点击button 验证倒计时

 <script type="text/javascript">
        var countdown = 60;
        function settime(val) {
            if (countdown == 0) {
                val.removeAttribute("disabled");
                val.value = "免费获取验证码";
                countdown = 60;
            }
            else {
                val.setAttribute("disabled", true);
                val.value = "重新发送(" + countdown + ")";
                countdown--;
            }
            setTimeout(function(){
                settime(val)
            },1000)
        }
         </script>

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