TypeScript Union Merge

unionMerge.d.ts

type CommonKeys<T extends object> = keyof T;

type AllKeys<T> = T extends any ? keyof T : never;

type Subtract<A, C> = A extends C ? never : A;

type NonCommonKeys<T extends object> = Subtract<AllKeys<T>, CommonKeys<T>>;

type PickType<T, K extends AllKeys<T>> = T extends { [k in K]?: any }
  ? T[K]
  : undefined;

type Merge<T extends object> = {
  [k in CommonKeys<T>]: PickTypeOf<T, k>;
} &
  {
    [k in NonCommonKeys<T>]?: PickTypeOf<T, k>;
  };

type PickTypeOf<T, K extends string | number | symbol> = K extends AllKeys<T>
  ? PickType<T, K>
  : never;