deentityfy.js

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

String.method('deentityify', function (  ) {

    var entity = {
        quot: '"',
        lt:   '<',
        gt:   '>'
    };

    return function (  ) {

        return this.replace(/&([^&;]+);/g,
            function (a, b) {
                var r = entity[b];
                return typeof r === 'string' ? r : a;
            }
        );
    };
    
}() );

console.log( '&lt;&quot;&gt;'.deentityify() );