JS验证码倒计时

<input type="button" id="btn" value="免费获取验证码" />  <script type="text/javascript">  var wait=60;  function time(o) {      if (wait == 0) {         o.removeAttribute("disabled");                   o.value="免费获取验证码";         wait = 60;         } else {            o.setAttribute("disabled", true);            o.value="重新发送(" + wait + ")";            wait--;            setTimeout(function() {                 time(o)            },             1000)       }     }  document.getElementById("btn").onclick=function(){time(this);}  </script>