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>