How can I assert the contents of a function in conjunction with `ReturnType<T>`? | TypeScript Community | Page 1

sweet spruce Jul 4, 2023, 7:34 AM

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idle trailBOT Jul 4, 2023, 7:34 AM

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giraffeunleashed#0

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Preview:```ts
type F = {
k: <K>(arg: K) => K
}

type Test<O extends Record<any, any>> = {
[Key in keyof O]: O[Key] extends (
arg: number
) => number
? ReturnType<O[Key]>
: null
}

type Res = Test<F>```

coarse nimbus Jul 4, 2023, 7:37 AM

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The type of K isn't determined until you execute the function, so it can't really reason about the the types

sweet spruce Jul 4, 2023, 7:37 AM

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warped sandalBOT Jul 4, 2023, 7:37 AM

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u.mergeDocument(true)

coarse nimbus Jul 4, 2023, 7:37 AM

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If you specify the type on F then it can infer the types:


type F<K> = {
    k: (arg: K) => K
}

type Test<O extends Record<any, any>> = {
    [Key in keyof O]: O[Key] extends (arg: number) => number ? ReturnType<O[Key]> : null
}

type Res1 = Test<F<string>>
type Res2 = Test<F<number>>

sweet spruce Jul 4, 2023, 7:41 AM

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coarse nimbus If you specify the type on `F` then it can infer the types: ```ts type F<K> = ...

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coarse nimbus Jul 4, 2023, 7:44 AM

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Correct, because the type belongs to the function, so it means the type is only known when the function is called

sweet spruce Jul 4, 2023, 7:55 AM

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warped sandalBOT Jul 4, 2023, 7:55 AM

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const entitites = createServicesFramework({

sweet spruce Jul 4, 2023, 4:58 PM

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#How can I assert the contents of a function in conjunction with `ReturnType<T>`?