No Storybook tutorial would be complete without testing. Testing is essential to creating high-quality UIs. In modular systems, minuscule tweaks can result in major regressions. So far, we have encountered three types of tests:
Unfortunately, the aforementioned testing methods alone aren’t enough to prevent UI bugs. UIs are tricky to test because design is subjective and nuanced. Manual tests are, well, manual. When used for UI, snapshot tests trigger too many false positives, and pixel-level unit tests are of poor value. A complete Storybook testing strategy also includes visual regression tests.
Visual regression tests, also called visual tests, are designed to catch changes in appearance. They work by capturing screenshots of every story and comparing them commit-to-commit to surface changes. It's perfect for verifying graphical elements like layout, color, size, and contrast.
Storybook is a fantastic tool for visual regression testing because every story is essentially a test specification. Each time we write or update a story, we get a spec for free!
There are several tools for visual regression testing. We recommend Chromatic, a free publishing service made by the Storybook maintainers that runs visual tests in parallelized cloud. It also allows us to publish Storybook online, as we saw in the previous chapter.
Visual regression testing relies on comparing images of the newly rendered UI code to the baseline images. If a UI change is caught, we'll get notified.
Let's see how it works by tweaking the background of the
Start by creating a new branch for this change:
git checkout -b change-task-background
src/components/Task to the following:
<input type="text" :value="task.title" readonly placeholder="Input title" + style="background: red;" />
This yields a new background color for the item.
Add the file:
git add .
git commit -m "change task background to red"
And push the changes to the remote repo:
git push -u origin change-task-background
Finally, open your GitHub repository and open a pull request for the
Add a descriptive text to your pull request and click
Create pull request. Click on the "🟡 UI Tests" PR check at the bottom of the page.
It will show you the UI changes caught by your commit.
There are a lot of changes! The component hierarchy where
Task is a child of
Inbox means one small tweak snowballs into major regressions. This circumstance is precisely why developers need visual regression testing in addition to other testing methods.
Visual regression testing ensures components don’t change by accident. But it’s still up to us to determine whether changes are intentional or not.
If a change is intentional, we'll need to update the baseline to compare future tests to the latest version of the story. If a change is unintentional, it needs to be fixed.
Since modern apps are constructed from components, it’s important that we test at the level of the component. Doing so helps us pinpoint the root cause of a change, the component, instead of reacting to symptoms of a change: the screens and composite components.
When we’ve finished reviewing, we’re ready to merge UI changes with confidence--knowing that updates won’t accidentally introduce bugs. If you like the new
red background, then accept the changes. If not, revert to the previous state.
Storybook helps us build components; testing helps us maintain them. The four types of UI testing covered in this tutorial were manual, snapshot, unit, and visual regression testing. You can automate the last three by adding them to a CI as we've just finished setting up, and it helps us ship components without worrying about stowaway bugs. The whole workflow is illustrated below.