what is a true multi dimensional array?

Question

I was reading a book about Javascript and saw this line;

JavaScript does not support true multidimensional arrays, but you can approximate them with arrays of arrays.

What's the difference?

Answer 1

A true multi-dimensional array must be indexed with multiple indices. An array of arrays can be indexed with a single index, which will return another array. A true multi-dimensional array stores all its data contiguously. An array of arrays stores all its constituent arrays arbitrarily scattered around. This can improve iteration performance because of cache effects for true arrays.

Answer 2

(a visual explanation that complements an excellent answer of @recursive)

In some languages (C#) there are both. The difference is in "shape" of such arrays.

int[3, 4] // true two-dimensional array

// it will "look" like this, rectangular shape
[[0, 0, 0, 0]
 [0, 0, 0, 0]
 [0, 0, 0, 0]]

But when you define an array of arrays, it can easily (especially in javascript) look like this. It's called a jagged array.

[[0, 0]
 [0, 0, 0, 0, 0, 0]
 [0, 0, 0]]