maxBy

1/**
2 * Returns the item with the maximum value based on a selector function.
3 *
4 * @param arr - Array of items.
5 * @param fn - Function that returns a numeric value from each item.
6 * @returns The item with the largest selected value, or undefined if empty.
7 */
8export function maxBy<T>(arr: T[], fn: (item: T) => number): T | undefined {
9  if (arr.length === 0) return undefined;
10
11  return arr.reduce((max, curr) =>
12    fn(curr) > fn(max) ? curr : max
13  );
14}

1test('maxBy - basic usage', () => {
2  const data = [{ x: 3 }, { x: 5 }, { x: 2 }];
3  expect(maxBy(data, item => item.x)).toEqual({ x: 5 });
4});
5
6test('maxBy - same values', () => {
7  const data = [{ x: 2 }, { x: 2 }];
8  expect(maxBy(data, item => item.x)).toEqual({ x: 2 });
9});
10
11test('maxBy - negative numbers', () => {
12  const data = [{ n: -5 }, { n: -1 }, { n: -10 }];
13  expect(maxBy(data, item => item.n)).toEqual({ n: -1 });
14});
15
16test('maxBy - empty array', () => {
17  expect(maxBy([], x => 1)).toBeUndefined();
18});