If I get you correctly, you want to know when a tab/window is effectively closed. Well, AFAIK the only way in Javascript
to detect that kind of stuffs are onunload
& onbeforeunload
events.
Unfortunately (or fortunately?), those events are also fired when you leave a site over a link
or your browsers back button. So this is the best answer I can give, I don’t think you can natively detect a pure close
in Javascript. Correct me if I’m wrong here.