js控制30秒倒计时代码

请等待<span id="dd">15</span>秒
<script type="text/javascript">
function run(){
var s = document.getElementByIdx_x("dd");
if(s.innerHTML == 0){
   window.location.href='regform.shtml';
   return false;
}
s.innerHTML = s.innerHTML * 1 - 1;
}
window.setInterval("run();", 1000);
</script>