Authentication with xState, Firebase and Next.js

TL;DR

If you just want to see the code, it is here.

Background

In our team, we've been using xState and Firebase for the past year for the development of a React Native application, and the time for a web version is drawing closer. Although we haven't officially settled on a framework yet, Next.js 13 is definitely on the table, and I decided that it's worth spending some time experimenting with the basic techniques and workflows so that we can make an informed decision. To start with the authentication seems like the obvious choice.

Use case

To experiment with the authentication capabilities of the stack, we will build a simple website with two pages: a Sign In screen and a Dashboard page accessible only to authenticated users.

Disclaimers

Integrating Firebase in Next.js is quite straightforward and covered in enough online materials, so we won't discuss it in this post. The focus will be on integrating both technologies with xState. Just for clarity, here's the Firebase config file:

import { initializeApp, getApps } from "firebase/app";

const firebaseConfig = {
  apiKey: process.env.NEXT_PUBLIC_FIREBASE_API_KEY,
  authDomain: process.env.NEXT_PUBLIC_FIREBASE_AUTH_DOMAIN,
  projectId: process.env.NEXT_PUBLIC_FIREBASE_PROJECT_ID,
  storageBucket: process.env.NEXT_PUBLIC_FIREBASE_STORAGE_BUCKET,
  messagingSenderId: process.env.NEXT_PUBLIC_FIREBASE_MESSAGING_SENDER_ID,
  appId: process.env.NEXT_PUBLIC_firebaseApp_ID,
};

let firebaseApp =
  getApps().length === 0 ? initializeApp(firebaseConfig) : getApps()[0];

export default firebaseApp;

Also, to simplify things further, we will use the signInAnonymously method, which works the same way as authenticating with email/password or phone but doesn't require user input.

Implementation

xState machine

Usually, when working with xState and React, I reach a point where I need a globally accessible machine. That's why I start by creating an appMachine and pass it to the createActorContext method so that it can be used from the React context. This way, we can keep the authentication logic in a single place and send events to/from any page.

Key parts of the machine are the root GO_TO_AUTHENTICATED and GO_TO_UNAUTHENTICATED events. They lead the user to the correct state and, respectively, to the correct screen. They are intentionally flagged as internal to prevent infinite loops.

The userSubscriber service (which will be explained in a bit) plays the role of an orchestrator, which is in charge of listening for the user object from Firebase and targeting the appropriate state with the one of the already mentioned events.

The machine consists of three main states. loading is the initial state that is active until we know the user's status. After that, we transition to one of the other two states - authenticated or unauthenticated. They both have substates and are idle initially. When the user is authenticated, they can call the SIGN_OUT event to transition to the signingOut substate, which is in charge of invoking the signOut method. The unauthenticated structure is similar, with the difference that instead of signing out, it contains the signing-in logic.

import React, { PropsWithChildren } from "react";
import { createMachine } from "xstate";
import { createActorContext } from "@xstate/react";

const appMachine = createMachine({
  invoke: { src: "userSubscriber" },
  on: {
    GO_TO_AUTHENTICATED: { target: "authenticated", internal: true },
    GO_TO_UNAUTHENTICATED: { target: "unauthenticated", internal: true },
  },
  initial: "loading",
  states: {
    loading: { tags: "loading" },
    authenticated: {
      on: { SIGN_OUT: { target: ".signingOut" } },
      initial: "idle",
      states: {
        idle: {},
        signingOut: {
          invoke: { src: "signOut" },
          onDone: { target: "idle" },
        },
      },
    },
    unauthenticated: {
      on: { SIGN_IN: { target: ".signingIn" } },
      initial: "idle",
      states: {
        idle: {},
        signingIn: { invoke: { src: "signIn" }, onDone: { target: "idle" } },
      },
    },
  },
  {
  /* machine options */
  }
});

export const AppContext = createActorContext(appMachine);

export function AppProvider({ children }: PropsWithChildren<{}>) {
  return <AppContext.Provider>{children}</AppContext.Provider>;
}

Firebase

In order to retrieve the current user, Firebase recommends using the onAuthStateChanged observer, which is a perfect fit for a callback service. That simplifies our work drastically. In the callback, we just have to check the user value. If it is null, the user is unauthenticated; otherwise, we trigger the GO_TO_AUTHENTICATED event.

For the signIn service, as mentioned before, we go with the signInAnonymously method, and for the signOut service, we resolve the auth.signOut() promise. Both of these will reflect on the user that is being observed in the userSubscriber service.

import { onAuthStateChanged, getAuth, signInAnonymously } from "firebase/auth";
import firebaseApp from "@/firebase";

const auth = getAuth(firebaseApp);

const appMachine = createMachine(
  {
  /* machine definition */
  },
  {
    services: {
      userSubscriber() {
        return (sendBack) => {
          const unsubscribe = onAuthStateChanged(auth, (user) => {
            if (user) {
              sendBack({ type: "GO_TO_AUTHENTICATED" });
            } else {
              sendBack({ type: "GO_TO_UNAUTHENTICATED" });
            }
          });
          return () => unsubscribe();
        };
      },
      async signIn() {
        await signInAnonymously(auth);
      },
      async signOut() {
        await auth.signOut();
      },
    },
  }
);

Next.js

From here, we can continue by consuming the context. Wrapping the children with AppProvider in the RootLayout gives access to the global machine from all layouts and pages.

import { AppProvider } from "@/contexts/app";
import { Loader } from "@/components/Loader";

export default function RootLayout({
  children,
}: {
  children: React.ReactNode;
}) {
  return (
    <html lang="en">
      <body className={inter.className}>
        <AppProvider>
          <Loader>{children}</Loader>
        </AppProvider>
      </body>
    </html>
  );
}

The purpose of the <Loader> component is to prevent pages from rendering before the user data is loaded. The AppContext.useActor() hook always updates on state changes, and when the state is loading, we just display a placeholder screen.

"use client";

import { PropsWithChildren } from "react";
import { AppContext } from "@/contexts/app";

export function Loader({ children }: PropsWithChildren<{}>) {
  const [state] = AppContext.useActor();

  return state.matches("loading") ? (
    <main>
      <div>Loading...</div>
    </main>
  ) : (
    children
  );
}

Once one of the authenticated or unauthenticated states is active, the corresponding page will be loaded. If the user already exists, they will be navigated to the dashboard, which is located at the root - /. Otherwise, they should be redirected to the /sign-in page.

From my past experience, mixing xState with the navigation lifecycle of another framework is one of the most troublesome parts when setting the initial application architecture.

I had a couple of ideas for different compositions before settling on the current implementation, but I found that I have minimal control when navigating with useRouter() in the new App Router architecture.

I wanted to trigger an event when the router.push() method is done, but it turned out that it no longer returns a promise. Also, the alternative of using the router.events didn't provide the fine control that I expected, so there was again flickering when navigating between pages.

Despite my efforts to abstract the routing in a single component or context, I ended up using protected routes. They never scale well for me, but I think they are sufficient in this scenario.

In order to prevent premature loading of the screen, we assume with the isRedirecting flag that the user is on the wrong page until proven wrong. We listen for state.value changes and redirect the user to the sign-in url if the state is unauthenticated.

In order to prevent premature loading of the screen, we assume with the isRedirecting flag that the user is on the wrong page until proven otherwise. We listen for state.value changes and redirect the user with the sign-in href if the state is unauthenticated.

export default function Home() {
  const [state, send] = AppContext.useActor();
  const router = useRouter();
  const [isRedirecting, setIsRedirecting] = useState(true);

  useEffect(() => {
    if (state.matches("unauthenticated")) {
      router.push("/sign-in");
    } else if (state.matches("authenticated")) {
      setIsRedirecting(false);
    }
  }, [state.value]);

  if (isRedirecting) {
    return null;
  }

  return (
    <main>
      <h3>Dashboard:</h3>
      <button
        onClick={() => {
          send({ type: "SIGN_OUT" });
        }}
      >
        {state.matches("authenticated.signingOut") ? "...loading" : "sign out"}
      </button>
    </main>
  );
}

Conclusion

From the little that I tried out, I'm satisfied with the results so far, but I still have concerns about how the application will grow when adding more pages and interactions with Firebase.