What exactly is a dense array?

Question

The explanation for a dense array that I read from a few pages seem to be in contradiction to one another. I\'d like some help understanding what it is.

While some l

Answer 1

"Dense" is in opposition to "sparse", and generally is used when talking about storage. For example, this array is dense:

a = [undefined, undefined, 2]

It can be stored in memory exactly like that: a sequence of three locations, the first two being undefined, the third being 2.

This array is sparse:

a = []
a[100000000] = 100000000

It is not stored in memory as a sequence of 100000001 locations, as it would be horribly inefficient. It is definitely not 100000000 places of undefined followed by 100000000. Rather, it just says 100000000th one is 100000000, and there is no space allocated to the first 100000000 elements.

(Actually, try to do this with 2 instead of 100000000, and you'll notice a curious thing: Chrome will display the dense array as [undefined, undefined, 2], but the sparse one as [undefined × 2, 2].)

Answer 2

Those articles say you can create an array being dense. This means that, at the time of creation, such arrays are dense, since in arrays like:

var a = new Array("foo", "bar", "baz");
var b = [2, 3, 5];

every element is set: from 0 to length-1, there is no undefined value. Or better said: every position from 0 to length-1 was assigned a value (even if the value is, actually, undefined).

However, you can make those arrays not dense anymore, by doing something like this:

a[20] = "bat";

That array, which was dense, is not dense anymore since the elements 0 1 2 and 20 (unlike elements in 3 to 19) are set to a value (this array has 4 elements, not 21).