JS arrays 3 deep getting lost

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Learing JS by game. and of course the simplist is a clicker :-) OK, so I have had lots of help with this, but the array section is just not sinking in. JS arrays do not work like arrays I am used to. two part question: 1. Why does the following code keep saying weaponlevelIfo[0] undefined, when it is defined? Please explain your answer, don't just correct mine LOL 2. I am more interesting in populating the code at runtime

As stated, all the research I am coming across as well as videos, talk about static data, i.e. it is put in at programing level, not run time.

I have had a really patient community person that has tried to help me understand JS arrays, but I must be blind as I am not seeing it. I can do in in other language just fine. but JS? nope.

// produces weaponLevelNfo[weaponId][level][cost] and [goldperclick]
// weapon, level 1-9, cost/goldperclick on each level
var weaponLevelNfo = new Array(14); // Outter array comprised of weapons 0-14
function initGame() {

    for (let i=0; i <= weaponLevelNfo.length; i++) {

        weaponLevelNfo[i] = new Array(9); // create leves array under weaponid array
        for (let j = 0; j < weaponLevelNfo[i].length; j++) {    
            // loop through each 9 levels changing as needed
            weaponLevelNfo[i][j] = new Array(2); // create an object for readability


initGame();// added - forgot to add this in the original post (sorry)
// always gives me weaponLevelNfo[0] not defined

I prefer the results to be as such


but am quite happy with


But as you can see from the code, assigning them direct or at runtime, I get the not defined error

What is missing to allow me to assign these at run time?

3 Answers

trincot On Best Solutions

Two issues:

  • You need to call initGame to create all those subarrays, otherwise weaponLevelNfo[0] is not defined and so weaponLevelNfo[0][0] will trigger the error you get.

  • Your outer loop performs one iteration too many (<=). Change:

    for (let i=0; i <= weaponLevelNfo.length; i++) {


    for (let i=0; i < weaponLevelNfo.length; i++) {

    Without that change, the last iteration is actually adding an element to the array in slot i, and so the length of the array increases... the loop becomes infinit.

Note that there are shorter ways to define such a nested array. For instance:

var weaponLevelNfo = Array.from({length:14}, () => Array.from({length:9}, () => [0, 0]));
kristech On

When you create an array in javascript with new Array(2) this is an array with two positions. In this example (new Array(2)), you can access it at index 0 and index 1. Consider the following:

var newArr = new Array(2);

You can then access the two positions by the following:

var position1 = newArr[0];
var position2 = newArr[1];

So when you try this:

var position = newArr[2];

This will throw an undefined exception.

You can change the end of your example code to this:

Kamil Kiełczewski On

You define array with two elements Array(2) with indexes 0 and 1 but you use index 2.

To initialize multidimensional array filled by zeros (if not remove .fill(0)) use


let a=[...Array(14)].map(x=>[...Array(9)].map(y=>Array(2).fill(0)));


console.log('Last element value:',a[13][8][1]);