describe("flatten an array of arbitrarily nested arrays of integers into a flat array of integers", function () {
  it("Should return empty array when given empty array provided", function () {
    expect(flatten([])).toEqual([]);
  });

  it("Should return a flat array when given a flat array provided", function () {
    expect(flatten([1])).toEqual([1]);
  });

  it("Should return an empty array when given undefined", function () {
    expect(flatten(undefined)).toEqual([]);
  });

  it("Should return [1,2,3,4] on [[[1],2,3,4]] provided", function () {
    expect(flatten([[[1], 2, 3, 4]])).toEqual([1, 2, 3, 4]);
  });

  it("Should return [1,2,3,4] on [[1,2,[3]],4] provided", function () {
    expect(flatten([[1, 2, [3]], 4])).toEqual([1, 2, 3, 4]);
  });
  it("Should return [1,2,3,4] on [[1,2,[3],4]] provided", function () {
    expect(flatten([[1, 2, [3], 4]])).toEqual([1, 2, 3, 4]);
  });
});


function flatten(arr) {
  if (!Array.isArray(arr)) return [];
  return arr.reduce(function (acc, x) {
    return acc.concat(Array.isArray(x) ? flatten(x) : [x]);
  }, []);
};