Mapped types doesn't map the key value for methods when mapping over a generic instantiated with an array

[nicolo-ribaudo]

🔎 Search Terms

mapped type array tuple prototype object

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAyghgWwgaQiAzgIQK7Ewe3wBsI4A7ANTiOwnQB4AVAPigF4oBvKAbWSgCWZKAGs0+AGZRGAXQBcUAEaES5KAF8AUJpLAoANwCMC+ElQYceFaUrVaDGBGD10wAE5CA5s2Y8ARBL4bgCicADGABZ+MuxQ7rQA3JoA9MlQAHoA-DpOBgBMJogoaFi4BMQ2VDR09ACCbm5wIC7uXj7+gSHhUTEc8RBJqRnZQA

💻 Code

type SameKeysButBooleanValues<T> = { [K in keyof T]: boolean }

let v1: SameKeysButBooleanValues<Set<string>>["forEach"] = true;
// ^?
let v2: SameKeysButBooleanValues<Array<string>>["forEach"] = true;
// ^?

🙁 Actual behavior

• v1 has type boolean
• v2 has type (callbackfn: (value: boolean, index: number, array: boolean[]) => void, thisArg?: any) => void

🙂 Expected behavior

v2 has type boolean, same as v1

Additional information about the issue

This looks closely related to #27995, but I believe that it is different. My issue is not about the set of keys being mapped, but about some of them not being mapped but just copied as-is from the original type.

[nicolo-ribaudo]

This only happens if I'm mapping over a generic type argument. If I inline it it works properly:

type ArrayKeysButBooleanValues = { [K in keyof Array<string>]: boolean }

let v2: ArrayKeysButBooleanValues["forEach"] = true;
// ^? boolean

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAggTnAhiA0hEBnAQgV2FgewIBsJEA7ANUWJwgygF4oBvKAbRSgEtyoBrdAQBmsBMgA8GYHF4BzAHwBdAFxQARkVIUoAXwBQ+0sCgA3AExr4SVOmx5CJMlRp0M7AETCCcAKKIAYwALDyUmKBk6AG59AHpYqAA9AH4NLWd9IA

and

type NotT = Array<string>;
type SameKeysButBooleanValues<T> = { [K in keyof NotT]: boolean }

let v2: SameKeysButBooleanValues<Array<string>>["forEach"] = true;
// ^? boolean

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAcg9sAKlAvFAggJ0wQxAHgGdhMBLAOwHMA+AbgChRIoBlHAWwgGkIRCAhAK7B+cOABsIOcgDUc4wREL5E1VFADeUANpcoFKAGtecAGawEiALoAuKACMxk6VAC+9epOBQAbgCY7Nk4ePiERJylZeUVlLFwCYjIqamptACJTOEwAURwAYwALNKt1EkUGAHoKqAA9AH56IA

[jcalz]

Generic mapped array/tuples are not supposed to map over all the keys. It’s in the TS3.1 release notes. TS is behaving as intended; it’s not a bug.

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-1.html#mapped-types-on-tuples-and-arrays

[nicolo-ribaudo]

Then is the bug the behavior I see in #58694 (comment)?

[jcalz]

#27995, yes.

[nicolo-ribaudo]

Oh, closing as a duplicate of #27995 then :)

Btw, my original use case was that I was trying to convert an array type into an array-like (i.e. same type, but without the array methods). I tried something like this but it obviously didn't work:

type ToArrayLike<T extends unknown[]> = {
  [K in keyof T]: K extends `${bigint}` ? T[K] : never
}

let v!: ToArrayLike<["a", "b", "c"]>;

I then relaized that this is better:

type ToArrayLike<T extends unknown[]> = {
  [K in (keyof T) & `${bigint}`]: T[K]
}

let v!: ToArrayLike<["a", "b", "c"]>;