Flatten nested promises with asyncawait + then or plain synchronous looking asyncawait.

flattened-nested-promises-with-asyncawait.js

let wait = (s, data) =>
    new Promise(resolve => setTimeout(resolve.bind(null, data),
                                     s * 1000));

let loc = (l) =>
    ({ location: l });

let region = (r) =>
    ({ region: r,
       locations: [wait(3, loc(`${r} loc 1`)),
                   wait(4, loc(`${r} loc 2`))]});


let county = () =>
    Promise.resolve([wait(1, region('reg 1')),
                     wait(2, region('reg 2'))]);


let thenable = async () =>
    await county()
      .then(county => Promise.all(county))
      .then(regions => Promise.all(
        regions.map(region => region.locations)))
      .then(locations => Promise.all(
        locations.map(loc => Promise.all(loc))));

console.time('thenable');
thenable()
  .then(JSON.stringify)
  .then(console.log) // => [[{"location":"reg 1 loc 1"},{"location":"reg 1 loc 2"}],[{"location":"reg 2 loc 1"},{"location":"reg 2 loc 2"}]]
  .then(_ => console.timeEnd('thenable')); // => thenable: 4003.666015625ms


let asyncawait = async function () {
  let c = await county();
  let regions = await Promise.all(c);
  let locations = await Promise.all(
    regions.map(region => region.locations));
  return await Promise.all(
    locations.map(async (loc) => await Promise.all(loc)));
};

console.time("asyncawait");
asyncawait()
  .then(JSON.stringify)
  .then(console.log) // => [[{"location":"reg 1 loc 1"},{"location":"reg 1 loc 2"}],[{"location":"reg 2 loc 1"},{"location":"reg 2 loc 2"}]]
  .then(_ => console.timeEnd('asyncawait')); // => asyncawait: 4004.31005859375ms