What exactly is the use case for `Set.prototype.map` returning a set?

[dead-claudia]

I'm having a tough time figuring out what a set.map(func) -> set is for. I could see value in a set.map(func) -> iterator inherited from an %IterablePrototype% as it makes no sense to special case it there, but I'm not sure how useful a standalone, deduplicating set.map(func) -> set would be.

[zloirock]

I need set.map(func) -> set too often and I don't remember any use case of set.map(func) -> iterator. If you need it, use set.values().map(func) -> iterator from this proposal.

[ljharb]

Virtually every use case for mapping over an array applies to a Set, and vice versa.

If you’re asking about whether it should return an iterator instead of reifying a new Set, that question applies to every method in this proposal, and the answer is “methods on X typically produce a new X”. The iterator helpers proposal seems like it’d be what you want.

[dead-claudia]

@ljharb @zloirock

I'm familiar with the iterator helpers proposal, and that's precisely why I was asking what the use case for set.map was. I've not encountered a single scenario, either in my own code or anyone else's I've read, where this was actually useful.

[@ljharb:] Virtually every use case for mapping over an array applies to a Set, and vice versa.

I can see this in the abstract, but what does this mean concretely?

[@zloirock:] I need set.map(func) -> set too often

Could you give me examples of when you need this?

[zloirock]

For example, I need to do something with a collection (for example, extract property from an object), store the result and use .has method with O(1) complexity on it. At least 3 cases for 2 previous days.

[ljharb]

• I have a Set of HTML class names, and I want to map them to a Set of HTML elements.
• I have a Set of objects with a "role" property, and I want to map them to a Set of roles.
• I have a Set of thunks, and I want to map them to a set of values.

[dead-claudia]

So the new Set(set.values().map(...)) (borrowing from the iterator helpers) is common enough to warrant inclusion? If so, I'll go ahead and close this issue.

[ljharb]

Even if we had iterator helpers first, imo we'd still want .map on arrays - the existence of a generic solution doesn't preclude the usefulness of a specific one.