Displaying a confirmation box inside javascript unload event

HI,
I have added an unload event to my page like :
window.addEventListener('unload', function(event) {
    if(cancel === false) {
        return "Changes you made will not be saved";
    }
});

Bascially when unloading happens i want a confirmation dialog to appear
with two buttons ok and cancel. And if user presses ok then proceed with the normal flow otherwise stop closing of the current window.

But somwhow the dialog itself is not appearing. Many places say that
you cannot have confirmation dialog from inside an unload event so i tried returning a string but that also didnt work.
I am making it work in chrome .

Thanks