well. it could be a hash table, but then it's even worse (for big n-s)
as with hash table and big n-s there will be hash collisions, so it needs to do a find in the collision list
best case would be a balanced tree
Hmm, I wonder at what n you start to get enough collisions for it to matter in practice. I guess it is up to the implementation, and up to the browsers to create a good heuristic.
When I have used SpiderMonkey (and node.js to some extent) solve problems on kattis, I have never run into a problem of object lookup being a bottleneck. Maybe in some special case?
in practice it never causes problem
i suspect modern js engines are able to switch implementation based on the object "shape"
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