60秒倒计时

<input type="button" id="btn" value="点击获取验证码"/>
<script>
var btn = document.getElementById("btn");
var num = 60;
btn.onclick = function settime(){
if (num == 0){
btn.removeAttribute('disabled');
btn.value('点击获取验证码');
num = 60;
return;
}else{
btn.setAttribute('disabled',true);
btn.value = "重新获取" + num;
num--
}
setTimeout(function(){
settime()
},1000)
}


</script>