jQuery 实现验证码倒计时

jQuery 版本==jquery-1.9.1.min.js
实现代码如下：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<script src="jquery-1.9.1.min.js"></script>
<script type="text/javascript"> 
let countdown=60;
   $("#btn").click(function () {
       let obj = $(this);
       settime(obj);
   });

    function settime(obj) { //发送验证码倒计时
    if (countdown === 0) {
        obj.attr('disabled',false);
        //obj.removeattr("disabled");
        obj.text("免费获取验证码");
        countdown = 60;
        return;
    } else {
        obj.attr('disabled',true);
        obj.text("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" /> 
  
</body>
</html>