concurrency.js

const state = {
  counter: 0
};

const randomInt = (min, max) =>
  Math.floor(Math.random() * (max - min + 1)) + min;

const sleep = (min, max = min) =>
  new Promise(resolve => {
    setTimeout(resolve, randomInt(min, max));
  });

const incrementCounter = () => {
  const currentCounter = state.counter;
  sleep(1000, 2000).then(() => {
    state.counter = currentCounter + 1;
  });
};

incrementCounter();
incrementCounter();

sleep(3000).then(() => console.log(state.counter)); // aléatoirement 1 ou 2