I have a type that's defined in a third party library. Let's say its shape is:

type Foo = {
  id: string
  results: Array<{ bar: string }>
}

Is there a way to derive the type { bar: string }?

I'd like to use it like so:

type Baz = /* derived type */

const baz: Baz = { bar: 'abc' }

I was looking at utility types... but couldn't figure out how to target that part of the type definition.

CodePudding user response：

Arrays indexed by number give you the member type of the array, since arrays are indexed by integers.

type Foo = {
  id: string
  results: Array<{ bar: string }>
}

type Baz = Foo['results'][number]
const baz: Baz = { bar: 'abc' }

Playground

CodePudding user response：

Got some help from a TS wizard. This is a working solution:

type Foo = {
  id: string
  results: Array<{bar: string}>
}

type ElementsOf<T extends unknown[]> = T extends ReadonlyArray<infer E> ? E : never;

// type Baz = { bar: string; } 
type Baz = ElementsOf<Foo["results"]>;

Would love if someone could help explain the ternary though.