一个带倒计时按钮的代码片段

<div>
                        <input type="submit" value="免费获取验证码" id="btn" onclick="settime(this)" disabled="disabled">
                    </div>

var countdown = 60;

            function settime(val) {
                if(countdown == 0) {
                    val.removeAttribute("disabled");
                    val.value = "免费获取验证码";
                    countdown = 60;
                    return;
                } else {
                    val.setAttribute("disabled", true);
                    val.value = "重新发送(" + countdown + ")"+"S";
                    countdown--;
                }
                setTimeout(function() {
                    settime(val)
                }, 1000)
            }