Optimized for sparse arrays if separator is ''.
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, '');
}