Teeing an asynchronous iterator

tee.js

function tee(asyncIterable) {
  let source = asyncIterable[Symbol.asyncIterator]()

  return [[], []].map((buffer, i, buffers) => ({
    async next() {
      if (0 in buffer) return buffer.shift()

      let item = await source.next()

      if (!item.done) buffers[1 - i].push(item)

      return item
    },

    [Symbol.asyncIterator]() {
      return this
    }
  }))
}