groupByMulti

1/**
2 * Groups items into multiple buckets based on multiple keys per item.
3 * Each item can appear in multiple groups if the key extractor returns an array.
4 *
5 * @param arr - The array to group.
6 * @param fn - A function that returns one or more keys for each item.
7 * @returns An object where each key maps to an array of grouped items.
8 */
9export function groupByMulti<T, K extends string | number>(
10  arr: T[],
11  fn: (item: T) => K | K[]
12): Record<K, T[]> {
13  return arr.reduce((acc, item) => {
14    const keys = fn(item);
15    const keyList = Array.isArray(keys) ? keys : [keys];
16    for (const key of keyList) {
17      if (!acc[key]) acc[key] = [];
18      acc[key].push(item);
19    }
20    return acc;
21  }, {} as Record<K, T[]>);
22}

1test('groupByMulti - group by multiple tags', () => {
2  const posts = [
3    { title: 'Post 1', tags: ['react', 'javascript'] },
4    { title: 'Post 2', tags: ['css', 'design'] },
5    { title: 'Post 3', tags: ['javascript'] },
6  ];
7
8  expect(groupByMulti(posts, post => post.tags)).toEqual({
9    react: [{ title: 'Post 1', tags: ['react', 'javascript'] }],
10    javascript: [
11      { title: 'Post 1', tags: ['react', 'javascript'] },
12      { title: 'Post 3', tags: ['javascript'] },
13    ],
14    css: [{ title: 'Post 2', tags: ['css', 'design'] }],
15    design: [{ title: 'Post 2', tags: ['css', 'design'] }],
16  });
17});
18
19test('groupByMulti - single key fallback', () => {
20  const data = ['apple', 'banana', 'cherry'];
21  expect(groupByMulti(data, word => word[0])).toEqual({
22    a: ['apple'],
23    b: ['banana'],
24    c: ['cherry'],
25  });
26});