Promise.all using only async await.

await-all.js

const delay = ms => new Promise(r => setTimeout(r, ms))

async function AwaitAll() {
  try {
    const promises = [
      delay(1000),
      delay(250),
      delay(500),
    ]
    for (let p of promises) await p
    console.log("success")
  } catch (e) {
    console.log("failed")
  } finally {
    console.log("done")
  }
}

explanation.js

const delay = ms => new Promise(r => setTimeout(r, ms))

// First we need to understand that await runs in sequence.

async function AwaitInSequence() {
  console.time()
  await delay(1000)
  await delay(250)
  await delay(500)
  console.timeEnd() // ~1750ms, since promises ran in sequence.
}

// This can be avoided by creating the promises before awaiting them.

async function AwaitInParallel() {
  console.time()
  const a = delay(1000)
  const b = delay(250)
  const c = delay(500)
  await a
  await b
  await c
  console.timeEnd() // ~1000ms, since promises ran in parallel.
}

// Promise.all works by resolving an array of promises in parallel.

function PromiseAll() {
  const promises = [
    delay(1000).then(() => console.log(1000)),
    delay(250).then(() => console.log(250)),
    delay(500).then(() => console.log(500)),
  ]
  return Promise
    .all(promises)
    .then(() => console.log("success"))
    .catch(() => console.log("failed"))
    .then(() => console.log("done"))
}

// Here's an incorrect way of recreating Promise.all.
// Each promise will run in sequence (1750ms total delay).

async function AwaitAll() {
  try {
    await delay(1000).then(() => console.log(1000)),
    await delay(250).then(() => console.log(250)),
    await delay(500).then(() => console.log(500)),
    console.log("success")
  } catch (e) {
    console.log("failed")
  } finally {
    console.log("done")
  }
}

// Here's the correct way of recreating Promise.all.
// Each promise runs in parallel (1000ms total delay).

async function AwaitAll() {
  try {
    const promises = [
      delay(1000).then(() => console.log(1000)),
      delay(250).then(() => console.log(250)),
      delay(500).then(() => console.log(500)),
    ]
    for (let p of promises) await p
    console.log("success")
  } catch (e) {
    console.log("failed")
  } finally {
    console.log("done")
  }
}