javaScript实现简单网页倒计时代码

<div id="button">

<input type="button" value="同意" id="b01" disabled="disabled" onclick="window.close()"/>
</div>
 </body>
</html>
<script>
var a = document.getElementById("b01");
var i = 10;
function sub(){
setTimeout("sub()",1000);

if(i == 1){
a.value="同意";
a.removeAttribute("disabled");
return;
}else{
a.value="同意"+"("+(--i)+")";
}

}
</script>