I would not trust navigator.online I believe those things are not reliable under various circumstances (eg wifi up - but routing down, flaky cell connection etc etc.)
guess that is an additional advantage to the ajax request method.. since a timeout or any error (anything but 20x OK, or 401) means you are offline..

(btw remembered why I do the auth check in an ajax call - it’s because I can server side invalidate tokens (guardian_db))

So you could just verify token client side - but if you also want a reliable online/offline check I think the ajax call solves both issues in one go..

I don’t see a way to return a message in {:error, message} in phoenix, it expects {:ok, socket} or :error.

If you’re using a jwt you can check if it’s expired on the frontend. Where to do the check depends on your app. But you can always add the onError callback to the socket.

socket.onError((error) => { 
    // check jwt expiration and redirect to login
    // or
    // indicate to the user they are "offline"
   })

Yeah, it is pretty generic. There are a few jwt libraries out there that would help. Visit https://jwt.io/ and see which one fits your use case. Pretty much any of them from Auth0 are solid. I’ve had success with jsonwebtoken.

Or as @outlog mentioned you could make a request on error and let the server do that work.