Hashtable

The table behind Map and Set: a hash map for lookup plus a doubly linked list for insertion order.

The linked list is the reason this is not a LinkedHashMap. JavaScript lets a script add to, delete from or clear a collection while an iterator is walking it, and the iterator has to keep going from where it was. A deleted node keeps its next pointer so any iterator sitting on it can still walk forward; only the prev pointers are unlinked, so new iterators never see it.