How to transform union type into union of arrays in typescript?

Question

I have a Foo type which is a union of types

eg

type Foo = string | number

And I want to receive that type and transform it in an union of arrays of the types

type TransformedFoo = ToUnionOfArray<Foo> // => string[] | number[]

Usually, you can do something like Foo[] but it isn't what I want, because that would become (string | number)[] .

How can I have an union and transform it into a union of arrays of the types?

Answer 1

You can use distributive conditional types :

type Foo = string | number;

type Arrays<T> = T extends any ? T[] : never;

type TransformedFoo = Arrays<Foo>; // string[] | number[]

Playground

When conditional types act on a generic type, they become distributive when given a union type

Answer 2

You can use distributive conditionals in this case:

type ToUnionOfArray<T> = T extends infer I ? I[] : never;

When T is a naked type parameter in a conditional statement the conditional applies (or distributes) over every value in the input union T.

More info on how that works here: Typescript Distributive Conditional Types