function StringIteratorNext() {
  var iterator = this;
  var value = UNDEFINED;
  var done = true;

  if (!IS_RECEIVER(iterator) ||
      !HAS_DEFINED_PRIVATE(iterator, stringIteratorNextIndexSymbol)) {
    throw MakeTypeError(kIncompatibleMethodReceiver,
                        'String Iterator.prototype.next');
  }

  var s = GET_PRIVATE(iterator, stringIteratorIteratedStringSymbol);
  if (!IS_UNDEFINED(s)) {
    var position = GET_PRIVATE(iterator, stringIteratorNextIndexSymbol);
    var length = TO_UINT32(s.length);
    if (position >= length) {
      SET_PRIVATE(iterator, stringIteratorIteratedStringSymbol, UNDEFINED);
    } else {
      var first = %_StringCharCodeAt(s, position);
      value = %_StringCharFromCode(first);
      done = false;
      position++;

      if (first >= 0xD800 && first <= 0xDBFF && position < length) {
        var second = %_StringCharCodeAt(s, position);
        if (second >= 0xDC00 && second <= 0xDFFF) {
          value += %_StringCharFromCode(second);
          position++;
        }
      }

      SET_PRIVATE(iterator, stringIteratorNextIndexSymbol, position);
    }
  }
  return %_CreateIterResultObject(value, done);
}
function StringIteratorNext() {
  var iterator = this;
  var value = UNDEFINED;
  var done = true;

  if (!IS_RECEIVER(iterator) ||
      !HAS_DEFINED_PRIVATE(iterator, stringIteratorNextIndexSymbol)) {
    throw MakeTypeError(kIncompatibleMethodReceiver,
                        'String Iterator.prototype.next');
  }

  var s = GET_PRIVATE(iterator, stringIteratorIteratedStringSymbol);
  if (!IS_UNDEFINED(s)) {
    var position = GET_PRIVATE(iterator, stringIteratorNextIndexSymbol);
    var length = TO_UINT32(s.length);
    if (position >= length) {
      SET_PRIVATE(iterator, stringIteratorIteratedStringSymbol, UNDEFINED);
    } else {
      var first = %_StringCharCodeAt(s, position);
      value = %_StringCharFromCode(first);
      done = false;
      position++;

      if (first >= 0xD800 && first <= 0xDBFF && position < length) {
        var second = %_StringCharCodeAt(s, position);
        if (second >= 0xDC00 && second <= 0xDFFF) {
          value += %_StringCharFromCode(second);
          position++;
        }
      }

      SET_PRIVATE(iterator, stringIteratorNextIndexSymbol, position);
    }
  }
  return %_CreateIterResultObject(value, done);
}
function StringIteratorNext ( ) { var iterator = this ; var value = UNDEFINED ; var done = true ; if ( ! IS_RECEIVER ( iterator ) || ! HAS_DEFINED_PRIVATE ( iterator , stringIteratorNextIndexSymbol ) ) { throw MakeTypeError ( kIncompatibleMethodReceiver , 'String Iterator.prototype.next' ) ; } var s = GET_PRIVATE ( iterator , stringIteratorIteratedStringSymbol ) ; if ( ! IS_UNDEFINED ( s ) ) { var position = GET_PRIVATE ( iterator , stringIteratorNextIndexSymbol ) ; var length = TO_UINT32 ( s . length ) ; if ( position >= length ) { SET_PRIVATE ( iterator , stringIteratorIteratedStringSymbol , UNDEFINED ) ; } else { var first = % _StringCharCodeAt ( s , position ) ; value = % _StringCharFromCode ( first ) ; done = false ; position ++ ; if ( first >= 0xD800 && first <= 0xDBFF && position < length ) { var second = % _StringCharCodeAt ( s , position ) ; if ( second >= 0xDC00 && second <= 0xDFFF ) { value += % _StringCharFromCode ( second ) ; position ++ ; } } SET_PRIVATE ( iterator , stringIteratorNextIndexSymbol , position ) ; } } return % _CreateIterResultObject ( value , done ) ; }
ES6 section 21.1.5.2.1 %StringIteratorPrototype%.next ( )
ES6 section 21 . 1 . 5 . 2 . 1 %StringIteratorPrototype% . next ( )