How can I get the destination URL for the onbeforeunload event?

I've searched for hours, but I couldn't find a solution for this.

window.onbeforeunload = warn;

This doesn't work:

function warn (e) 
{ 
   var destination = e.href;
   alert(destination );
}

Okay, so to clear the things. If the user clicks on a link on the page itself, it is easy, because you can add an eventhandler to all of the links onclick event, but. I want to catch the address, what the user types into the url box of the browser.