实现发送短信验证码后60秒倒计时

<! DOCTYPE html>
< html >
< head >
< script src = "http://libs.baidu.com/jquery/1.10.2/jquery.min.js" ></ script >
< script type = "text/javascript" >
var countdown=60;
function settime(obj) {
    if (countdown == 0) {
        obj.removeAttribute("disabled");
        obj.value="免费获取验证码";
        countdown = 60;
        return;
    } else {
        obj.setAttribute("disabled", true);
        obj.value="重新发送(" + countdown + ")";
        countdown--;
    }
setTimeout(function() {
    settime(obj) }
    ,1000)
}

</ script >
< body >
< input type = "button" id = "btn" value = "免费获取验证码" onclick = "settime(this)" />

</ body >
</ html >