如何在JavaScript中创建多维数组？

Question

How can I create a multidimensional array in JavaScript? I have:

var m = 4;
for (var i = 0; i < m; i++){
    groupsData.name_of_bar = [];
    groupsData.name_of_bar[i]['a'] = data[i].a;
    groupsData.name_of_bar[i]['ab'] = data[i].ab;
    groupsData.name_of_bar[i]['de'] = data[i].de;
    groupsData.name_of_bar[i]['gh'] = data[i].gh;
    groupsData.name_of_bar[i]['xy'] = data[i].xy;
}

If I do:

groupsData.name_of_bar[0]

I get errors:

TypeError: Cannot read property '0' of undefined

TypeError: Cannot set property 'a' of undefined

What am I doing wrong?

Answer 1

JavaScript doesn't support multidimensional arrays per se. The closest you can come is to create an array where the values in it are also arrays.

// Set this **outside** the loop so you don't overwrite it each time you go around the loop
groupsData.name_of_bar = [];

for (var i = 0; i < m; i++){
    // Create a new "array" each time you go around the loop
    // Use objects, not arrays, when you have named properties (instead of ordered numeric ones)
    groupsData.name_of_bar[i] = {};

    groupsData.name_of_bar[i]['a'] = data[i].a;
    groupsData.name_of_bar[i]['ab'] = data[i].ab;
    groupsData.name_of_bar[i]['de'] = data[i].de;
    groupsData.name_of_bar[i]['gh'] = data[i].gh;
    groupsData.name_of_bar[i]['xy'] = data[i].xy;
}

Answer 2

The way you are declaring your objects are a little off. It looks like you are attempting to create an array of objects.

var groupsData = {name_of_bar: []}, 
    m = 4, 
    i = 0;
for(; i < m; i++) {
    groupsData.name_of_bar.push({
        a: data[i].a, 
        ab: data[i].ab, 
        de: data[i].de, 
        gh: data[i].gh, 
        xy = data[i].xy
    });
}

Answer 3

Each iteration through the loop, you are doing groupsData.name_of_bar = []; . This removes whatever else is already in there and replaces it with a blank array.

Also, when you do groupsData.name_of_bar[i]['a'] , you need to create groupsData.name_of_bar[i] first.

A way to do this is:

groupsData.name_of_bar = [];

var m = 4;
for (var i = 0; i < m; i++){
    groupsData.name_of_bar.push({
        a: data[i].a,
        ab: data[i].ab,
        ab: data[i].ab,
        de: data[i].de,
        gh: data[i].gh,
        xy: data[i].xy,
    });
}

Note that in JavaScript, arrays can only be numerically indexed. If you want string indexes, you need to use an object.

Also, if there are no other values in data[i] , then you can simplify this even further by doing:

groupsData.name_of_bar = [];

var m = 4;
for (var i = 0; i < m; i++){
    groupsData.name_of_bar.push(data[i]);
}

Heck, why not just use groupsData.name_of_bar = data; and lose the loop altogether?