export function flatten(input) {
  let output = [];

  input.forEach((item) => {
    if (Array.isArray(item)) {
      output = output.concat(flatten(item));
    } else {
      output = output.concat([item]);
    }
  });

  return output;
}

describe('Array flatten', () => {
  it('should flatten an array no matter the type and/or depth of the entries', () => {
    expect(flatten([1, 2, 3, 4])).toEqual([1, 2, 3, 4]);
    expect(flatten([1, 2, [3], 4])).toEqual([1, 2, 3, 4]);
    expect(flatten([1, 2, [3], [4, 5, 6]])).toEqual([1, 2, 3, 4, 5, 6]);
    expect(flatten([1, 2, [3], [4, [5, 6, [7, 8, [9]]], 10]])).toEqual([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
    expect(flatten([1, 2, [3, 'str'], { prop: 'value' }])).toEqual([1, 2, 3, 'str', { prop: 'value' }]);
  });
});