A simple flatten array

gistfile1.txt

/**
 * Given a array of array values, will return a new instance with a flatten array.
 *
 * @param {Array} The array which will be flatten
 * @param {Array} The new flatten array                               (private)
 * @param {number} A position index that use to traverse given array. (private)
 * @return {Array} The new array flatten
 * @public
 */
function flatten(numbers, result, index) {
  // init
  if (result === undefined) {
    result = [];
    index = 0;
    return flatten(numbers || [], result, index);
  }

  // end
  if (!numbers || numbers.length === index) {
    return result;
  }

  // action
  let currentNode = numbers[index];
  if (Array.isArray(currentNode)) {
    currentNode = flatten(currentNode, [], 0);
    result = result.concat(currentNode);
  } else {
    result.push(currentNode);
  }
  return flatten(numbers, result, ++index);
}


describe("Flatten array", () => {

  it("should flatten array with single nested level", () => {
     const SUT = [[1, 2, 3], 4];
     
     const result = flatten(SUT);

     expect(result[0]).toEqual(1);
     expect(result[1]).toEqual(2);
     expect(result[2]).toEqual(3);
     expect(result[3]).toEqual(4);
     expect(result.length).toEqual(4);
  });

  it("should flatten multi-level nested array with single item", () => {
    const SUT = [[[3]]];

    const result = flatten(SUT);

    expect(result[0]).toEqual(3);
    expect(result.length).toEqual(1);
  });

  it("should have tolerance to non-array", () => {
    const result = flatten();

    expect(result.length).toEqual(0);
  });

  it("should have tolerance to an empty array", () => {
    const SUT = [];

    const result = flatten(SUT);

    expect(result.length).toEqual(0);
  });
});