添加收藏夹，只运行一次的javascript代码

function get_cookie(Name)
{
var search = Name + "="
var returnvalue = "";
if (document.cookie.length > 0)
{
    var offset = document.cookie.indexOf(search)
    if (offset != -1)
    {
        offset += search.length
        end = document.cookie.indexOf(";", offset);
    if (end == -1)
        end = document.cookie.length;
       
    returnvalue=unescape(document.cookie.substring(offset, end))
    }
}
return returnvalue;
}
function WindowClose()
{
    var s = get_cookie('bookmarkit')
    alert(s)
    if (s=='')
    {
        if( confirm("感谢您访问本网站。您愿意收藏天气在线吗？"))
        {
            document.cookie="bookmarkit=yes"
            JSAddFavorite()
         }
     }
}