js实现短信验证码读秒功能

<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" />