show alert before close the window by javascript

window.onbeforeunload = function (evt) { 
  var showmessage = 'Are you sure you want to leave?'; 
  if (typeof evt == 'undefined') { 
    evt = window.event; 
  } 
  if (evt) { 
    evt.returnValue = showmessage; 
  } 
  return showmessage ; 
}