That's just how it optimizes their usage under-the-hood, but as far as JS is concerned they are all hash maps.

For example, try this:

    var foo = [1,2,3];  // or `foo = new Array(1,2,3)` also works
    foo['bar'] = 'test';
    console.log(foo);

and you get:

    0:1
    1:2
    2:3
    bar:"test"