popup message on closing browser

Question

When I am closing my browser I want a confirmation message.

 

 

I am using

 

window.onbeforeunload = confirmExit;
function confirmExit() {
return "You will Lost this test. Are you sure you want to exit this page?";
}

 

But this  function is also working on page postback on my page.

I want this message on window cclose only

What should I have to do.????