Composition pattern to extend library objects

// new class-like function
var Container = function(id) {
    // jQuery object
    this._element = $('#' + id);
};
// add your own method
Container.prototype.changeBackground = function(bg) {
    this._element.css(background:  bg);
};

// use the new class
var container = new Container;
// change background to red
container.changeBackground('#f00');