js实现免费获取手机验证码倒计时效果

<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=5;
document.getElementById("btn").disabled = false;  
function time(o) {
        if (wait == 0) {
            o.removeAttribute("disabled");          
            o.value="免费获取验证码";
            wait = 5;
        } 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>
</div>