实现发送短信验证码后60秒倒计时

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<!DOCTYPE html>
<html>
<head>
<script src="http://libs.baidu.com/jquery/1.10.2/jquery.min.js"></script>
<script type="text/javascript"> 
var countdown=60; 
function settime(obj) { 
	if (countdown == 0) { 
		obj.removeAttribute("disabled");    
		obj.value="免费获取验证码"; 
		countdown = 60; 
		return;
	} else { 
		obj.setAttribute("disabled", true); 
		obj.value="重新发送(" + countdown + ")"; 
		countdown--; 
	} 
setTimeout(function() { 
	settime(obj) }
	,1000) 
}

</script>
<body> 
<input type="button" id="btn" value="免费获取验证码" onclick="settime(this)" /> 

</body>
</html>