Can't assign req.session.user when is called fs.mkdir

This is the code of the app.post that calls fs.mkdir by the function that I made, newdir:

app.post('/register', express.bodyParser(), function (req, res, next){

 var newu = new UserModel({});

 newu.user = req.body.nuser;
 newu.pass = req.body.npass;
 newu.mail = req.body.nmail;


 UserModel.find({ user: req.body.user }, function (err, user){

    if (user.lenght == 1) {

        res.redirect('/');

    }

    else {    

    newdir(req.body.nuser);   

    next()   

    if (err) throw err;

          newu.save(function (err, newu){

               req.session.user = newu.user;  
               res.redirect('/home')


           });

     }

  });

});

This is the code of newdir:

function newdir (username){

    var pathu = __dirname + '/users/' + username;

    fs.mkdir(pathu, function (err){

       if (err) throw err;

    });
}

An this is the code of /home:

app.get('/home', function (req, res){

  console.log(req.session.user);

  res.send('Welcome ' + req.session.user + '!');

});

I can assign a req.session.user in all app.post/get that I want, for example when I verify the user with this, I can assign the req.session.user correctly:

app.post('/verify', express.bodyParser(), function (req, res){

  UserModel.find({ user: req.body.user }, function (err, user){

    if (user[0] == undefined) {

         res.redirect('/');

    }

     else{

       if (user[0].pass == req.body.pass) {

         req.session.user = user[0].user;
         res.redirect('/home');

       }

       else{

         res.redirect('/');

       }

   }

  if (err) throw err;

  });

});

But when I try to assign req.session.user in the same app.post where's it's called fs.mkdir, always req.session.user is undefined. Maybe I should create a module that makes the fs.mkdir call? I don't know what to do!