Not necessarily. Map/Set is a counterexample. There’s no substitute for the JIT knowing how to reason about the internal aliasing properties of your hash table implementation.

We’ll keep working on more optimizations for ES-6 so there will be more counter examples.

To be clear, do you mean that Map/Set is faster than {} even for plain idiomatic hashes with string keys (as opposed to pathological cases)?

When I last tested in other JS engines I found plain {} hashes were still moderately faster. (Haven't tried Safari since it doesn't support what I've been working on)

No, I don't mean that. If you're using string or integer keys, then I think that {} still have a perf advantage.

But ES6 Map/Set is almost useless as it has no custom hash nor equality function. The key is either numeric, string, or object identity, not anyway better than the builtin {}.

I've found them to be useful when I write ES6 code. I agree that custom hash and equality functions would make these classes a lot better.

I agree too. I wonder if this has been discussed at TC39.