code: function join(iterable, separator = ',') {
    const iterator = iterable[Symbol.iterator]();
    let head = iterator.next();
    if (head.done)
      return "";
  
    let s = head.value.toString();
  
    head = iterator.next();
    while(!head.done) {
      s += separator;
      s += head.value.toString();
      head = iterator.next();
    } // while
  
    return s;
  }
docs: test