实现倒计时效果获取验证码

    <form id="form1" runat="server">  
       <div class="input">  
           <input type="button" id="btn" class="btn_mfyzm" value="免费获取验证码" />  
           <script type="text/javascript">  
               var wait = 60;  
               document.getElementById("btn").disabled = false;  
               function time(o) {  
                   if (wait == 0) {  
                       o.removeAttribute("disabled");  
                       //$(o).attr("disabled", false);  jquery  
                       o.value = "免费获取验证码";  
                       wait = 60;  
                   } else {  
                       o.setAttribute("disabled", true);  
                       o.value = wait + "秒后,重新获取验证码";  
                       wait--;  
                       setTimeout(function () {  
                           time(o)  
                       },  
               1000)  
                   }  
               }  
               document.getElementById("btn").onclick =  
          function () {  
              time(this);  
          }  
           </script>  
       </div>  
       </form>