> Invoking Array(length) or new Array(length) (with a number argument) creates a fully sparse array:

In my experience the common javascript engines are so accelerated that in practice, before anything happens to the array it is uninitialised and the engine uses the length parameter as a future hint. If the array is promptly filled contiguously it will perform as a dense array. The types which the array is filled with and subsequently maintains can have a greater impact on performance. An array that only contains integers performs faster than one containing doubles or strings, and once types are changed or mixed in the array it slows down, generally by a factor of 2 or 3.