cartesianProduct.js

const cartesianProduct = arrays => {
  if (arrays.length === 0) {
    return [];
  }

  // Find the next set of combinations after this first array.
  const nextCombinations = cartesianProduct(arrays.slice(1));

  // Add values from the current array to the next combinations.
  return arrays[0].reduce(
    (result, value) => [
      ...result,
      ...(nextCombinations.length
        ? nextCombinations.map(combination => [value, ...combination])
        : [[value]]),
    ],
    [],
  );
};

export default cartesianProduct;

cartesianProduct.spec.js

import cartesianProduct from './cartesianProduct';

it('handles empty arrays correctly', () => {
  expect(cartesianProduct([[]])).toEqual([]);
  expect(cartesianProduct([[], [], []])).toEqual([]);
});

it('handles a single populated array correctly', () => {
  expect(cartesianProduct([[1, 2, 3]])).toEqual([[1], [2], [3]]);
});

it('handles mixed empty and populated arrays correctly', () => {
  expect(cartesianProduct([[1, 2, 3], [], [], []])).toEqual([[1], [2], [3]]);
});

it('handles multiple populated arrays', () => {
  expect(cartesianProduct([[1, 2, 3], [4, 5, 6]])).toEqual([
    [1, 4],
    [1, 5],
    [1, 6],
    [2, 4],
    [2, 5],
    [2, 6],
    [3, 4],
    [3, 5],
    [3, 6],
  ]);
});

it('handles multiple populated arrays with mixed types', () => {
  expect(cartesianProduct([['a', 'b', 3], [undefined, true]])).toEqual([
    ['a', undefined],
    ['a', true],
    ['b', undefined],
    ['b', true],
    [3, undefined],
    [3, true],
  ]);
});