Accessing Array beyond its size in javascript

Question

I read in a particular book that an array in JavaScript can hold 4,294,967,295 items and would throw exception if the number reaches beyond that. I tried out the functionality using the following code:

var a = ["a","b","c"];
a[4294967300] = "d";
console.log(a[4294967300]);

It shows the output "d" and no exception or error. Am I missing something here? Can someone put some light on the topic and share some knowledge regarding max array items in JavaScript and various scenarios related to it?

Denys Séguret · Accepted Answer · 2014-05-26T09:28:41.633

7

An array doesn't have to hold all the items from 0 to N to contain one with index N.

That's because arrays in JavaScript engines can switch to a dictionnary mode when the holes are too big, those arrays are called sparse arrays (vs dense arrays).

It's important to know this distinction because the implementation is leaking on one point : performance. You should read this on this topic : http://www.html5rocks.com/en/tutorials/speed/v8/

But regarding indexes starting at 2³², sebcap26 is right, there's a distinction due to the fact the index is handled as a string. This distinction is important and can be verified by logging a.length : you'll see the length isn't impacted by such an element. There's no exception or error per se but it makes it impossible to use normal array operations like iterating up to the length or using array functions like map or filter (the elements with index greater than the numeric index limit are ignored by those functions).

edited May 26 '14 at 09:28

answered May 26 '14 at 09:11

Denys Séguret

372,613
87
782
758

So you mean to say index is not relevant, but the size is? – Kazekage Gaara May 26 '14 at 09:12
I mean the internal representation in all JS engines makes it possible to hold an element with index 4294967300 without allocating space for all elements with a smaller index. – Denys Séguret May 26 '14 at 09:13
Can you please elaborate on this issue? – Yair Nevet May 26 '14 at 09:13

Sebastien C. · Answer 2 · 2014-05-26T09:20:26.833

6

If I understand well the ECMAScript specifications, an index which is not in [0 .. 2^32-1] is converted into a String and used as an Object key, not as an Array index.

A property name P (in the form of a String value) is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 2^32−1.

edited May 26 '14 at 09:20

answered May 26 '14 at 09:14

Sebastien C.

4,649
1
21
32

So all normal operations on that element work fine if Object key is used in place of Array index ? – tejas_spy007 May 26 '14 at 09:21
@tejas_spy007 No, all operations don't work fine. To start with you won't iterate the same way and you won't have a correct array length (see my updated answer). – Denys Séguret May 26 '14 at 09:25

score 2 · Answer 3 · edited May 23 '17 at 12:05

Try running this code: fiddle : http://jsfiddle.net/vXtfE/

var a = ["a","b","c"];
a[4294967300] = "d";
console.log(a.length);
console.log(a);
console.log(a[4294967300]);

You will see this output:

3 
["a", "b", "c", 4294967300: "d"] 
d

The initial items get stored as array elements, but for large index, the storage changes to hash based sparse array. Hence, it is a mix of both in your case.

Good explanation of this :

Why is array.push sometimes faster than array[n] = value?

Accessing Array beyond its size in javascript

3 Answers3