js 倒计时 倒计时60秒

<input type="button" id="btn" value="免费获取验证码" onclick="settime(this)" />
<script type="text/javascript">
var countdown=60;
function settime(val) {
if (countdown == 0) {
val.removeAttribute("disabled");
val.value="免费获取验证码";
countdown = 60; 
return;                                      ///////////////////////注意加return;
} else {
val.setAttribute("disabled", true);
val.value="重新发送(" + countdown + ")";
countdown--;
}
setTimeout(function() {
settime(val)
},1000)
}
</script>