TypeScript Generic Factory Function Type, matching array element order

Question

I want to build some kind of FactoryFactory: Basically a generic function that returns a factory function. Writing the function itself is simple, but I can't figure out how to do the TypeScript typings for it.

The function should be used like this:

const stubFactoryFunction = (...props) => (...values) => ({ /* ... */ });

const factory = stubFactoryFunction("prop1", "prop2");
const instance = factory("foo", 42);
console.log(instance); // { prop1: "foo", prop2: 42 }

At first I tried to provide the value types as an array:

type FactoryFunction<T extends any[]> =
  <K extends string[]>(...props: K) =>
    (...values: T[number]) =>
      {[key in K[number]]: T[number]}

But this will result in { prop1: string | number, prop2: string | number}, because the type doesn't match the array indexes.

Next I tried to provide the whole object as generic type:

type FactoryFunction<T extends {[key: string]: any}> =
  (...props: (keyof T)[]) =>
    (...values: ???) =>
      T

And here I got a similar problem: values must somehow match the order of props.

Is this possible at all?

Bonus 1: Don't allow duplicate props.

Bonus 2: Enforce the provide all non-optional props from T.

Answer 1

This is another solution, generic this time and finally not so complicated, eventhough some intermediary types are necessary. The base ideas are:

• Having an object type with keys in a string tuple type P (for "Props") (extends string[]) is done with a mapped type { [K in P[number]]: ... }
• The difficulty is to get the index of K (one of the "Props") in P. It's done using another mapped type IndexOf, itself using a third mapped type Indexes.
• Then, the object type values are given by V[IndexOf<P, K>] which can be acceptable only if IndexOf<P, K> is an index of V (for "Values"), hence the conditional type IndexOf<P, K> extends keyof V ? V[IndexOf<P, K>] : never. We will never have never since both P and V array types have the same length due to the constraint V extends (any[] & { length: P['length'] }).

// Utility types
type Indexes<V extends any[]> = {
  [K in Exclude<keyof V, keyof Array<any>>]: K;
};

type IndexOf<V extends any[], T> = {
  [I in keyof Indexes<V>]: V[I] extends T ? T extends V[I] ? I : never : never;
}[keyof Indexes<V>];

type FactoryFunctionResult<P extends string[], V extends (any[] & { length: P['length'] })> = {
  [K in P[number]]: IndexOf<P, K> extends keyof V ? V[IndexOf<P, K>] : never;
};

// Tests
type IndexesTest1 = Indexes<['a', 'b']>; // { 0: "0"; 1: "1" }

type IndexOfTest1 = IndexOf<['a', 'b'], 'b'>; // "1"
type IndexOfTest2 = IndexOf<['a', 'b'], string>; // never
type IndexOfTest3 = IndexOf<[string, string], 'a'>; // never
type IndexOfTest4 = IndexOf<[string, string], string>; // "0" | "1"

type FactoryFunctionResultTest1 = FactoryFunctionResult<['a'], [string]>; // { a: string }
type FactoryFunctionResultTest2 = FactoryFunctionResult<['a', 'b'], [string, number]>; // { a: string; b: number }
type FactoryFunctionResultTest3 = FactoryFunctionResult<['a', 'b', 'c'], [string, number, boolean]>; // { a: string; b: number; c: boolean }
type FactoryFunctionResultTest4 = FactoryFunctionResult<['a', 'b', 'c', 'd'], [string, number, boolean, string]>; // { a: string; b: number; c: boolean; d: string }