Error: req#logout requires a callback function

Since version 0.6.0 (which was released only a few days ago by the time of writing this), req.logout is asynchronous. This is part of a larger change that averts session fixation attacks.

See the release announcement:

The other major change is that that req.logout() is now an asynchronous function, whereas previously it was synchronous. For instance, a logout route that was previously:

app.post('/logout', function(req, res, next) {
  req.logout();
  res.redirect("https://stackoverflow.com/");
});

should be modified to:

app.post('/logout', function(req, res, next) {
  req.logout(function(err) {
    if (err) { return next(err); }
    res.redirect("https://stackoverflow.com/");
  });
});

Jared Hanson mentioned that the docs are not up to date yet:

It is necessary in order to improve the security of how sessions are managed during logout. Upgrading to 0.6.0 will require applications to pass a callback to req#logout. I’m still working on updating the docs and examples.