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>