Determin user's intention when onunload fires

I want to use onunload to capture user's action when he
clicks on the "x" button, the close window button on the
upper-right corner of the brower window, to close the
browser. Here is how I capture the event:
<SCRIPT FOR="window" EVENT="onunload">
myScript();
</SCRIPT>

While myScript() does get called when "x" gets clicked, it
is also get called when the user leaves the current page
and navigate to another page within our web application.
My question is how do I determin if onunload is fired
because of an "x" clicking, an attempt to go out of
current web application, or simply go to another page
within our application.

Any input will be highly appreciated.

Thanks