Use window.onbeforeunload event to stop browser from closing or Disable Button before Page PostBack

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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; 
};