How to Implement Offline-First Authentication in PWAs

Progressive Web Apps (PWAs) are designed to deliver a seamless user experience, whether online or offline. One crucial aspect of this experience is authentication. Implementing offline-first authentication ensures that users can access certain features of your application, even without a reliable internet connection. Here’s how you can apply offline-first authentication in your PWAs.

Understanding Offline-First Strategy

Before diving into implementation, it's essential to understand the offline-first approach. This strategy prioritizes the user experience by ensuring the app remains functional without an internet connection. When the device is offline, the app utilizes local resources and data and syncs with the server once the connection is re-established.

Utilizing Service Workers

One of the key components of PWAs is the service worker. This is a script that your browser runs in the background, separate from a web page, and it enables features like caching and background sync. For offline-first authentication, you’ll want to set up your service worker to handle caching authentication responses.

Setting Up the Service Worker

Begin by registering your service worker in your main JavaScript file:


if ('serviceWorker' in navigator) {
    window.addEventListener('load', () => {
        navigator.serviceWorker.register('/sw.js').then((registration) => {
            console.log('Service Worker registered with scope:', registration.scope);
        }).catch((error) => {
            console.error('Service Worker registration failed:', error);
        });
    });
}

Caching Authentication Data

Modify the service worker to cache authentication tokens. When a user logs in, intercept the request, and store the token in the Cache Storage API.


self.addEventListener('fetch', (event) => {
    if (event.request.url.endsWith('/login')) {
        event.respondWith(
            fetch(event.request).then(response => {
                return response.clone().text().then(body => {
                    // Extract a token from the response and cache it
                    const token = JSON.parse(body).token;
                    caches.open('auth-cache').then(cache => {
                        cache.put('token', new Response(token));
                    });
                    return response;
                });
            })
        );
    }
});

Implementing Local Storage

In addition to caching tokens, consider using local storage or IndexedDB for storing user credentials or session information. This allows the user to remain authenticated even when offline. Here’s a basic implementation example:


function storeAuthToken(token) {
    localStorage.setItem('authToken', token);
}

Call this function after a successful login to save the token locally.

Handling Authentication State

To maintain an authenticated state, check local storage upon loading the app to see if a valid token exists:


function checkAuth() {
    const token = localStorage.getItem('authToken');
    if (token) {
        // Validate token or update user state accordingly
    } else {
        // Redirect to login page or show login prompt
    }
}
window.onload = checkAuth;

Syncing with the Server

Once the user is back online, synchronize any offline data with your backend server. Implement a background sync event to manage this:


self.addEventListener('sync', (event) => {
    if (event.tag === 'sync-auth') {
        event.waitUntil(syncAuthData());
    }
});

This might involve sending queued login requests or any user activity that occurred while offline.

Conclusion

Implementing offline-first authentication in PWAs enhances user experience by providing access to essential features without constant internet reliance. By leveraging service workers, local storage, and synchronization techniques, you can create a robust authentication system suited for both online and offline scenarios. This approach not only improves usability but also increases user engagement and satisfaction.

How to Implement Offline-First Authentication in PWAs