Leave site? Changes you made may not be saved.

I see this at Reddit when I try to bail on changes in Google Chrome. Wrangling the WindowEventHandlers.onbeforeunload in JavaScript seems to be the way to approach making something like this possible. How else can a web site know that you are clicking on bookmark and trying to abandon ship? See:

• https://developer.mozilla.org/en-US/docs/Web/API/WindowEventHandlers/onbeforeunload
• https://code-maven.com/prevent-leaving-the-page-using-plain-javascript