Object.prototype.only.js

(function() {
    function only(propertiesToKeep){
	var object = this;
	
	if (object === window) return object;
 
	Object.keys(object).forEach(function(key) {
	    if (propertiesToKeep.indexOf(key) === -1) delete object[key];
	});
	
	return object;
}

if ('defineProperty' in Object) {
    Object.defineProperty(Object.prototype, 'only', {
        value: only,
        enumerable: false
    });
} else {
    Object.prototype.only = only;
}

})();