export default function flattenOnce(arr) {
  // TODO: Flatten one level deep
  throw new Error('Not implemented');
}

import flattenOnce from './flattenOnce';

describe('flattenOnce', () => {
  test('flattens a simple nested array', () => {
    expect(flattenOnce([1, [2, 3], [4, [5]]])).toEqual([1, 2, 3, 4, [5]]);
  });

  test('returns same array when already flat', () => {
    expect(flattenOnce([1, 2, 3])).toEqual([1, 2, 3]);
  });

  test('handles mixed array contents', () => {
    expect(flattenOnce([[1], 2, [3, [4]], 5])).toEqual([1, 2, 3, [4], 5]);
  });

  test('returns empty array for empty input', () => {
    expect(flattenOnce([])).toEqual([]);
  });
});