Typescript Custom Type Modifiers

tsModifiers.ts

export type ModifyKey<I, K extends keyof I, V> = Omit<I, K> &
  { [K in keyof I]: V };

export type PickAndModifyKey<I, K extends keyof I, V> = ModifyKey<
  Pick<I, K>,
  K,
  V
>;

// key should be an interface/object
export type PickApartKey<
  I,
  K extends keyof I,
  P extends keyof I[K]
> = PickAndModifyKey<I, K, Pick<I[K], P>>;

type Unboxed<T> = T extends (infer U)[] ? U : T;

export type PickApartArrayKey<
  I, // interface
  K extends keyof I, //key(array) which needs to be changed
  P extends keyof Unboxed<I[K]> //keys of K that are needed
> = PickAndModifyKey<I, K, Pick<Unboxed<I[K]>, P>[]>;

https://stackoverflow.com/questions/60067100/why-is-the-infer-keyword-needed-in-typescript
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html