groupBy

1/**
2 * Groups items in an array based on a key returned by a callback.
3 *
4 * @param arr - The array to group.
5 * @param fn - A function that returns the key for grouping each item.
6 * @returns An object where each key maps to an array of grouped items.
7 */
8export function groupBy<T, K extends string | number>(
9  arr: T[],
10  fn: (item: T) => K
11): Record<K, T[]> {
12  return arr.reduce((acc, item) => {
13    const key = fn(item);
14    (acc[key] ||= []).push(item);
15    return acc;
16  }, {} as Record<K, T[]>);
17}

1test('groupBy - string length', () => {
2  const data = ['apple', 'banana', 'cherry'];
3  expect(groupBy(data, fruit => fruit.length)).toEqual({
4    5: ['apple'],
5    6: ['banana', 'cherry'],
6  });
7});
8
9test('groupBy - objects by field', () => {
10  const users = [
11    { name: 'Alice', role: 'admin' },
12    { name: 'Bob', role: 'user' },
13    { name: 'Eve', role: 'admin' },
14  ];
15  expect(groupBy(users, user => user.role)).toEqual({
16    admin: [
17      { name: 'Alice', role: 'admin' },
18      { name: 'Eve', role: 'admin' },
19    ],
20    user: [
21      { name: 'Bob', role: 'user' },
22    ],
23  });
24});
25
26test('groupBy - empty array', () => {
27  expect(groupBy([], x => x)).toEqual({});
28});