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Prevent visual regression bugs with BackstopJS

saniadsouza profile image sania-dsouza Updated on ・3 min read

Testing could be a lot of work if you are responsible for the visual (or even functional) feels of a website. And picking out visual differences after seemingly-unrelated code changes could be a pain. BackstopJS is one of the tools that could help automate this bit.

Following is a short tutorial on how to set up Backstop for your node project.

Quick facts:

  • BackstopJS automates visual regression testing of a responsive web UI by comparing DOM screenshots over time.
  • It includes an in-browser reporting feature, which allows you to check layout settings for print and screen, test approving, inspection, etc
  • Docker rendering for cross-platform tests
  • Simulating user interactions using Puppeteer

A quick test:

  • Install BackstopJS :
npm install -g backstopjs
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  • Generate a Backstop config file:
backstop init 
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The file generated is backstop.json. This has some default config settings that Backstop looks for when it runs.

Some of the important config properties:

  • id : used for screenshot naming
  • viewports : array of viewport sizes ; at least one must be specified
  • scenarios: specifies different user flows for example. Section of the config file is shown below.

sampleinit

This test config file specifies a single scenario. All it does is navigate to the URL against url.

  • Run the test
backstop test
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This first test fails since it didn’t find a reference image to compare the test screenshot with.

samplefail1

The report generated on browser (since the ‘report’ property was set to ‘browser’ in the config file) looks like this:

samplefailreport1

You might've noticed that when you ran backstop init, a set of folders was created. One of these was bitmaps_test. This folder holds the test screenshots.

Screen Shot 2021-03-22 at 1.00.58 PM

  • To make a test file the reference for future tests, run:
backstop approve
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This 'approves' the previous test screenshot as a standard or reference to compare future test screenshots with. It copies the screenshot from the bitmaps_test folder to the bitmaps_reference folder.

samplereference1

  • Run the test again:
backstop test
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This time, the test passes.

samplepass1

The corresponding browser report:

samplepassreport1

Another test(with user interaction)

Puppeteer is used to simulate user scenarios.
Add another scenario to the scenarios array in the config file:

...
"scenarios": [
    {
      "label": "BackstopJS Homepage",
      "cookiePath": "backstop_data/engine_scripts/cookies.json",
      "url": "https://garris.github.io/BackstopJS/"
    },
     {
       "label": "BackStopJS Interaction scenario",
       "url": "https://garris.github.io/BackstopJS/",
       "clickSelector": ".cta"
     }
  ],
...
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Here, the URL as specified by url is opened and when the element specified by the selector clickSelector is available, it is clicked; and then Backstop takes a screenshot.

The first time you run backstop test, the test will fail as it doesn't find the reference image. Run backstop approve and backstop test like before to get your test to pass.

In my test, this second test fails as well because of a difference between the reference and the test screenshot. This is indicated by the hot pink text on the difference screenshot.

samplefailreport2

At this point, I could either fix what changed in the code or update my reference using backstop approve.

Backstop also has a scrubber utility that displays the difference between the reference and test screenshots by moving a mapper cursor, which is pretty intuitive.

samplescrubber

You could automate more complex user flows, add more viewports, tune performance and other good stuff using Backstop; their documentation is great and they have a healthy user base also.

Backstop niceties:

  • Easy to set-up
  • Good support for various viewports and Docker integration in case of cross-platform issues
  • Intuitive reporting and inspection
  • Easy Puppeteer scripts to simulate user interaction
  • Variable image difference sensitivity

Not a con, but a housekeeping task:

Outdated screenshots will have to be cleared manually or pushed to a .gitignore file to ensure they don't make it into the remote repo.

PS: If you are anything (read curious) like me, that cutie on the Backstop logo is a ring-tailed lemur ;).

Try these visual testing tools too:

  1. Jest(jest-image-snapshot specifically)
  2. Aye-spy

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