Function.prototype.method = function (name, func) {
        this.prototype[name] = func;
        return this;
    };

String.method('deentityify', function () {
        // The entity table. It maps entity names to
        // characters.
        var entity = {
            quot: '"',
            lt: '<',
            gt: '>'
        };
        return function () {
            // This is the deentityify method. It calls the string
            // replace method, looking for substrings that start
            // with '&' and end with ';'. If the characters in
            // between are in the entity table, then replace the
            // entity with the character from the table. It uses
            // a regular expression (Chapter 7).
            return this.replace(/&([^&;]+);/g,
                    function (a, b) {
                        var r = entity[b];
                        return typeof r === 'string' ? r : a;
                    }
            );
        };
    }());

'<">'.deentityify()

String.method('deentityify', function () {
        // The entity table. It maps entity names to
        // characters.
        var entity = {
            quot: '"',
            lt: '<',
            gt: '>'
        };
        return function () {
            // This is the deentityify method. It calls the string
            // replace method, looking for substrings that start
            // with '&' and end with ';'. If the characters in
            // between are in the entity table, then replace the
            // entity with the character from the table. It uses
            // a regular expression (Chapter 7).
            return this.replace(/&([^&;]+);/g,
                    function (a, b) {
                        var r = entity[b];
                        return typeof r === 'string' ? r : a;
                    }
            );
        };
    }());

'any string'.deentityfy()

String.deentityfy()