原生js 按钮点击 倒计时

var countdown=5;
    function settime(val) {
      if (countdown == 0) {
        val.removeAttribute("disabled");
        val.value="获取验证码";
        countdown=5;
      } else {
        val.setAttribute("disabled", true);
        val.value="重新发送(" + countdown + ")";
        countdown--;
        setTimeout(function() {
          settime(val)
        },1000)
      }
    }

效果如下图：