PromiseSeq.js

/**
 * Promise sequence
 * @description Handles the sequence of Promise.
 * @param {Array<Function: Promise>} promises Collection of functions which should return instance of Promise.
 * @return {Promise}
 */
function PromiseSeq(promises) {
  return promises.reduce((promisesPool, promise) =>
    promisesPool.then((result) => {
      /* Explicitly check if function (returning promise), since you can pass {true} to it
       * to be resolved instantly (conditional Promise). */
      const isFunction = (currentPromise instanceof Function);
      const appendResult = () => Array.prototype.concat.bind(result);
      return isFunction ? currentPromise().then(appendResult()) : appendResult();
    }), Promise.resolve([]));
  }
}

/**
 * Usage example
 */
const promise1 = () => Promise(resolve => setTimeout(() => {
  console.log('promise 1 is resolved');
  resolve();
}, 3000));

const promise2 = () => Promise(resolve => setTimeout(() => {
  console.log('promise 2 is resolved');
  resolve();
}, 1500));

PromiseSeq([promise1, promise2]).then(() => {
  console.log('All promises have been resolved sequentially!');
});