code: function AdvanceStringIndex(string, index, unicode) {
    var increment = 1;
    if (unicode) {
      var first = %_StringCharCodeAt(string, index);
      if (first >= 0xD800 && first <= 0xDBFF && string.length > index + 1) {
        var second = %_StringCharCodeAt(string, index + 1);
        if (second >= 0xDC00 && second <= 0xDFFF) {
          increment = 2;
        }
      }
    }
    return increment;
  }
docs: ES#sec-advancestringindex AdvanceStringIndex ( S, index, unicode )