Complete implementation of Promise.map

Promise.map.js

Promise.map = function (iterable, mapper, options = {}) {
  let concurrency = options.concurrency || Infinity

  let index = 0
  const results = []
  const pending = []
  const iterator = iterable[Symbol.iterator]()

  while (concurrency-- > 0) {
    const thread = wrappedMapper()
    if (thread) pending.push(thread)
    else break
  }

  return Promise.all(pending).then(() => results)

  function wrappedMapper () {
    const next = iterator.next()
    if (next.done) return null
    const i = index++
    const mapped = mapper(next.value, i)
    return Promise.resolve(mapped).then(resolved => {
      results[i] = resolved
      return wrappedMapper()
    })
  }
}