each遍历数组前端出来undefined，是什么原因？

 var categories = [];
    var datas1 = [];
    var datas2 = [];
    var datas3 = [];

    //Ajax 获取数据并解析创建Highcharts图表
    function queryData(index) {
        $.ajax({
            url : 'servlet/HighCharts',
            type : 'get',
            dataType : "json",
            success : function(data) {

                $.each(data, function(i, n) {
                    alert(n["ym_datetime"]);
                    categories[i] = n["ym_datetime"];
                    datas1[i] = n["electric_A"];
                    datas2[i] = n["electric_B"];
                    datas3[i] = n["electric_C"];
                });


                options.series[0].data = datas1;
                options.series[1].data = datas2;
                options.xAxis.categories = categories;
                options.series[2].data = datas3;

                chart = new Highcharts.Chart(options);
            }
        });
    }

你先把data在成功函数中输出一下看看结构。。，这么吧代码贴出来没办法知道原因哎