Create the caresian product of TypeScript arrays

cartesian-product.ts

function product<K extends string, T>(arrays: Record<K, T[]>): Record<K, T>[] {
  return Object.entries(arrays).reduce(
    // @ts-expect-error Object.entries is resolving K to string
    (acc, [field, arr]: [K, T[]]): Record<K, T>[] =>
      arr.flatMap((value) =>
        acc.map((existing) => ({ ...existing, [field]: value })),
      ),
    [{}] as Record<K, T>[],
  ) as Record<K, T>[];
}

cartesianProduct({ a: [1, 2], b: [1, 2] }); // [{ a: 1, b: 1 }, { a: 2, b: 1 }, { a: 1, b: 2 }, { a: 2, b: 2 }]

tests.ts

describe('cartesian-product', () => {
  test('small', () => {
    const res = product({ a: [1, 2], b: [1, 2] });
    console.log(res);
    expect(res).toEqual([
      { a: 1, b: 1 },
      { a: 2, b: 1 },
      { a: 1, b: 2 },
      { a: 2, b: 2 },
    ]);
  });
  test('full', () => {
    const res = product({ a: [1, 2], b: [3, 4], c: [5, 6] });
    expect(res).toEqual([
      { a: 1, b: 3, c: 5 },
      { a: 2, b: 3, c: 5 },
      { a: 1, b: 4, c: 5 },
      { a: 2, b: 4, c: 5 },
      { a: 1, b: 3, c: 6 },
      { a: 2, b: 3, c: 6 },
      { a: 1, b: 4, c: 6 },
      { a: 2, b: 4, c: 6 },
    ]);
  });
  test('length', () => {
    expect(
      product({ a: [1, 2, 3], b: [3, 4, 5], c: [5, 6, 7, 8, 9] }),
    ).toHaveLength(45);
  });
});