const join = (arr, separator = ',', end = separator) =>
  arr.reduce(
    (acc, val, i) =>
      i === arr.length - 2
        ? acc + val + end
        : i === arr.length - 1
          ? acc + val
          : acc + val + separator,
    ''
  );

join(['pen', 'pineapple', 'apple', 'pen'], ',', '&'); // "pen,pineapple,apple&pen"
join(['pen', 'pineapple', 'apple', 'pen'], ','); // "pen,pineapple,apple,pen"
join(['pen', 'pineapple', 'apple', 'pen']); // "pen,pineapple,apple,pen"