groupBy

/**
 * Creates an object composed of keys generated
 * from iteratee results and values containing
 * arrays of the original elements.
 *
 * Time Complexity:
 * O(n)
 *
 * Space Complexity:
 * O(n)
 *
 * @param {Array} array
 * @param {(value: any) => any} iteratee
 * @returns {Object}
 */
export default function groupBy(array, iteratee) {
  const result = {};

  for (const item of array) {
    // Generate grouping key
    const key = iteratee(item);

    // Initialize bucket if missing
    if (!(key in result)) {
      result[key] = [];
    }

    // Store original item
    result[key].push(item);
  }

  return result;
}

/**
groupBy([6.1, 4.2, 6.3], Math.floor);
// => { '4': [4.2], '6': [6.1, 6.3] }

groupBy([{ n: 3 }, { n: 5 }, { n: 3 }], (o) => o.n);
// => { '3': [{ n: 3 }, { n: 3 }], '5': [{ n: 5 }] }

groupBy([], (o) => o); // => {}

groupBy([{ n: 1 }, { n: 2 }], (o) => o.m); // => { undefined: [{ n: 1 }, { n: 2 }] }
 */