Gtk.IconFactory.prototype.add

function add(stock_id:String, icon_set:Gtk.IconSet):void {
    // Gjs wrapper for gtk_icon_factory_add()
}

Adds the given icon_set to the icon factory, under the name stock_id. stock_id should be namespaced for your application, e.g. "myapp-whatever-icon". Normally applications create a Gtk.IconFactory, then add it to the list of default factories with Gtk.add_default. Then they pass the stock_id to widgets such as Gtk.Image to display the icon. Themes can provide an icon with the same name (such as "myapp-whatever-icon") to override your application's default icons. If an icon already existed in factory for stock_id, it is unreferenced and replaced with the new icon_set.