Javascript Array Arbitrary Index Memory Allocation

Question

is declaring an array and initializing some arbitrary indexes allocate all the array elements in the memory even the undefined ones?

Example:

var users = [];

function addUser(userID, name, address) {
   if (typeof (users[userID]) === 'undefined')
       users[userID] = new User(userID, name, address)
}

function User (userID, name, address) {
    this.userID = userID;
    this.name = name;
    this.address = address;
}

$(function () {
   addUser(63, 'John', 'VA');
   addUser(5, 'Kate', 'NY');
   addUser(895, 'Yaz', 'DC');
});

So in the above example, will the browser allocate 896 instances of User in the memory (only 3 are defined) or only 3?

Thanks,

What browser are you using? Also, did you intend to have a close parentheses and semicolon at the end of the sample code? — Maz
– Maz, Commented Feb 13, 2014 at 5:30
I'm targeting all the major browsers. I fixed my jQuery syntax error — yazanpro
– yazanpro, Commented Feb 13, 2014 at 5:32
If you do var arr = []; a[1000] = 42; console.dir(a);, then you will see that there is only one element in the array. — Felix Kling
– Felix Kling, Commented Feb 13, 2014 at 5:49

Mulan · Accepted Answer · 2014-02-13 05:37:53Z

1

Nope

JavaScript doesn't care what you put in the array, and it's not going to auto-populate it with values you didn't give it.

If you add 3 users to the array, you will only have 3 users in memory.

The indices in the gaps will just be undefined

var x = [];
// undefined

x[0] = "user1";
// 'user1'

x[3] = "user2";
// 'user2'

x[10] = "user3";
// 'user3'

x;
// ['user1',,,'user2',,,,,,,'user3']

All of that said, you might be better off using an Object ({})

var users = {};

function addUser(userID, name, address) {
  if (!(userID in users)) {
    users[userID] = new User(userID, name, address)
  }
}

You will have an object that looks like this

{"63": [object User], "5": [object User], "895": [object User]}

edited Feb 13, 2014 at 5:37

answered Feb 13, 2014 at 5:32

Mulan

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6 Comments

yazanpro Over a year ago

I couldn't understand the difference. Can you please elaborate more? I can see {} over there, aren't you declaring an array too?

Mulan Over a year ago

@yazanpro, the first example uses an Array the second example uses an Object. The object won't have "gaps" like the array will. I recommend reading up on both on MDN.

Felix Kling Over a year ago

Technically speaking there is no differences between arrays and objects in that case. You say that the gaps are undefined, but that's the same case for objects: non-existing properties are always undefined.

yazanpro Over a year ago

@FelixKling so would you suggest a better approach to achive the purpose?

Felix Kling Over a year ago

@yazanpro: Since the you actually have user IDs, an object feels more appropriate. Objects are typically used as key -> value maps whereas arrays are more of a set of elements. And it looks like you rather want a map here.

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