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Chrome has this text:
A window’s onbeforeunload property may be set to a function that returns a string that is shown to the user in a dialog box to confirm that the user wants to navigate away. This was intended to prevent users from losing data during navigation. Unfortunately, it is often used to scam users.
Starting in Chrome 51, a custom string will no longer be shown to the user. Chrome will still show a dialog to prevent users from losing data, but it's contents will be set by the browser instead of the web page.
With this change, Chrome will be consistent with Safari 9.1 and later, as well as Firefox 4 and later.