code: function GetIterator(obj, method) {
    if (IS_UNDEFINED(method)) {
      method = obj[iteratorSymbol];
    }
    if (!IS_CALLABLE(method)) {
      throw MakeTypeError(kNotIterable, obj);
    }
    var iterator = %_Call(method, obj);
    if (!IS_RECEIVER(iterator)) {
      throw MakeTypeError(kNotAnIterator, iterator);
    }
    return iterator;
  }
docs: ---------------------------------------------------------------------------- Iterator related spec functions. ES6 7.4.1 GetIterator(obj, method)