In all of the sample code I have seen, it appears that the only function of onbeforeunload is to serve up an alert dialog box prior to the person leaving the page. Is that the only thing that can be triggered by the event or is it possible to do something else, like an unobtrusive function that sends off partial form data?
I am trying to capture abandoned shopping carts in Yahoo! Small Business and unfortunately I do not have access to any server side scripting, so I'm forced to work client-side only.
I was also thinking of doing an ajax posting of data after the email field was changed, and then comparing the list of all forms partially submitted against completed carts to determine which were incomplete.
You can save the partial form data in localStorage. Then, when another page is loaded, you could check for the presence of that data and AJAX it to the server, deleting it from localStorage on success. Or you might be able to just use that data in JavaScript, without involving the server, but that depends on your setup.