Flatten Array

test.js

import { flatten } from 'utils';

describe('flatten', () => {
  it('flatten', () => {
    expect(flatten([])).toEqual([]);
    expect(flatten([[], []])).toEqual([]);
    expect(flatten([[1, 2, [3]], 4])).toEqual([1, 2, 3, 4]);
    expect(flatten([[1, 2, 3], [4, [5, 6, [7, [8]]]], 9])).toEqual([1, 2, 3, 4, 5, 6, 7, 8, 9]);
  });
});

utils.js

// flatten an array of arbitrarily nested arrays 
export function flatten(array) {
  if (!array.length) { return []; }

  return array.reduce((prev, el) => {
    if (Array.isArray(el)) {
      return prev.concat(flatten(el));
    }

    return prev.concat(el);
  }, []);
}