sequential-concurrent.js

const assert = require('assert');

//========== Helper functions

/**
 * Resolves after `ms` milliseconds
 */
function delay(ms) {
  return new Promise((resolve, _reject) => {
    setTimeout(resolve, ms);
  });
}
async function paused(id) {
  console.log('START ' + id);
  await delay(10); // pause
  console.log('END ' + id);
  return id;
}

//========== Main code

async function sequentialAwait() {
  const result1 = await paused('first');
  assert.equal(result1, 'first');
  
  const result2 = await paused('second');
  assert.equal(result2, 'second');
}
// Output:
// 'START first'
// 'END first'
// 'START second'
// 'END second'

async function concurrentPromiseAll() {
  const result = await Promise.all([
    paused('first'), paused('second')
  ]);
  assert.deepEqual(result, ['first', 'second']);
}
// Output:
// 'START first'
// 'START second'
// 'END first'
// 'END second'

async function concurrentAwait() {
  const resultPromise1 = paused('first');
  const resultPromise2 = paused('second');
  
  assert.equal(await resultPromise1, 'first');
  assert.equal(await resultPromise2, 'second');  
}
// Output:
// 'START first'
// 'START second'
// 'END first'
// 'END second'