It's very important to ensure that the value of hash for a given entity doesn't change if it is to be relied upon, otherwise things like Map or Set will not fun

It's very important to ensure that the value of hash for a given entity doesn't change if it is to be relied upon, otherwise things like Map or Set will not function properly. Therefore, one should be very careful in implementing it yourself and ideally one should use immutable hashable structures provided by the language/stdlib (like tuples or sets in Python). Until Javascript has such structures which should be enough for majority of use cases (there is record and tuple language proposal, but it's in limbo) I think it's unwise to provide a way to sloppily emulate it in user code.