join

Joins all elements of an array into a string and returns this string. Uses a separator and an end separator.

• Use Array.prototype.reduce() to combine elements into a string.
• Omit the second argument, separator, to use a default separator of ','.
• Omit the third argument, end, to use the same value as separator by default.

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'