Alright, I've been in the industry a very long time but moved to backend when it got complex. I am wondering why you need a decent hash map or set implementation. This is supposed to be done on the backend. The frontend is for presentation. Your backend should not give you elements with duplicates (why you don't need set). "Decent hash map" sounds like you just don't like the hash map implementation because you are trying to do something complex with the Map implementation, in which case it belongs on the backend.