/**
* @param {any[]} input
* @param {Function} iterator
* @param {any} initivalValue
* @returns {Promise}
*/
function asyncReduce(input, iterator, initialValue) {
  const arr = input.map(item => item());

  return Promise.all(arr).then(results => {
    let acc = initialValue;

    results.forEach((item, i, arr) => {
      acc = iterator(acc, item);
    });

    return acc;
  });
}

const a = () => Promise.resolve('a');
const b = () => Promise.resolve('b');
const c = () => new Promise(resolve => setTimeout(() => resolve('c'), 100));

asyncReduce(
  [a, b, c],
  (acc, curr) => [...acc, curr],
  ['d']
).then(results => {
  console.log(results); // ['d', 'a', 'b', 'c'];
});