Logging user in after user have registered

Question

I am reading an article about using passport-local-mongoose for authentication. Article

In this article I found this line of code that do register the user and log them in

app.post('/register', function(req, res) {
    Account.register(new Account({ username : req.body.username }), req.body.password, function(err, account) {
        if (err) {
            return res.render('register', { account : account });
        }

        passport.authenticate('local')(req, res, function () {
          res.redirect('/');
        });
    });
  });

And the part that confuse me is this

passport.authenticate('local')(req, res, function () {
   res.redirect('/');
});

First, I don't understand the part that go after passport.authenticate(). It doesn't look like a callback or IIFE. Second, as far as I know, passport.authenticate() is a middleware (sorry if I'm wrong). Should we use it as a function like that.

My best explanation for this is because in the article have defined above

passport.use(new LocalStrategy(Account.authenticate()))

Because of that, passport.authenticate() now will call Account.authenticate(). The strange part is, authenticate() require a password, which we didn't provide it any. Maybe it return a Promise, but the part I said above doesn't look like a Promise.

So what is that part is, and is it a good practice to actually sign a user in?

Answer 1

Lets talk about that weird part first:

TLDR: As the first commenter mentioned passport.authenticate return a function.

How do we call a function? Like this: functionName(param1, param2, callbackMaybe); . You see no problem with that right?

Now, take a look at the code below

function a() {
  return function b() {
    console.log("Hi there!");
  }
}

Function a returning function b. How would you the the Hi there! in your console? You have to do it like this a()(). You can se it in this way:

let b = a(); // Since, a returns a function so, b is now a function.
b();

Same here, passport.authenticate('local') returning a function that takes 3 parameters where last one is a callback function. In terms of express the 3rd parameter is the next() function. I believe you know the usage of next() function. You are sending function () { res.redirect('/'); } as a callback. What this callback does it redirect you to this '/' route.

Second part of your question

Now a days, I do not see any website auto login user after signing up.

Hope it helps.