JS Unique with Recursion (es6)

unique.js

// unique.js
const unique = (array, newArray = [], index = 0) => {
  if (index === array.length) return newArray;

  if (!newArray.includes(array[index])) newArray.push(array[index]);
  index += 1;
  return unique(array, newArray, index);
}

export default unique;

// unique.spec.js
import unique from './unique';

describe('unique', () => {
  describe('array with zero elements', () => {
    test('returns an empty array', () => {
      expect(unique([])).toEqual([]);
    });
  });

  describe('array with one element', () => {
    test('returns an array with one element', () => {
      expect(unique([1])).toEqual([1]);
    });
  });

  describe('array with more than one element', () => {
    test('returns an array with unique elements', () => {
      expect(unique([1, 2, 2, 3, 4, 5, 5])).toEqual([1, 2, 3, 4, 5]);
    });
  });
});