unionBy

1/**
2 * Returns an array of distinct elements from both arrays using a key selector.
3 * If elements from both arrays share the same key, only the first occurrence is kept.
4 *
5 * @param a - First array.
6 * @param b - Second array.
7 * @param keyFn - Function to extract a comparable key from each element.
8 * @returns A new array with unique elements by key.
9 */
10export function unionBy<T>(
11  a: T[],
12  b: T[],
13  keyFn: (item: T) => string | number
14): T[] {
15  const seen = new Set<string | number>();
16  const result: T[] = [];
17
18  for (const item of [...a, ...b]) {
19    const key = keyFn(item);
20    if (!seen.has(key)) {
21      seen.add(key);
22      result.push(item);
23    }
24  }
25
26  return result;
27}

1test('unionBy - with objects and id key', () => {
2  const arr1 = [{ id: 1 }, { id: 2 }];
3  const arr2 = [{ id: 2 }, { id: 3 }];
4  expect(unionBy(arr1, arr2, item => item.id)).toEqual([
5    { id: 1 },
6    { id: 2 },
7    { id: 3 },
8  ]);
9});
10
11test('unionBy - using length as key', () => {
12  const arr1 = ['a', 'bb'];
13  const arr2 = ['ccc', 'dd'];
14  expect(unionBy(arr1, arr2, str => str.length)).toEqual(['a', 'bb', 'ccc']);
15});
16
17test('unionBy - one array empty', () => {
18  expect(unionBy([], [{ id: 1 }], item => item.id)).toEqual([{ id: 1 }]);
19});
20
21test('unionBy - both arrays empty', () => {
22  expect(unionBy([], [], item => item)).toEqual([]);
23});