Hidden class optimizations just make JavaScript objects behave a little more like Java class instances, where the VM knows where to find each field, rather than having to look it up like a map.
replyIt doesn't make JS faster than Java, it makes it almost as fast in some cases.
I can only say what I observed in testing, and that's that having millions of instances of a class like Point3D{x, y, z} in JS uses significantly less memory than in Java (this was tested on Android, not sure if relevant). It was quite some time ago so I don't remember the details.
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