离开画面时的JavaScript确认提示

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

onbeforeunload 事件参考地址

http://msdn.microsoft.com/en-us/library/ms536907(VS.85).aspx