I am using HTTP Basic Auth to authenticate the user. Once authenticated, I set req.session.loggedIn = true on the server-side. If a user makes requests after being logged in, for security purposes, should I just check the session loggedIn value or send the basic auth header on each request and then verify credentials on each request on my backend?
As mti2935 mentions, the Basic Auth header is automatically sent by the browser with every request. That means setting a session cookie is redundant.
However, usage of HTTP Basic Auth should be discouraged, as is discussed in this answer.
If your variable, req.session.loggedIn is client-side controllable, then you have a gaping authentication flaw.
Assuming that is the case -- you need to generate a unique session ID after successful login. When a user requests a page, you will check if the session ID is valid.
It's worth noting: we want to reduce transmittance of credentials wherever possible. Generally speaking, session ID usually less valuable than credentials.