Weird `join` implementation

join.ts

const dogs = [
  { name: 'Fido', favorites: ['sam', 'mark', 'bob'] },
  { name: 'Happy', favorites: ['mark'] },
  { name: 'Buddy', favorites: ['bob'] },
  { name: 'Oscar', favorites: ['mark', 'sam', 'bob'] },
];

const friends = [
  { name: 'sam', age: 22, toes: 5 },
  { name: 'mark', age: 37, toes: 9 },
  { name: 'bob', age: 28, toes: 3 },
];

join(friends)
  .onto(dogs)
  .match(({ name }, { favorites }) => favorites.includes(name))
  .object({
    key: (friend) => friend.name,
    value: (dog) => dog.favorites,
  });

function join<T>($: T[]) {
  return {
    onto<U>(onto: U[]) {
      return {
        match(match: (e: T, o: U) => boolean) {
          const entries = $.map(
            (e) => [e, onto.filter((o) => match(e, o))] as [T, U[]]
          );

          const _extras = {
            object: <R>({
              key,
              value,
            }: {
              key: (e: T) => string | number;
              value: (o: U) => R;
            }) =>
              Object.fromEntries(
                entries.map(([e, o]) => [key(e), o.map(value)])
              ),
          };

          return Object.assign(entries, _extras);
        },
      };
    },
  };
}