js 验证码 倒计时60秒

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