sumBy

1/**
2 * Calculates the total sum of values in an array based on a selector function.
3 *
4 * @param arr - The input array.
5 * @param fn - A function that extracts a number from each item.
6 * @returns The sum of selected values.
7 */
8export function sumBy<T>(arr: T[], fn: (item: T) => number): number {
9  return arr.reduce((total, item) => total + fn(item), 0);
10}

1test('sumBy - sums numeric properties', () => {
2  const data = [
3    { amount: 10 },
4    { amount: 5 },
5    { amount: 3 },
6  ];
7  expect(sumBy(data, x => x.amount)).toBe(18);
8});
9
10test('sumBy - works with empty array', () => {
11  expect(sumBy([], x => x as number)).toBe(0);
12});
13
14test('sumBy - works with negative values', () => {
15  const data = [{ val: -2 }, { val: 4 }, { val: -1 }];
16  expect(sumBy(data, x => x.val)).toBe(1);
17});
18
19test('sumBy - handles floats correctly', () => {
20  const items = [{ price: 1.5 }, { price: 2.5 }];
21  expect(sumBy(items, x => x.price)).toBe(4.0);
22});