uniq.util.js

const unique = (list, key) =>
  key
    ? list.map(e => e !== null && e[key])
      .map((e, i, final) => final.indexOf(e) === i && i)
      .filter(e => e !== null && list[e])
      .map(e => list[e])
    : [...new Set(list)].filter(entry => entry !== null)

describe('uniq()', () => {
  it('should work as expected', () => {
    assert.deepStrictEqual(helpers.uniq([1, 1, 2, 1, 3, 3]), [1, 2, 3])
    assert.deepStrictEqual(helpers.uniq([1, 2, 3, null]), [1, 2, 3])
    assert.deepStrictEqual(helpers.uniq(
      [{ id: 1 }, { id: 1 }, { id: 2 }, { id: 2 }, { id: 1 }, { id: 3 }, null],
      'id'
    ), [{ id: 1 }, { id: 2 }, { id: 3 }])
  })
})