js手机验证码倒计时

<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>