code: function StringCodePointAt(pos) {
    CHECK_OBJECT_COERCIBLE(this, "String.prototype.codePointAt");
  
    var string = TO_STRING(this);
    var size = string.length;
    pos = TO_INTEGER(pos);
    if (pos < 0 || pos >= size) {
      return UNDEFINED;
    }
    var first = %_StringCharCodeAt(string, pos);
    if (first < 0xD800 || first > 0xDBFF || pos + 1 == size) {
      return first;
    }
    var second = %_StringCharCodeAt(string, pos + 1);
    if (second < 0xDC00 || second > 0xDFFF) {
      return first;
    }
    return (first - 0xD800) * 0x400 + second + 0x2400;
  }
docs: ES6 Draft 05-22-2014, section 21.1.3.3