JS瀏覽器關閉時清空cookie

function addCookie(objName,objValue,objHours)
{
    var str = objName + "=" + escape(objValue);
    if(objHours > 0){//等於0時，關閉瀏覽器自動清除Cookies.
     var date = new Date();
     var ms = objHours*3600*1000;
     date.setTime(date.getTime() + ms);
     str += "; expires=" + date.toGMTString();
    }
    document.cookie = str;
   }