js implements SMS verification code countdown function

<script type="text/javascript">   
  var countdown=60;   
  function settime(obj) {   
      if (countdown == 0) {   
          obj.removeAttribute("disabled");      
          obj.value="免费获取验证码";   
          countdown = 60;   
          return;  
      } else {   
          obj.setAttribute("disabled", true);   
          obj.value="重新发送(" + countdown + ")";   
          countdown--;   
      }   
   setTimeout(function() { 
   settime(obj) }   
   ,1000);  
    
  }  
  function dd(){
   var obj = document.getElementById("test1");
   obj.innerHTML="";
   var nullOp = document.createElement("option");
   nullOp.setAttribute("value", "");
   nullOp.appendChild(document.createTextNode("请选择"));
   obj.appendChild(nullOp);
   for ( var int = 0; int < 1; int++) {
    var op = document.createElement("option");
    op.setAttribute("value", "123");
    op.appendChild(document.createTextNode("123"));
    obj.appendChild(op);
   }
  });
 </script>
html部分
<input type="button" id="btn" value="免费获取验证码" onclick="settime(this)" /> 
<input type="text" id="dd" onclick="dd()" value="sdf" />