I would certainly say "no" to having your basic array type be a hashmap as a hashmap is several orders of magnitude slower than an array for all operations on small arrays and for access for all sizes, although there is nothing wrong with also having hashmap and linked list collections if you need O(1) inserts.

However, what I was referring to is much worse than that:

JavaScript arrays are defined as a normal hashmap with string keys with hooks on assignment to parse the key as a uint32 and update the length property if the value is larger than the current one, and on length assignment iterate over all keys and delete those that successfully parse as a uint32 and has a numeric value larger than the one assigned to length.