serialPromises.js

// Callback must returns a promise.
// This function, unlike `Promise.all`, will resolve
// one promise at a time, in the original array order
const serial = (arr, callback) => {
  return new Promise((resolve, reject) => {
    arr.reduce((previous, current, index) => {
      return previous.then(() => callback(current)).then(() => {
        if (index === (arr.length - 1)) resolve();
      }).catch(e => reject(e));
    }, Promise.resolve());
  });
};