You can implement one

String.prototype.htmlEncode = function () {
  return this.replace(/./g, function (c) { return '&#' + c.charCodeAt()
+ ';'; });
}

var s = 'ö';
s.htmlEncode()

if you want. Note that the above escapes any character, you might want
to change that depending on which characters you regard as 'special'.

And passing a function to the replace method is not supported in JScript
5 but only with JScript 5.5 and later.