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Suppose I have such type with several members:

type TheType = {
    PK_1: number;
    PK_2: number;    // The type may same as other
    PK_3: boolean;
    PK_4: string;
}

I'd like to get a tuple of type which contains each type of each member above such as:

// Should return as [number, number, boolean, string]
type TypeTuple = ToTuple<TheType>;  // How to make this ToTuple method?
jayatubi
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1 Answers1

1
type TheType = {
    PK_1: number;
    PK_2: number;    // The type may same as other
    PK_3: boolean;
    PK_4: string;
}

// credits goes to https://stackoverflow.com/a/50375286
type UnionToIntersection<U> = (U extends any ? (k: U) => void : never) extends (
    k: infer I
) => void
    ? I
    : never;

// credits goes to https://github.com/microsoft/TypeScript/issues/13298#issuecomment-468114901
type UnionToOvlds<U> = UnionToIntersection<
    U extends any ? (f: U) => void : never
>;

type PopUnion<U> = UnionToOvlds<U> extends (a: infer A) => void ? A : never;

// credit goes to https://stackoverflow.com/questions/53953814/typescript-check-if-a-type-is-a-union#comment-94748994
type IsUnion<T> = [T] extends [UnionToIntersection<T>] ? false : true;

type UnionToArray<T, A extends unknown[] = []> = IsUnion<T> extends true
    ? UnionToArray<Exclude<T, PopUnion<T>>, [PopUnion<T>, ...A]>
    : [T, ...A];

type MapPredicate<Obj, Key> = Key extends keyof Obj ? Obj[Key] : never

// credit goes to https://catchts.com/tuples#map
type Mapped<
    Obj,
    Arr extends Array<unknown>,
    Result extends Array<unknown> = [],
    > = Arr extends []
    ? []
    : Arr extends [infer H]
    ? [...Result, MapPredicate<Obj, H>]
    : Arr extends [infer Head, ...infer Tail]
    ? Mapped<Obj, [...Tail], [...Result, MapPredicate<Obj, Head>]>
    : Readonly<Result>;

type Result = Mapped<TheType, UnionToArray<keyof TheType>>; // [number, number, boolean, string]

Playground

Regarding converting union to array you can find more explanation here

Because this answer is a mix of already existing solutions you can find more explanation in appropriate links which I've been left above each helper

captain-yossarian from Ukraine
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  • For the `UnionToIntersection` does that mean: When an union distributes to function it would produce another single function which only contain one parameter type of the intersection of the union? – jayatubi Aug 27 '21 at 09:39
  • @jayatubi `UnionToIntersection` just converts union to intersection. For example: `{a:1} | {b: 1}` to `{a:1,b:1}` see this link https://stackoverflow.com/a/50375286 for more info – captain-yossarian from Ukraine Aug 27 '21 at 09:42
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    This answer works perfectly. It took me a few hours to understand every line of the answer. To much new knowledge for me to learn. Thanks very much! – jayatubi Aug 27 '21 at 11:55
  • Unfortunately it is not trivial. Every time it will take less and less time to understand such utilities – captain-yossarian from Ukraine Aug 27 '21 at 12:04