js短信验证码读秒

js部分

<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" />