Accumulator

function joinElements(arr, joinString) {
  function recurse(i, resultSoFar) {
    resultSoFar += arr[i];
    console.log(resultSoFar);
    if (i === arr.length - 1) {
      return resultSoFar;
    } else {
      return recurse(i + 1, resultSoFar + joinString);
    }
  }
  return recurse(0, "");
}

joinElements(["s", "cr", "et cod", " :) :)"], "e");