filterObject

1/**
2 * Filters object properties based on a predicate function.
3 *
4 * @param obj - The source object.
5 * @param fn - A predicate function that returns true to keep a key-value pair.
6 * @returns A new object with only the properties for which the predicate returns true.
7 */
8export function filterObject<T>(
9  obj: Record<string, T>,
10  fn: (val: T, key: string) => boolean
11): Record<string, T> {
12  return Object.keys(obj).reduce((acc, key) => {
13    if (fn(obj[key], key)) {
14      acc[key] = obj[key];
15    }
16    return acc;
17  }, {} as Record<string, T>);
18}

1test('filterObject - filters values based on predicate', () => {
2  const input = { a: 1, b: 2, c: 3 };
3  const result = filterObject(input, val => val % 2 === 1);
4  expect(result).toEqual({ a: 1, c: 3 });
5});
6
7test('filterObject - predicate can use keys', () => {
8  const input = { foo: 1, bar: 2 };
9  const result = filterObject(input, (_, key) => key.startsWith('f'));
10  expect(result).toEqual({ foo: 1 });
11});
12
13test('filterObject - returns empty object when none match', () => {
14  const input = { a: 1, b: 2 };
15  const result = filterObject(input, () => false);
16  expect(result).toEqual({});
17});
18
19test('filterObject - works with empty input', () => {
20  expect(filterObject({}, () => true)).toEqual({});
21});
22
23test('filterObject - original object is not mutated', () => {
24  const input = { a: 1 };
25  const copy = { ...input };
26  filterObject(input, () => true);
27  expect(input).toEqual(copy);
28});