code: 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);
  }
docs: ES6 section 21.1.5.2.1 %StringIteratorPrototype%.next ( )