javascript - 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

The "javascript" answer is that it returns a function which is called again with those 2nd set of arguments.

That function is the "accumulator for failures from each strategy in the chain".

https://github.com/jaredhanson/passport/blob/master/lib/middleware/authenticate.js