If you encountered an error message like:
Uncaught ReferenceError: MutationEvent is not defined
or an unexpected error window related to MutationEvent when opening a page or system, then this article is for you.
📅 Updated: May 2025
This issue affects regular users, not just developers accessing old systems or web apps that still rely on outdated technology. The error started to appear more frequently in Google Chrome and Microsoft Edge from mid-2024 onwards. In this article, I explain:
- Why it appears,
- How to eliminate it,
- And which browser version lets you keep using the system without errors.
Why does this error appear?
For many years, browsers allowed websites to use a technology called Mutation Events to detect automatic changes on a page (like new messages, alerts, or data).
However, this technology became obsolete and caused performance and security issues. Therefore, it was removed in the latest versions of Chrome and Edge.
Starting with version 127 (released in July 2024), Chrome and Edge stopped supporting this feature. When an old website tries to use it, you’ll see the error:
MutationEvent is not defined
This error is not your fault
The error is not caused by you, nor by anything you did wrong. It appears because:
- You’re using a site, platform, or system that has not been updated;
- You’re using a newer version of Chrome or Edge where this feature has been disabled.
How to eliminate the error (quick fix)
If you urgently need to access the system without seeing this error, you can temporarily fix the issue by using an older version of the browser, where the feature still existed.
The last compatible version was version 126 of both Chrome and Edge.
Download a compatible version
Google Chrome 126 (Windows 64-bit)
- Version: 126.0.6478.57
- Download (offline installer)
Microsoft Edge 126 (Windows 64-bit)
- Version: 126.0.2592.68
- Download (offline installer)
⚠️ These versions do not receive security updates. Use them only in controlled environments or to access specific systems.
Permanent alternative: the system must be updated
The only way to fix this issue permanently is for the system you’re using to be updated by its developers.
The "MutationEvent is not defined" error only disappears for good if the site or application code is updated internally, replacing the old feature with a modern technology called MutationObserver.
If you work with the system or know who develops it, you can send them this explanation:
Starting from version 127 of Chrome and Edge, the MutationEvent API has been completely removed from browsers. To maintain compatibility with modern browsers, the system must replace DOMNodeInserted, DOMNodeRemoved, DOMSubtreeModified, and similar events with MutationObserver.
📅 Issue Timeline
| Event | Version | Date |
|---|---|---|
| Last version where it worked | 126 | June 2024 |
| Error began appearing | 127 | June 2024 |
| Experimental support (enterprise only) ended | 136 | May 2025 |
| Feature completely removed | 137 | May/June 2025 |
Conclusion
This error is increasingly common in legacy systems, and many regular users are caught by surprise — especially in schools, enterprise software, or internal platforms.
If you’ve encountered this error, you can still access the system temporarily using Chrome or Edge 126. However, the real solution depends on the developers of the site or application updating the code to modern technologies.
- If you’re a regular user, send this article to the system’s technical support.
- If you’re a developer, I recommend replacing Mutation Events with MutationObserver.
🎥 Practical Fix Video (for version 126)