It's like map, but you get the whole list.

andThen.ts

interface Array<T> {
  andThen: <U>(this: T[], fn: (items: T[]) => U) => U;
}

Array.prototype.andThen = function<T, U>(this: T[], fn: (items: T[]) => U): U {
  return fn(this);
};

example.ts

const users = await ["userid1", "userid2"].map(fetch).andThen(Promise.all);