How to group an array by key in javascript in a functional style

groupArrayByKey.js

/**
 * groupArrayByKey
 * @param array
 * @param {function(object):number | string} keyExtractorCallback - function to calculate the key
 * @returns {*}
 */
export const groupArrayByKey = (array, keyExtractorCallback) =>
  array.reduce((accumulator, item) => {
    const key = keyExtractorCallback(item);
    accumulator[key] = (accumulator[key] || []).concat(item);
    return accumulator;
  }, {});