How we built it

Custom messages in onbeforeunload dialogs (removed)

A window’s onbeforeunload property may be set to a function that returns a string. If the function returns a string, then before unloading the page, a dialog is shown to have the user confirm that they indeed want to navigate away. The string provided by the function will no longer be shown in the dialog. Rather, a generic string not under the control of the webpage will be shown.


This shipped in Safari 9.1, and has been shipping in Firefox since Firefox 4. Safari considers this a security fix and assigned it CVE-2009-2197 (see ). Approved with the intent .


Established standard

Status in Chromium

Removed (launch bug) in:

  • Chrome for desktop release 51
  • Chrome for Android release 51
  • Android WebView release 51
  • Opera release 38
  • Opera for Android release 38

Consensus & Standardization

  • Shipped
  • No public signals
  • Shipped
  • Negative


Last updated on 2016-06-02