Flatten

flatten-test.js

const flatten = require('./flatten');

describe('flatten', () => { 
  it('should flatten deeply nested arrays of integers', () => {
    const arrays = [1, 2, 3, [4, 5, 6, [7, 8, 9], [10, 11, 12]]]
    const expected = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12];

    expect(flatten(arrays)).toEqual(expected);
  });
});

flatten.js

module.exports = function flatten(arrays) {
  return arrays.reduce((acc, item) => {
    const isArray = Array.isArray(item);
    const newItems = isArray ? flatten(item) : [item];

    return acc.concat(newItems);
  }, [])
}

See a running implementation at https://repl.it/@Quiver/FlattenArrays