Sveltekit + Supabase Auth Starter

This is a starter template for SvelteKit apps with Supabase authentication. Uses the new Supabase SSR libraries rather than the deprecated auth helpers.

Running locally

Install the supabase cli: (https://supabase.com/docs/guides/cli/getting-started)
Copy .env.example to .env
Start supabase services in your project directory: supabase start
Update the values for PUBLIC_SUPABASE_URL and PUBLIC_SUPABASE_ANON_KEY with the values from the generated supabase services.
Run your dev server: npm run dev

Demonstration

Navigate to the dev server at (http://localhost:5173)
Either navigate to the protected route, or click 'sign in'. Either will bring you to the login page. (NOTE: you may experience a reload in development only when the supabase auth helper dependency is re-optimized by vite - this does not occur in production builds.)
In the login page, enter a test email.
Supabase sends email events in development to a locally hosted mail service at (http://localhost:54324/monitor). Click the link in the test email to log in.

How it works

The supabase client is first initialized on the server in hooks.server.ts to load the authentication information from the request. This client is attached to event.locals (defined in src/app.d.ts) so that it can be used in any request handling functions.

The client is then used in the root layout server load function, +layout.server.ts, to provide the session information to the browser client. The browser client is created in the root layout load function, +layout.ts, and returned so that all child pages and routes can access the supabase client and the session.

The root layout, +layout.svelte, subscribes to the supabase client's auth state changes and triggers an invalidation of all auth information when necessary. This is done via a dependency specified in appropriate areas on supabase:auth. This ensures that e.g. the auth guard on the /protected route is re-run whenever a session change is detected, so the user can be navigated away if they are no longer logged in.

Magic Link Auth Notes

Magic links work by saving a code verifier in the browser that requests the link, and using that along with the query params in the generated email link to verify the user.

This can cause problems when users open the link in a different browser or device than the one they requested the link from - in particular, mobile app email clients use in-app browsers with different cookie stores than the main browser, so the code verifier is not present and the link will not work.

To mitigate this, the auth/callback route conditionally renders a page if the code verifier is not present in the browser's cookies, rather than immediately attempting to call supabase.auth.exchangeCodeForSession which will fail if the code verifier is absent, and mark the link as used in the process (meaning the user cannot recover from this error by opening the current page in a new browser window). By deferring this call until the code verifier is present, we give the user an opportunity to open the link in the correct browser or device.

To test this, open the magic link (from inbucket) in a different browser or device than the one you requested it from. You should see a page with a message about the code verifier not being present. Then, open the link in the same browser or device you requested it from, and you should be logged in.

Notes on session / user safety and object member use

Supabase recently added warnings when a user object is used from a fetched session on the server, since the session is restored from cookies without being validated. This occurs even if the user is validated (as they recommend) due to the way that the warning was implemented. See: supabase/auth-js#873

In this starter kit, the problem is resolved in the following way: