[asonge]

Just to be clear, Javascript's arrays will, depending on the implementation, dynamically switch between sparse and compact implementations. A sparse javascript array literally behaves like a javascript object or a map with an integer index. Ruby and Python's arrays will grow as long as the allocator can alloc more memory onto the end, but has to copy the entire array if there's memory fragmentation...it's not quite the same as a C array.

Because of the immutable data, your best hopes are actually vectors or maps.

Btw, the erlang library you mention implements arrays on top of a tree of 10-ary(-ish) tuples. It'd need to be measured, but I'd be willing to bet that the native maps are faster at access, insert, and delete. The maps are implemented in C, and worst case for all operations would be O(n log n) or similar.