Implementations of functions that work on Promise arrays, including `all` and `race`.

all.js

const all = promises => {
  if (promises.length === 0)
    return Promise.resolve([])
  let [head, ...tail] = promises
  return head.then(h =>
    all(tail).then(t =>
      [h].concat(t)))
}

allSuccessful.js

const allSuccessful = promises => {
  if (promises.length === 0)
    return Promise.resolve([])
  let [head, ...tail] = promises
  return head.then(h =>
    allSuccessful(tail)
      .then(
        t => [h].concat(t)),
        () => allSuccessful(tail))
}

race.js

const race = promises =>
  new Promise((resolve, reject) =>
    promises.forEach(promise =>
      promise.then(resolve, reject)))