index.mjs

// Express Node app
app.use('/vue', express.static(__dirname + '/node_modules/vue/dist/vue.esm.browser.js'));
app.use('/vue-router', express.static(__dirname + '/node_modules/vue-router/dist/vue-router.esm.js'));

index.pug

script(src="/main.js" type="module")

main.js

import Vue from "./vue"
import VueRouter from "./vue-router";
Vue.use(VueRouter);
const routes = [...];
const router = new VueRouter({
    mode: "history",
    routes: routes
});
let app = new Vue({ router });
app.$mount('#jg-app');

main.works.js

import Vue from "./vue";
let app = new Vue();
app.$mount('#jg-app');