#1 ― async / await (KharkivJS)

index.js

function squareAfter2Seconds(x) {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(x * x);
    }, 2000);
  });
}

async function calc() {
  const a = await squareAfter2Seconds(10);
  const b = await squareAfter2Seconds(20);
  const c = await squareAfter2Seconds(30);

  console.log(a + b + c);
}

calc();

@bozheville

Если совсем упороться, можно еще и без переменных )
(btw но так делать не нужно никогда)

async function calc() {
  console.log(
    await [ 10, 20, 30 ]
    .map(async x => await squareAfter2Seconds(x))
    .reduce(async (a, b) => await a + await b)
  )
}