jq倒计时

            var step = 59;
            $('#btn').val('重新发送60');
            var _res = setInterval(function()
            {   
                $("#btn").attr("disabled", true);//设置disabled属性
                $('#btn').val('重新发送'+step);
                step-=1;
                if(step <= 0){
                $("#btn").removeAttr("disabled"); //移除disabled属性
                $('#btn').val('获取验证码');
                clearInterval(_res);//清除setInterval
                }
            },1000);