This is a simple implementation of Promise.reduce, that is available in Bluebird

Installation

npm install reduce-for-promises

Usage

import reduce from 'reduce-for-promises';

Example

Callback function will iterate to next item only after previous item's promise is resolved

const result = await reduce([1, 2, 3, 4], function (acc, item) {
  return new Promise(function (resolve) {
    setTimeout(function () {
      resolve(acc + item);
    }, 1000);
  });
}, 0);
console.log(result); // 10

You can set fourth argument to reduce if you want to set step size. For example, if you want two items to be processed at a time, here is the example

const delays = [1000, 100, 300, 200, 100];
const { sum } = await reduce([1, 2, 3, 4, 5], function (acc, item, i) {
  return new Promise(function (resolve) {
    setTimeout(function () {
      acc.sum += item;
      resolve(acc);
    }, delays[i]);
  });
}, {sum: 0}, 2);
console.log(sum); // 15

Please note that if you set step argument, make sure to use object as accumulator. Otherwise it won't be carried properly between callback iterations