Show alert when back button is pressed

<script>
 window.close = function(){alert("your message");}
</script>

Don't you mean window.onunload?
Although, even clicking a link or going forward will make the event fire.

Dosn't work

> .. show a message box when the back button is pressed?
impossible (except patching the browser)

window.onunload() does not work if you navigate with the browser's back button from http://..../this#that to http://..../this

Hence this is not a working answer if you read the question verbatim.
Just my 2 pence ...

Hehe, yep... it's really not the fix for that. In fact, onunload is a very unreliable way to detect back navigations, I admit.
If you want to detect back navigations from this.htm#that to this.htm, I'd put a timer that checks window.location.hash -- I believe this method is being used by a few Flash-based websites to detect back button clicks.

Hmm..

that window.location.hash is interesting,

but anyway the onunload is good enough for the page I want it on.

thanks guys