A webpage is not always broken because of a large mistake. Small details can produce exactly this problem: debugging: async script changes execution order. This guide shows you what to look for, a concrete fix, and how to check that it worked.
The Idea in Plain English
Async scripts execute when their downloads finish, not in document order. Avoid dependencies between async scripts.
HTML supplies the structure; JavaScript supplies the behavior described here. If you remove the script, check what remains usable for visitors.
A Working Example
The first part shows a problem. The second part shows a safer or more accurate version. Compare them before reading the explanation.
<!-- Problem -->
<script async src="/js/library.js"></script>
<script async src="/js/plugin.js"></script>
<!-- Corrected example -->
<script defer src="/js/library.js"></script>
<script defer src="/js/plugin.js"></script>What Each Part Does
- The script element can load code or hold data; the type attribute tells the browser how to treat its contents.
- src points to the asset the browser should load.
Before the fix, the browser can behave incorrectly or report a warning. After the change, test the exact failure rather than assuming the page is fixed.
Try It Yourself
Defer preserves the relative execution order of these classic deferred scripts.
Common Beginner Mistakes
- Check the underlying cause before changing unrelated tags.
- Copying the code without checking whether its linked files or endpoints exist; sample addresses and file paths need real resources.
- Choosing a tag only because its default appearance looks right, without checking whether it describes the content correctly.
Key Takeaways
- Async scripts execute when their downloads finish, not in document order. Avoid dependencies between async scripts.
- Before the fix, the browser can behave incorrectly or report a warning. After the change, test the exact failure rather than assuming the page is fixed.
- Test the markup in a browser, and check accessibility and real-world behavior where relevant.