Understanding Passportjs Custom Callback

Question

I'm experimenting with Passportjs and the code for a Custom Callback is:

app.get('/login', function(req, res, next) {
  passport.authenticate('local', function(err, user, info) {
    if (err) { return next(err); }
    if (!user) { return res.redirect('/login'); }
    req.logIn(user, function(err) {
      if (err) { return next(err); }
      return res.redirect('/users/' + user.username);
    });
  })(req, res, next);
});

I'm happy with all of this code except for the second to last line (req, res, next); - Could someone explain why these parameters are added on the end. This is probably more of a JS question than a Passport question but any help is much appreciated.

Answer 1

Connect/Express middleware function has signature:

function(req, res, next)

passport.authenticate() can be used as a middleware, e.g:

app.post('/login', passport.authenticate('local'), nextMiddleware);

This means authenticate() returns a middleware function object, which you can evoke with (req, res, next) parameters to continue the app's request-response cycle.