js实现弹出的提示框只弹出一次

 1 <script type="text/javascript">
 2 var ua = navigator.userAgent.toLowerCase();    
 3 if (/iphone|ipad|ipod/.test(ua)) {
 4 //alert("iphone");        
 5 } else if (/android/.test(ua)) {
 6 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!"
 7 var once_per_session=1
 8 function get_cookie(Name) {
 9   var search = Name + "="
10   var returnvalue = "";
11   if (document.cookie.length > 0) {
12     offset = document.cookie.indexOf(search)
13     if (offset != -1) {
14       offset += search.length
15       end = document.cookie.indexOf(";", offset);
16       if (end == -1)
17          end = document.cookie.length;
18       returnvalue=unescape(document.cookie.substring(offset, end))
19       }
20    }
21   return returnvalue;
22 }
23 function alertornot(){
24 if (get_cookie(‘alerted‘)==‘‘){
25 loadalert()
26 document.cookie="alerted=yes"
27 }
28 }
29 function loadalert(){
30 alert(alertmessage)
31 }
32 if (once_per_session==0)
33 loadalert()
34 else
35 alertornot()
36     }
37 </script>