utilities.js

/**
 * @description
 * Takes an Array<V>, and a grouping function,
 * and returns a Map of the array grouped by the grouping function.
 * Source: https://stackoverflow.com/questions/14446511/most-efficient-method-to-groupby-on-an-array-of-objects#answer-38327540
 *
 * @param list An array of type V.
 * @param keyGetter A Function that takes the the Array type V as an input, and returns a value of type K.
 *                  K is generally intended to be a property key of V.
 *
 * @returns Map of the array grouped by the grouping function.
 */
function groupBy(list, keyGetter) {
  const map = new Map();

  list.forEach((item) => {
    const key = keyGetter(item);
    const collection = map.get(key);

    if (!collection) {
      map.set(key, [item]);
    } else {
      collection.push(item);
    }
  });
  return map;
}

module.exports = { groupBy };