js 倒计时 倒计时60秒

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