js给dropdownlist添加选项的小例子

?<BR><BR><BR>Js代码<BR><BR><scripttype="text/javascript">
////创建新目录
       functionnewDir(){
           vartxt=document.getElementById("newDirtxt");
           vardirname=txt.value;
           if(dirname){
               alert("请填写目录名");
           }else{
               $.ajax({
                   type:"Post",
                   url:"/center/directorynew",
                   async:false,
                   data:{dirname:dirname},
                   success:function(msg){
                       if(msg.length<40){
                           varDirectoryID=document.getElementById("DirectoryID");
                           vartOption=document.createElement("Option");
                           tOption.text=dirname;
                           tOption.value=msg;
                           DirectoryID.appendChild(tOption);
                           txt.value="";
                           alert("增加成功");

                       }else{
                           alert("增加失败");
                       }
                   }
               });
           }//else
       }
   </script><BR><BR><BR>后台代码<BR><BR>返回刚刚插入的目录的id字符串<BR>