What does someFunction(args)(moreArgs); do in JavaScript?

Question

What does:

someFunction(args)(moreArgs);

Do in JavaScript?

I have a following working piece of code from the Passport.js documentation:

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);

});

In this case

passport.authenticate

is being called with args (the String

local

and a Function) then afterwards

(req, res, next)

exists.

In otherwords:

someFunction(args)(moreArgs);

This isn't an IIFE - there's no

()

around the function being ran. What is it?

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Answer 1

someFunction(args) returns a function, so (moreArgs) is just calling that returned function.

E.g:

function foo(a) {
    console.log(a);
    function bar(b,c){
        console.log(b, c);
    }
    return bar;
}

foo(20)("Thou shalt", "not steal");

What does someFunction(args)(moreArgs); do in JavaScript?

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