var popup = {
    init: function()
    {
        this.element = somelib.createElement('div');
        // ...
        this.element.addEventListener('click', function()
        {
            this.hide();
        }.bind(this));
    },
    show: function(){},
    hide: function(){}
};

popup.init();

var superPopup = somelib.extend(popup, {
    hide: function()
    {
        // ...
    }
});