how to get the confirmation box when we close by shortcut that is Alt+F4

document.onkeydown = keydown; 
function keydown(evt)
{ 
  if (!evt) 
     evt = event;
  if (evt.altKey && evt.keyCode==115)
  { 
     // ALT+F4 
     alert("ALT+F4"); // Do whatever you want here
  } 
} 

<script>      
var userIsEditingSomething; // set this if something crazy happens         
oldOnBeforeUnload = window.onbeforeunload;      
window.onbeforeunload = function () 
{         // attempt to handle a previous onbeforeunload        
    if ('function' === typeof oldOnBeforeUnload) 
    {             
        var message = oldOnBeforeUnload();             
        if ('undefined' !== typeof message) 
        {                 
           if (confirm('string' === typeof message ? message : 'Are you sure you want to leave this page?'))    
           {   
               return; // allow user to exit 
           }
        }
    }         
    if (userIsEditingSomething) 
    {
                 return 'Are you sure you want to exit?';        
    }
};  </script>

I got this script from :