Parallel vs serial async await

parallel-serial-async.js

const items = 10

async function delayedFunc(delay, i) {
  return new Promise((resolve, reject) => {
    console.log(`Start executing ${i}`)
    setTimeout(() => {
      console.log(`Finished ${i}`)
      resolve()
    }, delay)
  })
}

async function parallel(arr) {
  console.log('***Start Parallel')
  await Promise.all(arr.map(delayedFunc))
  console.log('***Finished Parallel\n')
  return await arr;
}

async function series(arr) {
  console.log('***Start Series')
  let counter = 0
  for (item of arr) {
    await delayedFunc(item, counter)
    counter++
  }
  console.log('***Finished Series\n')
  return await arr;
}

async function* genDelayedFunction(arr) {
  let counter = 0;
  for (const item of arr) {
    await delayedFunc(item, counter);
    yield `Delayed ${counter} by ${item}ms\n`
    counter++
  }

}

async function asyncIteration(arr) {
  // for-await-of is currently at stage-4 (2018/Dec)
  // for-await-of can be used within async functions and anywhere await calls can be used
  console.log('***Start asyncIteration')
  for await (const item of genDelayedFunction(arr)) {
    console.log(item);
  }
  console.log('***Finished asyncIteration\n')
  return await arr;
}

async function exec() {
  const arr = Array(items).fill(0).map((item, i) => Math.floor(i < items / 2) ? i * 1000 : (items - i) * 1000)
  await parallel(arr);
  await series(arr);
  await asyncIteration(arr);
  return;
}

(async function () {
  console.log('BEGIN\n')
  await exec()
  console.log('DONE')
})()