2631-group-by

2631. Group By (Medium) (https://leetcode.com/problems/group-by/)

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. Constraints: 0 <= array.length <= 10^5 fn returns a string

const result: Record<string, T[]> = {}
 
  for (const item of this) {
    const key = fn(item)
 
    if (key in result) {
      result[key].push(item)
    }
    else {
      result[key] = [item]
    }
  }
 
  return result
}
 
// Local check:
console.log([{ id: '1' }, { id: '1' }, { id: '2' }].groupBy(item => item.id))
console.log([[1, 2, 3], [1, 3, 5], [1, 5, 9]].groupBy(list => String(list[0])))
console.log([1, 2, 3, 4, 5, 6, 7, 8, 9, 10].groupBy(n => String(n > 5)))

Example 1:
 
    Input:
    array = [
      {"id":"1"},
      {"id":"1"},
      {"id":"2"}
    ],
    fn = function (item) {
      return item.id;
    }
    Output:
    {
      "1": [{"id": "1"}, {"id": "1"}],
      "2": [{"id": "2"}]
    }
    Explanation:
    Output is from array.groupBy(fn).
    The selector function gets the "id" out of each item in the array.
    There are two objects with an "id" of 1. Both of those objects are put in
    the first array. There is one object with an "id" of 2. That object is put
    in the second array.
 
  Example 2:
 
    Input:
    array = [
      [1, 2, 3],
      [1, 3, 5],
 
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      [1, 5, 9]
    ]
    fn = function (list) {
      return String(list[0]);
    }
    Output:
    {
      "1": [[1, 2, 3], [1, 3, 5], [1, 5, 9]]
    }
    Explanation:
    The array can be of any type. In this case, the selector function defines
    the key as being the first element in the array. All the arrays have 1 as
    their first element so they are grouped together.
 
  Example 3:
 
    Input:
    array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
    fn = function (n) {
      return String(n > 5);
    }
    Output:
    {
      "true": [6, 7, 8, 9, 10],
      "false": [1, 2, 3, 4, 5]
    }
    Explanation:
    The selector function splits the array by whether each number is greater than 5.