http://exploringjs.com/es2016-es2017/ch_async-functions.html

async-await.js

// Async function returns promise
async function asyncFunc() {
  return "123";
}

async function resolveAfter1Second(n) {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(n);
    }, 1000);
  });
}

const sumFunc = async() => {
    const n1 = await resolveAfter1Second(10);
    const n2 = await resolveAfter1Second(20);
    const n3 = await resolveAfter1Second(30);
    return n1 + n2 + n3;
}

// use sequential await
async function resolveAfterNSecond(num, sec) {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(num);
    }, sec * 1000);
  });
}

// Resolve promise in sequential order
const multFunc = async() => {
  const n1 = await resolveAfterNSecond(2, 1);
  const n2 = await resolveAfterNSecond(3, 2);
  const n3 = await resolveAfterNSecond(4, 3);
  return n1 * n2 * n3;
}

// Use promise.all to resolve promises in parallel
const multFuncParallel = async() => {
  const [n1, n2, n3] = await Promise.all([resolveAfterNSecond(2, 1), resolveAfterNSecond(3, 2), resolveAfterNSecond(4, 3)]);
  return n1 * n2 * n3;
}

module.exports = {
  asyncFunc,
  sumFunc,
  multFunc,
  multFuncParallel
};

// Call async functions in separate file
const a = require('./asyncFunc');

a.asyncFunc()
.then(console.log);

// expect 60
a.sumFunc()
.then(console.log)
.catch(console.error);

a.multFunc()
.then(console.log)
.catch(console.error);

a.multFuncParallel()
.then(console.log)
.catch(console.error);