The key to visual regression testing is to ensure that the appearance of the page remains unchanged after the code is updated. Its implementation can be achieved through the following points: 1. Select a tool that supports CI integration, can handle dynamic content and has clear reports; 2. Set a reasonable baseline from a small-scale core page and ignore non-critical changes; 3. Integrate the test into the PR process and the CI closed loop; 4. Set pixel thresholds to deal with false alarms, control the test environment, and pay attention to font loading issues.

The visual regression test of front-end UI is simply to ensure that the page looks the same after the code is updated. This doesn't sound complicated, but it's easy to cause problems. Especially when the project becomes larger and there are many components, it is almost impossible to rely on human eyes to check the changes in each page. So now more and more teams will introduce automation tools to assist.

The following directions are the most critical points in actual operation.

Why do visual regression tests

In front-end development, CSS changes, third-party library upgrades, and even layout structure adjustments may inadvertently affect the appearance of the UI. This part that "looks unchanged" is the easiest to be overlooked. Functional testing can ensure that the button can still be clicked, but it cannot tell you whether the button is tilted by 5 pixels.

To give a simple example: you changed the global color variable, and the colors of all buttons changed, and you didn't expect that those old pages were also using this variable. If you discover this kind of problem only after you go online, the repair cost will be high.

Common practices and tool selection

There are roughly two types of mainstream visual regression testing methods:

• Based on screenshot comparison : such as Percy, Applitools, BackstopJS, these tools will take screenshots and compare them with the benchmark graph to automatically mark the differences.
• Based on pixel/DOM structure comparison : Some self-developed solutions will choose to directly compare element sizes, positions, color values, etc., which are suitable for more refined control scenarios.

When choosing a tool, focus on three points:

• Whether it supports CI integration
• Is there a mechanism for dynamic content (such as time stamps, user avatars)
• Is the report clear and can the problem be located quickly

How to effectively implement visual regression testing

Having tools alone is not enough, and the execution method is also crucial. The following suggestions can help you avoid pitfalls:

• Start with a small scope : Don't run the entire website from the beginning. Choose a few core pages or components to ensure that the process is stable before expanding.
• Set a reasonable baseline picture : Make sure that the screenshot is fine when running the first time, otherwise the subsequent comparison will be wrong.
• Ignore unnecessary changes : such as ad spaces and dynamic data areas, which can be skipped through masking or exclusion rules.
• Use in combination with the PR process : screenshot comparison is automatically triggered every time a PR is submitted, and the review is passed and merged can be discovered in time.

In addition, it is recommended to use visual regression testing as part of the CI process rather than running independently. This makes it easier to form a closed loop.

Problems and solutions that are easy to ignore

Many people used it smoothly at the beginning, but later they found that there were too many false alarms or the maintenance cost was too high. These problems can actually be prevented in advance:

• Frequently changing pages are not suitable for full comparison → Only screenshots of static parts or split component-level tests
• Different browsers rendering effects are different → clearly specify the test environment (for example, only run on Chrome)
• Too many false alarms, no one reads the report → Set a reasonable pixel threshold, appropriately relax the tolerance, and avoid small changes and errors

There is another detail that is easily overlooked: the font loading problem . Sometimes screenshots are taken before the font is loaded, which will cause abnormal text display. You can add a hook to wait for the font to load before the screenshot, or use the system default font for testing.

Basically that's it. Visual regression testing is not omnipotent, but it can detect many problems that are difficult to detect with the naked eye in the early stage. If done well, it will save a lot of time in post-debugging.

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