silverstripe-webpack/site/src/js/main.js

import $ from 'jquery';

import Events from './_events';
// import an example component
import Spinner from './_components/_spinner';

const MainUI = (($) => {
    // Constants
    const NAME = 'MainUI';

    class MainUI {
        // Static methods
        static init() {
            this.destroy();
            console.log(`Initializing: ${NAME}`);

            Spinner.hide(() => {
                $('body').addClass('loaded');
            });
        }

        static destroy() {
            console.log(`Destroying: ${NAME}`);
            Spinner.show(() => {
                $('body').removeClass('loaded');
            });
        }
    }

    $(window).on(`${Events.AJAX} ${Events.LOADED}`, () => {
        MainUI.init();
    });

    return MainUI;
})($);

export default MainUI;