AsyncArray.js

class AsyncArray {
  constructor(value) {
    this.promise = Promise.resolve(value);
  }

  filter(fn) {
    const promise = this.promise.then(value => Promise.all([
      Promise.all(value.map(fn)),
      value,
    ]))
    .then(([bools, value]) => value.filter((item, index) => bools[index]));

    return new AsyncArray(promise);
  }

  map(fn) {
    const promise = this.promise.then(value => Promise.all(value.map(fn)));
    return new AsyncArray(promise);
  }

  reduce(fn, initial) {
    return this.promise.then(value => value.reduce((promise, item, index) =>
      promise.then(innerValue => fn(innerValue, item, index)),
      Promise.resolve(initial)
    ));
  }

  value() {
    return this.promise;
  }
}