表似乎是空的

Question

我有一个json文件：

[
    {"cat1":"A1","cat2":"B1","cat3":10},
    {"cat1":"A2","cat2":"B2","cat3":20},
    {"cat1":"A3","cat2":"B3","cat3":30}
]

我需要2个数组内容：

["A1","A2","A3"]
[10,20,30]

我的脚本应该做的工作：

var tab1 = [];
var tab2 = [];
var tab3 = [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18];

$.getJSON("myjsonfile.json", function(json) {
    $.each( json, function( key, value ) {
        tab1[key] = value["cat1"];
        tab2.push(value["cat1"]);
    });
});

但事实并非如此。

console.log(tab1[0]); // undefined
console.log(tab2[0]); // undefined
console.log(tab3[0]); // 0

我不明白为什么