Function groupBy

src/cli/utilities.js:17–31

* @template Obj * @template Key * @param {Array } array * @param {(value: Obj) => Key} iteratee * @returns {{[p in Key]: T}}

(array, iteratee)

 * @returns {{[p in Key]: T}}
 */
function groupBy(array, iteratee) {
  const result = Object.create(null);

  for (const value of array) {
    const key = iteratee(value);

    if (Array.isArray(result[key])) {
      result[key].push(value);
    } else {
      result[key] = [value];
    }
  }

  return result;
}

/**
 * @template Obj