How does babel implement Set / Map polyfills

Question

My question is related to computational complexity of Set / Map, Weak Set / Weak Map polyfills by Babel? Afaik there are no ES5 language features allowing to implement Set / Map directly, and so it might happen that Set / Map might use Array structure under the hood to implement lookup by object reference which would yield to O(N) lookup performance. And so the question is:

What is the computational complexity of Set / Map lookup operations?

Thank you in advance!

Answer 1

Babel uses core-js for its polyfill, from the GitHub repo:

core-js uses native collections in most case, just fixes methods / constructor, if it's required, and in old environment uses fast polyfill (O(1) lookup).

_{(Emphasis mine)}

And if you're interested in the exact lookup, it's in this file. It's not backed by an array.