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Key in mapped variadic tuple should not be literal after the variadic part #48856
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I don't think this is a bug. Consider:
type Box<T> = { value: T }; type Boxified<T> = { [P in keyof T]: Box<T[P]> }; type T0 = Boxified<[number, string, ...boolean[]]>; // [Box<number>, Box<string>, ...Box<boolean>[]]
With the proposed change,
Pwould benumberinstead of"2"for the rest element, which means the resulting rest element would be...Box<string | number | boolean>[]instead of just...Box<boolean[]>. Not only is that less correct, it is a breaking change. Effectively,numberis a less precise way of expressing "element at index 2 or above", whereas"2"gets much closer. This speaks to the fact that in homomorphic mappings over array and tuple types, the key types are just exemplars and not an exact representation of some specific property.I also thought about that. The problem is it's really hard to be correct for both the key type itself, and for homomorphic mapped types to continue be homomorphic. However, accessing
T[P]andPdirectly should be two orthogonal actions. In my actual case, I'm doingEntriesinstead ofKeys:export type Entries<O extends unknown[]> = { [K in keyof O]: [K, O[K]] };
In an ideal world, I'd like
Entries<[number, string, ...boolean[]]>to be[["0", number], ["1", string], ...[number, boolean][]]. Does that sound reasonable?If we have subtracted types, the index should rather by
Exclude<number, 0 | 1>, but since we are not there yet, I'd be more happy withnumberthan2as the index type.This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.
Joshua Chen (@Josh-Cena) FWIW, I now agree with your observations and suggested changes. See #57031. Better late than never! 😄
Reacted by Mateusz Burzyński, Andrii Dieiev, Joshua Chen and andrew jarrettReacted by Joshua Chen and andrew jarrett- locked as resolved and limited conversation to collaborators
on Oct 22, 2025
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Variadic tuple mapped type keyof
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🙁 Actual behavior
AandBboth have the variadic part typed as..."1"[], which is wrong.Bis further wrong because thenumberpart becomes"2", while in practice it would rarely be"2".🙂 Expected behavior
Ashould be["0", ...number[]];Bshould be["0", ...number[], number]