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"