Typescript convert string union type to intersections of another type

Question

I have the following union type:

type Foo = 'ol' | 'ul';

and I somehow want to convert that to this using a generic (as Foo might change to include others)

type Bar = OtherType<'ol'> & OtherType<'ul'>;

I'm not quite sure where to start...

EDIT: a more concrete example with React is:

type Elements = 'ol' | 'ul';

// ... somehow transform to:

type Props = JSX.IntrinsicElements['ol'] & JSX.IntrinsicElements['ul']

Then I'd want ```type Bar = OtherType<'ol'> & OtherType<'ul'> & OtherType<'other'>``` — leepowell, Jul 09 '21 at 09:47
Please share OtherType. Do you expect it to be some kind of object? — captain-yossarian from Ukraine, Jul 09 '21 at 09:53
You might be looking for smth like this https://www.typescriptlang.org/play?#code/C4TwDgpgBA8sAWEBOAVcEA8KoQB7AgDsATAZylOCQEtCBzAPigF4oADAEgG8UBfAWggAbCAFsiwNgCgpoSFABiAeyUtYCZGkgYA5EqE6oAHyg6ArgeOni1AG46GAbigB6F1CgAifYJHjCwJ5Wnha+YhJBJp42tmH+gVJAA — captain-yossarian from Ukraine, Jul 09 '21 at 09:54
you can do something like `type OtherType = T;` Your requirements aren't clear though — Amir Saleem, Jul 09 '21 at 09:54
Updated with a more concrete example of what I want to get to — leepowell, Jul 09 '21 at 09:59

score 0 · Accepted Answer · answered Jul 09 '21 at 10:07

import React from 'react'


// 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;

type OtherType<T extends keyof JSX.IntrinsicElements> = UnionToIntersection<JSX.IntrinsicElements[T]>

type Result = OtherType<'ol' | 'ul'>;

declare var x: Result
declare var y: JSX.IntrinsicElements['ol'] & JSX.IntrinsicElements['ul']

x = y // ok

y = x // ok

Playground

Union distribution will do the work for you

score 0 · Answer 2 · answered Jul 09 '21 at 10:11

Here's a solution which will work in general, when OtherType may not distribute over unions: it takes advantage of contravariance of function parameter types in order to convert a union to an intersection.

type MakeIntersection<T> = (T extends infer U ? (x: OtherType<U>) => void : never) extends (x: infer V) => void ? V : never

Playground Link

Typescript convert string union type to intersections of another type

2 Answers2