Get all value types of a double-nested object in TypeScript

Question

My question is similar to Types from both keys and values of object in Typescript, but with an additional layer of nesting:

interface Outer {
  a: {
    b: number
  },
  c: {
    d: string
  }
}

And I'm only looking to retrieve the union type of all values, here number | string.

Here's my stab:

type Values<T extends Record<string, any>> = T[keyof T]; // From previous SO answer

type InnerValues<
  T extends Record<string, Record<string, any>>,
  K extends keyof T
> = T[K][keyof T[K]];

type All = Values<Outer>;
type In = InnerValues<Outer, keyof Outer>; // expected number|string

but I have an error saying Outer doesn't have an index type.

Playground link.

Titian Cernicova-Dragomir · Accepted Answer · 2019-09-19 16:17:29Z

9

T[K][keyof T[K]] will not work because T[K] will be a union of all possible values in T (well the values specified by K anyway) and keyof T[K] will probably end up being never if the values have no common keys.

The solution is to take each constituent in the union T[K] and get its possible values. We can do this using a distributive conditional type

interface Outer {
  a: {
    b: number;
  };
  c: {
    d: string;
  };
}

type DistributiveValues<T extends Record<string, any>> = T extends T ? T[keyof T] : never;

type InnerValues<
  T extends Record<keyof T, object>,
  K extends keyof T
> = DistributiveValues<T[K]>;

type In = InnerValues<Outer, keyof Outer>; // is number|string

play

edited Sep 19, 2019 at 16:17

answered Sep 19, 2019 at 16:11

Titian Cernicova-Dragomir

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1 Comment

Jimmy Over a year ago

Would it be possible to make this recursive so it could give you a union of all the non-Record leaf value types in an arbitrarily deep nesting of objects? For example, if the property d in Outer was an object rather than a string.

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