JavaScript + onbeforeunload
window.onbeforeunload = function() { return 'You have unsaved changes!';}See the MSDN article on onbeforeunload
Also there is a similar question in SO
From my experience onunload works differently in different browsers. Why dont you use another event handler called onbeforeunload instead. It should work. Onbeforeunload will execute first before the window closes, so that should solve your problem.