code: function SparseJoin(array, keys, convert) {
    var keys_length = keys.length;
    var elements = new InternalArray(keys_length);
    for (var i = 0; i < keys_length; i++) {
      var e = array[keys[i]];
      elements[i] = IS_STRING(e) ? e : convert(e);
    }
    return %StringBuilderConcat(elements, keys_length, '');
  }
docs: Optimized for sparse arrays if separator is ''.