code: function (TO_STRING) {
    return function (that, pos) {
      var s = String(_defined(that));
      var i = _toInteger(pos);
      var l = s.length;
      var a, b;
      if (i < 0 || i >= l) return TO_STRING ? '' : undefined;
      a = s.charCodeAt(i);
      return a < 0xd800 || a > 0xdbff || i + 1 === l || (b = s.charCodeAt(i + 1)) < 0xdc00 || b > 0xdfff
        ? TO_STRING ? s.charAt(i) : a
        : TO_STRING ? s.slice(i, i + 2) : (a - 0xd800 << 10) + (b - 0xdc00) + 0x10000;
    };
  }
docs: true  -> String#at false -> String#codePointAt