however, if both screenshots match, the test is completed.
Advantages of Screenshot Testing
Let’s first understand the benefits of using Screenshot testing.
- Saves time: When screenshot testing was initially announced, it was revealed that testers were seeking more time in writing a test for a component than actually writing a component itself. Snapshot testing has shorter syntax and thus eases the process of testing and saves a good amount of time while testing.
- Easy to maintain: Screenshot testing usually contains only a few lines of code for testing. Thus, it becomes easier to maintain.
Disadvantages of Screenshot Testing
Despite all the comfort and benefits it provides, there are certain disadvantages to Screenshot testing.
- Small changes can lead to a failed test: As seen in the above example, where a slight change in the color of the Navbar, despite none of them being wrong, can lead to the failure of the test. The same happens very often at the production level of any application. A slight change in the application leads to failed screenshot testing. This makes our testing significantly fragile, and thus, developers have to manually test each and everything, which is time-consuming.
- Screenshot testing doesn’t provide meaningful output: The overall idea behind Screenshot testing is that it should always match the recorded value and shouldn’t ever change. This does not reflect any meaningful assertion or expectation, and ultimately developer has to manually go through the test to verify the output. This leads to another significant drawback that it does not go along with the dynamic content. Any application that has to produce random dynamic output might fail in screenshot testing because the test will only match it with previously stored value.
Tools for Performing Screenshot Testing
Let’s understand what tools you may use to perform quality screenshot testing of your application before launching it in front of real-world users. There are several tools. Let’s look at some of the most popular tools used to perform screenshot testing.
There are several other frameworks available. However, these are the most widely used. Moreover, all the above-mentioned frameworks will allow you to perform screenshot testing in the local system. Also, there are cloud-based services to run tests against several combinations of web browsers and operating systems simultaneously and could also store screenshots.
Let’s now perform the screenshot testing in a cloud-based service Browserstack.
Screenshot Testing using BrowserStack
Let’s now implement a practical example of screenshot testing using BrowserStack. BrowserStack is a cloud-based testing service that is very helpful in performing screenshot testing of websites under several combinations of different web browsers and operating systems.
Step 1: Navigate to BrowserStack’s Screenshot testing tool
Step 2: Enter the URL of the website you want to test and select the combinations of browsers and operating systems on which you want to perform testing.
Step 3: Click on the download button beside each Screenshot to download a screenshot individually, and click on the zip to download all the screenshots at once.
Screenshot testing is a great way to test the application’s stability while presenting it to real-world users. Snapshot testing has been adopted by numerous developers since its release, mainly because it has a shorter syntax to write tests than the traditional way. However, it could not replace them entirely due to its few disadvantages. The most prominent disadvantage of using screenshot testing is that it only matches the Screenshot with the previous file stored in the database and does not provide any valuable assertion on test failure.