modeBy

1/**
2 * Returns the most frequent item in an array based on a selector function.
3 * If there are multiple items with the same frequency, returns the first one encountered.
4 *
5 * @param arr - The input array.
6 * @param fn - A function that returns a comparable key for each item.
7 * @returns The most frequent item based on the selected key, or undefined if the array is empty.
8 */
9export function modeBy<T>(arr: T[], fn: (item: T) => string | number): T | undefined {
10  const counts = new Map<string | number, { count: number; item: T }>();
11
12  for (const item of arr) {
13    const key = fn(item);
14    if (!counts.has(key)) {
15      counts.set(key, { count: 1, item });
16    } else {
17      counts.get(key)!.count++;
18    }
19  }
20
21  let maxCount = -1;
22  let result: T | undefined;
23
24  for (const { count, item } of counts.values()) {
25    if (count > maxCount) {
26      maxCount = count;
27      result = item;
28    }
29  }
30
31  return result;
32}

1test('modeBy - returns most frequent by key', () => {
2  const data = [
3    { name: 'a' },
4    { name: 'b' },
5    { name: 'a' },
6    { name: 'c' },
7    { name: 'b' },
8    { name: 'a' },
9  ];
10  expect(modeBy(data, x => x.name)).toEqual({ name: 'a' });
11});
12
13test('modeBy - ties return first encountered', () => {
14  const data = [
15    { id: 1 }, { id: 2 }, { id: 2 }, { id: 1 }
16  ];
17  expect(modeBy(data, x => x.id)).toEqual({ id: 1 }); // 1 and 2 both occur twice
18});
19
20test('modeBy - works with numbers', () => {
21  expect(modeBy([1, 2, 2, 3, 1, 2], x => x)).toBe(2);
22});
23
24test('modeBy - empty array returns undefined', () => {
25  expect(modeBy([], x => x)).toBeUndefined();
26});