What is the professional way in javascript to manage user access? [closed]

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Hello, I have a web application in vanilla Javascript with Firebase. What I try to do is restrict access to users who are not registered or logged in. For this I simply ask in the dashboard.html if the user is logged in, but if not redirect to (login.html). If the user logs in then I do another redirect on that page, that sends it to dashboard.html

I suppose there is a better way to do this, I think I am not doing it well working with independent html pages

who can recommend me. thank you!