2631-group-by.js

/**
 * 2631. Group By
 * https://leetcode.com/problems/group-by/
 * Difficulty: Medium
 *
 * Write code that enhances all arrays such that you can call the array.groupBy(fn) method on any
 * array and it will return a grouped version of the array.
 *
 * A grouped array is an object where each key is the output of fn(arr[i]) and each value is an
 * array containing all items in the original array which generate that key.
 *
 * The provided callback fn will accept an item in the array and return a string key.
 *
 * The order of each value list should be the order the items appear in the array. Any order of
 * keys is acceptable.
 *
 * Please solve it without lodash's _.groupBy function.
 */

/**
 * @param {Function} fn
 * @return {Object}
 */
Array.prototype.groupBy = function(fn) {
  return this.reduce((grouped, item) => {
    const key = fn(item);
    grouped[key] = grouped[key] || [];
    grouped[key].push(item);
    return grouped;
  }, {});
};

/**
 * [1,2,3].groupBy(String) // {"1":[1],"2":[2],"3":[3]}
 */