SuperTokens is an open source authentication solution offering features like: Different types of login: Email / password, Passwordless (OTP or Magic link based).

SuperTokens.com

Is there a way to call a function on the frontend when the user is logged out due to the refresh (not access) token expiring?

When the refresh token expires, you get back a 401. You should redirect the user to the loving screen

Where do I get this 401 exactly? I have a function that I need to call when the refresh token expires

you will get a 401 as a result of the axios / fetch call

So currently I am using the provided axios interceptors

yea, so when u make the axios call, it will return a 401 (i think axios throws)