// Takes an array of arbitrary depth
// Returns a flattened array of all @array's elements
const arrayFlatten = (array) => {
  let flattened = [...array];
  while (flattened.some(el => Array.isArray(el))) {
    flattened = flattened.reduce((arr, el) => arr.concat(el), []);
  }
  return flattened;
}

// Example mocha tests
describe('arrayFlatten()', () => {
  it('returns unmodified array if already flat', () => {
    const array = [1,2,3,4,5,6];
    expect(arrayFlatten(array)).to.deep.equal(array);
  });

  it('flattens a 2D array of numbers', () => {
    const array2D = [[1,2],[3,4],[5,6]];
    const flattened = [1,2,3,4,5,6];
    expect(arrayFlatten(array2D)).to.deep.equal(flattened);
  });

  it('flattens an array of arbitrary depth', () => {
    const deepArray = [[[[1,2,[3,4,[5,6],7],8],9,[10,11],12,13],[14,15]],16,17];
    const flattened = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17];
    expect(arrayFlatten(deepArray)).to.deep.equal(flattened);
  });

  it('does not directly mutate the passed-in array', () => {
    const array = [[1,2],[3,4],[5,6]];
    const array2 = [[1,2],[3,4],[5,6]];
    arrayFlatten(array);

    expect(array).to.deep.equal(array2);
  })
});