How to make an array of all the strings in string union type?

Question

I want to take type stringUnion = "Value1"|"Value2" and make an array from all the strings in stringUnion

so the array looks like this ["Value1","Value2"], I want an array so I can iterate over it like this:

theArray.map((current, index)=>{

})

How do I do that?

Does this answer your question? [How to transform union type to tuple type](https://stackoverflow.com/questions/55127004/how-to-transform-union-type-to-tuple-type) — Oleg Valter is with Ukraine, Apr 04 '21 at 13:20

score 5 · Answer 1 · answered Apr 04 '21 at 09:40

Yes, it is possible.

// credits goes to https://stackoverflow.com/questions/50374908/transform-union-type-to-intersection-type/50375286#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;


// credits 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];

interface Person {
  name: string;
  age: number;
  surname: string;
  children: number;
}

type Result = UnionToArray<keyof Person>; // ["name", "age", "surname", "children"]

Here is an alternative way:

// credits goes to https://twitter.com/WrocTypeScript/status/1306296710407352321
type TupleUnion<U extends string, R extends any[] = []> = {
    [S in U]: Exclude<U, S> extends never ? [...R, S] : TupleUnion<Exclude<U, S>, [...R, S]>;
}[U];

interface Person {
    firstName: string;
    lastName: string;
    dob: Date;
    hasCats: false;
}

type keys = TupleUnion<keyof Person>;

Playground

More explanation you can find in my blog

Agree, it's brilliant! Would there be a way to make it independent of the order in the array? — jorgen, Dec 06 '22 at 10:42
@jorgen could you please elaborate? I mean with some example and expected result — captain-yossarian from Ukraine, Dec 06 '22 at 11:43
I mean (referring to the first method): type T = "a" | "b" means that const a: UnionToArray = ["a", "b"] is permissible but not ["b", "a"]. In our case the type T is derived (using amongst other things Exclude), and somehow the correct order seems to depend on environment (local vs CI).. Not sure exactly how that happens — jorgen, Dec 07 '22 at 09:31
Note that in our case the second method gives an error "Type instantiation is excessively deep and possibly infinite.". I guess our derived type is just not amenable to this! — jorgen, Dec 07 '22 at 09:35

score 0 · Answer 2 · answered Apr 04 '21 at 09:16

It is not possible to create a value out of a type. So you can't convert your union type to an array. If you wish to create an array type though. You can do it in 2 ways.

type YourArrayType = StringUnion[];
type YourArrayType = Array<StringUnion>;

This type will create the following type: ("value1" | "value2")[] and it will only allow you to have either "value1" or "value2" in your array with this type.

score 0 · Accepted Answer · answered Apr 04 '21 at 09:20

0

You can't use the stringUnion as a value in a variable (because types are not transpiled to js code).

But if this array is gonna be a constant, you can use:

const stringValues = ["value1", "value2"] as const;
type stringUnion = (typeof stringValues)[number];

Which will allow you to write the values only once.

answered Apr 04 '21 at 09:20

YohaiM

383
2
9

1

The OP is asking about transforming a union type into a tuple. – Oleg Valter is with Ukraine Apr 04 '21 at 13:22
@OlegValter From the question it's not clear if they ask about transforming it to a type of array or to a value of array... – YohaiM Apr 05 '21 at 04:50

How to make an array of all the strings in string union type?

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