jquery 手机短信倒计时

手机短信验证码倒计时,必须用button或者用input的type='button'

            var countdown=60;
            function settime(obj) {
                if (countdown == 0) {
                    obj.removeAttribute("disabled");
                    obj.innerHTML="免费获取验证码";
                    countdown = 60;
                    return;
                } else {
                    obj.setAttribute("disabled", true);
                    obj.innerHTML="重新发送(" + countdown + ")";
                    countdown--;
                }
            setTimeout(function() {
                settime(obj) }
                ,1000)
            }
            $('#btn').on('click',function(e){
                settime(this);
            })

例如

<button type="button" id="btn">免费获取验证码</button>