简单的倒计时按钮

<button>点击开始倒计时</button>
<script>
var btn = document.getElementsByTagName('button')[0];
var count=10;
var timer=null;
btn.onclick=function(){
clearInterval(timer);
this.disabled=true;
var _this=this;
function redown(){
count--;
if (count>=0) {
_this.innerHTML=count;
}else{
_this.innerHTML="请再次点击";
_this.disabled=false;
clearInterval(timer);
count=10;
}
}
timer=setInterval(redown, 500);
}
</script>