partition

1/**
2 * Splits an array into two subarrays based on a predicate function.
3 * The first array contains items that match the predicate,
4 * and the second contains those that do not.
5 *
6 * @param arr - The input array.
7 * @param fn - A predicate function to test each element.
8 * @returns A tuple of two arrays: [matched, unmatched].
9 */
10export function partition<T>(arr: T[], fn: (item: T) => boolean): [T[], T[]] {
11  return arr.reduce<[T[], T[]]>(
12    (acc, item) => {
13      acc[fn(item) ? 0 : 1].push(item);
14      return acc;
15    },
16    [[], []]
17  );
18}

1test('partition - even and odd numbers', () => {
2  expect(partition([1, 2, 3, 4], n => n % 2 === 0)).toEqual([[2, 4], [1, 3]]);
3});
4
5test('partition - all true', () => {
6  expect(partition(['a', 'b'], () => true)).toEqual([['a', 'b'], []]);
7});
8
9test('partition - all false', () => {
10  expect(partition(['a', 'b'], () => false)).toEqual([[], ['a', 'b']]);
11});
12
13test('partition - empty array', () => {
14  expect(partition([], () => true)).toEqual([[], []]);
15});
16
17test('partition - complex predicate', () => {
18  const users = [
19    { name: 'Alice', active: true },
20    { name: 'Bob', active: false },
21  ];
22  expect(partition(users, u => u.active)).toEqual([
23    [{ name: 'Alice', active: true }],
24    [{ name: 'Bob', active: false }],
25  ]);
26});