加急！javascript实现下拉列表框显示（输入拼音头显示选项）

一种自己写拼音<select name="username" onkeydown=go()>
     <option value="1" sp="ZS">张三</option>
     <option value="2" sp="ZSH">张三火</option>
     <option value="3" sp="LS">李四</option>
     <option value="4" sp="LSS">李四水</option>
</select><script>
var sel="",timer=null;
function go(){
       with(window.event){
          with(srcElement){
              if(keyCode<48)return;
              if(keyCode>95)keyCode-=48
              sel+=String.fromCharCode(keyCode)
              window.status=sel
              for(i=0;i<length;i++){
               if(options[i].sp.indexOf(sel)==0){selectedIndex=i;break;}
              }
          }
          returnValue=false;
          clearTimeout(timer)
          timer=setTimeout("sel=''",500);
     }
}
</script>