[destus]

Что за оптимальная загрузка?
Так работает?

<script src="assets/js/modules/config_modules.js"></script>
<script data-main="assets/js/page_js/basket.js" src="assets/js/require.js"></script>

и в config_modules.js

var require = {
    baseUrl: "assets/js/modules",
    paths: {
        constants: "constants",
        firebase: "firebase/firebase-app",
        firebase_auth: "firebase/firebase-auth",
        firebase_database: "firebase/firebase-database",
        databaseModule: "databaseModule",
        user_entity: "entities/user",
        order_entity: "entities/order"

    },
    callback: function(constants) {

        //console.log(constants.url);
        //console.log("eeee");
    }
};