How to send a flash error when passport.deserializeUser() fails? - node.js

Question

Using passport in an express app. For reasons, the session tokens expire after one hour.
If the user is active when the session expired, the deserialize function "fails", i.e., user is undefined.
passport.deserializeUser(function (id, done) {
const user = sessionManager.userLookup(id);
done(null, user);
});
The trouble is that when user is undefined, then there is no req.user for subsequent middleware. So to the code it simply appears that the user is not signed in, with no breadcrumbs to indicate that the session just expired. The app simply redirects all request from unauthenticated users to /login.
For a user in the middle of a workflow, this experience is sub-optimal.
The expiration can be detected within passport.deserializeUser() like this:
passport.deserializeUser(function (id, done) {
const user = sessionManager.userLookup(id);
const errorInfo = ( expire logic check ) ? 'session has expired' : null;
done(errorInfo, user);
});
I can get the logic check right with the sessionManager. The trouble with this solution is that passport sends the user a 500 Internal Server Error, which is also sub-optimal.
What I would like is for the app to send a flash error saying the session has expired. But passport.deserialize() has no visibility to the req object for calling req.flash().
At this point the only way I can think to resolve the issue is to insert a middleware before passport, where the code would lookup the user in the session manager and call req.flash() if the session has expired. It seems like passport should provide a better way to handle such errors.
Answers would be extra-helpful if they include a link to documentation for passport.deserialize(). The only docs I have found here make no mention of how passport handles errors or if it is possible to configure or override the behavior.
UPDATE
After some reflection, flash is not the best mechanism for reporting the session expiration. The app should instead redirect to a "session expired" page. However, the main question still stands. The call changes from req.flash() to res.redirect(), but neither of these objects is available in passport.deserialize().

Answer 1

you can add req as a first parameter in the function, like this:
(also, recommend you use arrow function)
passport.deserializeUser(async (req, id, done) => {
req.flash('error', {});
(...)
});