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Mapped types doesn't map the key value for methods when mapping over a generic instantiated with an array #58694
Closed as not planned
Description
🔎 Search Terms
mapped type array tuple prototype object
🕗 Version & Regression Information
- This is the behavior in every version I tried
⏯ Playground Link
💻 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
v1has typebooleanv2has 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.
Activity
nicolo-ribaudo commentedon May 29, 2024
This only happens if I'm mapping over a generic type argument. If I inline it it works properly:
https://www.typescriptlang.org/play/?#code/C4TwDgpgBAggTnAhiA0hEBnAQgV2FgewIBsJEA7ANUWJwgygF4oBvKAbRSgEtyoBrdAQBmsBMgA8GYHF4BzAHwBdAFxQARkVIUoAXwBQ+0sCgA3AExr4SVOmx5CJMlRp0M7AETCCcAKKIAYwALDyUmKBk6AG59AHpYqAA9AH4NLWd9IA
and
https://www.typescriptlang.org/play/?#code/C4TwDgpgBAcg9sAKlAvFAggJ0wQxAHgGdhMBLAOwHMA+AbgChRIoBlHAWwgGkIRCAhAK7B+cOABsIOcgDUc4wREL5E1VFADeUANpcoFKAGtecAGawEiALoAuKACMxk6VAC+9epOBQAbgCY7Nk4ePiERJylZeUVlLFwCYjIqamptACJTOEwAURwAYwALNKt1EkUGAHoKqAA9AH56IA
[-]Mapped types doesn't map the key value for methods when mapping over an array[/-][+]Mapped types doesn't map the key value for methods when mapping over a generic instantiated with an array[/+]jcalz commentedon May 29, 2024
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 commentedon May 29, 2024
Then is the bug the behavior I see in #58694 (comment)?
jcalz commentedon May 29, 2024
#27995, yes.
nicolo-ribaudo commentedon May 29, 2024
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:
I then relaized that this is better: