All modern browsers support the JavaScript's **window.onbeforeunload** event. It fires before the **unload** event when the page is unloaded. So this event always uses to stop a page from exit. The following codes illustrate how to implement this function:
window.onbeforeunload = function (e) { var message = 'Are you sure you want to leave?'; var e = e || window.event; // For IE and Firefox prior to version 4 if (e) { e.returnValue = message ; } // For Safari return message; };
and we can also use this event to disable button before postback is done or form is submitted so that user cannot click button again. It is very simple.
window.onbeforeunload = function () { document.getElementById("<%=Button1.ClientID %>").disabled = true; };