Comment by molf

With async/await this can become:

    const reduceP = async (fn, identity, listP) => {
      const values = await Promise.all(listP)
      return values.reduce(fn, identity)
    }

The whole thing feels like a synthetic and overcomplicated example, though. In practice I'm sure I'd just write:

    let total = 0
    while (listP.length > 0) {
      total += await listP.pop()
    }