auth.js

// import something here
import firebase from "firebase/app";
import "firebase/auth";
// "async" is optional
export default async ({ router, store /* app, router, Vue, ... */ }) => {
  // something to do

  firebase.auth().onAuthStateChanged(user => {
    store.dispatch("auth/signInFromSession", user);
  });

  router.beforeEach((to, from, next) => {
    return next();
  });
};