Undefined values when trying to create a multi-dimensional array

Question

I'm trying to create a multi-dimensional array.

My assumption that the following structure stuff['mykey1']['mykey2']['mykey3'] can be interpreted as stuff is an array of two-dimensional arrays. And stuff['mykey1'] will return me a two dimensional array with following keys ['mykey2']['mykey3']

I try to create this structure like so:

var stuff = null;

if(stuff === null) 
{
    stuff = []; // stuff is []
}

if(stuff[userId] === undefined)
{
    stuff[userId] = [];  // stuff is [undefined, undefined, undefined, 888087 more...]
}

if(stuff[userId][objectId] === undefined)
{
    stuff[userId][objectId] = [];
}

However, when I look at stuff array as I step through, I see that after stuff[userId] = []; stuff array is [undefined, undefined, undefined, 888087 more...]

I'm expecting [888087, []]

Where do the undefined values come from?

When you do `stuff[888087] = []` you're saying `stuff` is an array with a `length` of at least 888088. Are you sure that's what you want? Don't you want objects instead of arrays? — bfavaretto, Dec 18 '12 at 18:29
@bfavaretto: True, I forgot about the auto-sizing of arrays. — voithos, Dec 18 '12 at 18:30

score 4 · Accepted Answer · edited May 23 '17 at 11:58

Where do the undefined values come from?

You are using Arrays, not objects. If you add a numerical property on an Array object, it's length will be updated and the other indices stay unitialized (sparse array), but are displayed as undefined (see What is "undefined x 1" in JavaScript?).

Instead, use normal objects, where numerical properties have no special behavior:

var stuff = null;

if(stuff === null) 
{
    stuff = {}; // stuff is an empty object
}

if(stuff[userId] === undefined)
{
    stuff[userId] = {};  // stuff is now enriched with one property
}

if(stuff[userId][objectId] === undefined)
{
    stuff[userId][objectId] = {}; // or maybe you really want an array here?
}

score 0 · Answer 2 · answered Dec 18 '12 at 18:32

0

Its because of usage of arrays. The length of the remainin elements is made undefined. For example if a(1) is specified, a(0) will be undefined

answered Dec 18 '12 at 18:32

LPD

2,833
2
29
48

score 0 · Answer 3 · answered Dec 18 '12 at 18:48

You are trying to create associative arrays, and in JavaScript this is done with... objects, not arrays!

So at each step you need to use {} instead of [] to create the next level. And you need to use a for...in loop to iterate through the keys.

For more details, search the Web for JavaScript associative arrays". For example:

https://developer.mozilla.org/en-US/docs/JavaScript/Guide/Working_with_Objects

score 0 · Answer 4 · answered Sep 16 '15 at 20:09

The question is long answered, but I want to chip in with this shorthand, that really makes it more compact and readable:

stuff = stuff || {};

// if stuff is already defined, just leave it be. If not (||), define it as object

stuff[userId] = stuff[userId] || {};

// if stuff[userId] is already defined, define it as self (let it be unchanged). If not defined ( the || -signs ), define it as object.

stuff[userId][objectId] = stuff[userId][objectId] || {};

// and so on :)

Undefined values when trying to create a multi-dimensional array

4 Answers4