js实现弹出的提示框只弹出一次

 <script type="text/javascript">
 var ua = navigator.userAgent.toLowerCase();
 if (/iphone|ipad|ipod/.test(ua)) {
 //alert("iphone");
 } else if (/android/.test(ua)) {
 var alertmessage="温馨提示：您可以在右下角的菜单中下载最新版本的手机版APP哦！\n \n本提示只出现一次!"
 var once_per_session=1
 function get_cookie(Name) {
   var search = Name + "="
   var returnvalue = "";
   if (document.cookie.length > 0) {
     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 alertornot(){
 if (get_cookie('alerted')==''){
 loadalert()
 document.cookie="alerted=yes"
 }
 }
 function loadalert(){
 alert(alertmessage)
 }
 if (once_per_session==0)
 loadalert()
 else
 alertornot()
     }
 </script>

其实就是记录cookie。