浏览器退出提示

<script type="text/javascript"><!--  window.onbeforeunload = onbeforeunload_handler;window.onunload       = onunload_handler;document.onkeydown    = disableF5;function onbeforeunload_handler(){  if(1==1){  var warning="Close this window?";       return warning;  }       }function onunload_handler(){ }function disableF5(e)    {    if (!e)        {  var e = window.event;}    var keycode = e.keyCode;    if (e.which)        keycode = e.which;    var src = e.srcElement;    if (e.target)        {src = e.target;           }    // 116 = F5    if (116 == keycode)    {        // Firefox and other non IE browsers        if (e.preventDefault)        {            e.preventDefault();            e.stopPropagation();        }        // Internet Explorer        else if (e.keyCode)        {            e.keyCode = 0;            e.returnValue = false;            e.cancelBubble = true;        }        return false;    }}