es6 Map and Set complexity, v8 implementation

For people don't want to dig into the rabbit hole too deep:

1: We can assume good hash table implementations have practically O(1) time complexity.
2: Here is a blog posted by V8 team explains how some memory optimization was done on its hashtable implementation for Map, Set, WeakSet, and WeakMap: Optimizing hash tables: hiding the hash code

Based on 1 and 2: V8's Set and Map's get & set & add & has time complexity practically is O(1).

Is it a fair assumption that in v8 implementation retrieval / lookup is O(1)?

Yes. V8 uses a variant of hash tables that generally have O(1) complexity for these operations.

For details, you might want to have a look at https://codereview.chromium.org/220293002/ where OrderedHashTable is implemented based on https://wiki.mozilla.org/User:Jorend/Deterministic_hash_tables.