Gist for www.direcore.xyz/archives/35/

async-iter-await.js

const numbers = [1, 2, 3, 4, 5]

let start = Date.now()

async function someRandomWork() {
  for (let i = 0; i < 2e9; i++) {
  }
}

async function getCurrentTime() {
  return new Date(Date.now() - start)
    .toISOString()
    .split('T')[1]
    .split('Z')[0]
}

async function mapAsync() {
  await Promise.all(numbers.map(async i => {
    await someRandomWork();
    console.log(await getCurrentTime(), i);
  }))
  console.log('mapAsync finished!')
}

async function reduceAsync() {
  await numbers.reduce(async (promise, i) => {
    await promise;
    await someRandomWork();
    console.log(await getCurrentTime(), i);
  }, Promise.resolve());
  console.log('reduceAsync finished!')
}

async function forOfAsync() {
  for (const i of numbers) {
    await someRandomWork();
    console.log(await getCurrentTime(), i);
  }
  console.log('forOfAsync finished!')
}

async function forAwaitOfAsync() {
  for await (const i of numbers) {
    await someRandomWork();
    console.log(await getCurrentTime(), i);
  }
  console.log('forAwaitOfAsync finished!')
}

async function main() {
  console.log(`\nmap test`);
  start = Date.now();
  await mapAsync();

  console.log(`\nreduce test`);
  start = Date.now();
  await reduceAsync();

  console.log(`\nfor...of test`);
  start = Date.now();
  await forOfAsync();
}

main();

// expected output:
//
// map test
// 00:00:03.863 1
// 00:00:03.863 2
// 00:00:03.863 3
// 00:00:03.863 4
// 00:00:03.863 5
// mapAsync finished!

// reduce test
// 00:00:00.629 1
// 00:00:01.256 2
// 00:00:01.883 3
// 00:00:02.510 4
// 00:00:03.137 5
// reduceAsync finished!

// for...of test
// 00:00:00.627 1
// 00:00:01.255 2
// 00:00:01.881 3
// 00:00:02.507 4
// 00:00:03.134 5
// forOfAsync finished!

for await...of vs for...of.js

const numberPromises = [
  Promise.resolve(1),
  Promise.resolve(2),
  Promise.resolve(3),
  Promise.resolve(4),
  Promise.resolve(5)
]

let start = Date.now()

async function someRandomWork() {
  for (let i = 0; i < 2e9; i++) {
  }
}

async function getCurrentTime() {
  return new Date(Date.now() - start)
    .toISOString()
    .split('T')[1]
    .split('Z')[0]
}

async function forOfAsync() {
  for (const i of numberPromises) {
    await someRandomWork();
    console.log(await getCurrentTime(), i);
  }
  console.log('forOfAsync finished!')
}

async function forAwaitOfAsync() {
  for await (const i of numberPromises) {
    await someRandomWork();
    console.log(await getCurrentTime(), i);
  }
  console.log('forAwaitOfAsync finished!')
}

async function main() {
  console.log(`\nfor...of test`);
  start = Date.now();
  await forOfAsync();

  console.log(`\nfor await...of test`);
  start = Date.now();
  await forAwaitOfAsync();
}

main();

// expected output:
//
// for...of test
// 00:00:00.988 Promise { 1 }
// 00:00:01.933 Promise { 2 }
// 00:00:02.562 Promise { 3 }
// 00:00:03.189 Promise { 4 }
// 00:00:03.816 Promise { 5 }
// forOfAsync finished!

// for await...of test
// 00:00:00.627 1
// 00:00:01.254 2
// 00:00:01.882 3
// 00:00:02.509 4
// 00:00:03.136 5
// forAwaitOfAsync finished!