Summary

So far, a result of an event handler has been assigned to the returnValue of the original event by jQuery. Initially, one could pass a message the browser will then display to the user. Since that got abused a lot, every browser stopped using the provided string and they all now provide a generic message. From the browsers supported in v4, only IE 11 would still display the message.

Incidentally, IE 11 is the only browser from our supported ones which respects the value returned from a beforeunload handler attached by addEventListener; other browsers do so only for inline handlers, so not setting the value directly shouldn't reduce any functionality.

This looks like a good moment to stop passing the message through and just call event.preventDefault() without extra checks which is shorter. This used to not work in Chrome but it got implemented in Chrome 119.

Unfortunately, it's hard to test this event in unit tests since it blocks page dismissal.

Size difference:

main @667321eb2d1c4328b993c25fbe2342a01ec4f87f
   raw     gz     br Filename
   -37    -17    +15 dist/jquery.min.js
   -37    -18     -7 dist/jquery.slim.min.js
   -37    -17    +31 dist-module/jquery.module.min.js
   -37    -19    -10 dist-module/jquery.slim.module.min.js

Checklist

• New tests have been added to show the fix or feature works
• If needed, a docs issue/PR was created at https://github.com/jquery/api.jquery.com