promiseReduce.mjs

import isPromise from './isPromise';

/**
 * Similar to Array.prototype.reduce(), however the reducing callback may return
 * a Promise, in which case reduction will continue after each promise resolves.
 *
 * If the callback does not return a Promise, then this function will also not
 * return a Promise.
 */
export default function promiseReduce(values, callback, initialValue) {
  return values.reduce(function (previous, value) {
    return isPromise(previous) ? previous.then(function (resolved) {
      return callback(resolved, value);
    }) : callback(previous, value);
  }, initialValue);
}