Will this code block in node.js

When reigstering a user I request two reads from the database to see if there is a username and email and then I write if the checks pass.. Can anyone tell me if this code will block other users?

I am using node with express.js and mongojs.

I know I am saving passwords in plaintext.. This will be changed.

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

    var checked = 0;
    var errors = [];

    var finishedCheck = function() {
        checked++;
        register();
    }

    var register = function() {

        if(checked === 2){

            if(errors.length > 0) {
                console.log('errors', errors);
                res.statusCode = 409;
                res.send(errors);
            } else {

                var newuser = {
                    username: req.body.username,
                    uppercase: req.body.username.toUpperCase(),
                    password:req.body.password,
                    email: req.body.email.toLowerCase(),
                    userLevel: 0,
                    createdOn: new Date()
                };

                db.users.save(newuser, function(err, val){
                    if(!err) {
                        req.session.userid = val['_id'];
                        res.end();
                    } else {
                        res.statusCode = 406;
                        res.end(['Something went wrong', err]);
                    }
                });
            }
        }
    }

    var check = function(username, email) {
        console.log('chekc');
        db.users.ensureIndex({email:1},{unique:true});
        db.users.ensureIndex({uppercase:1},{unique:true});
        if(!username.match(/^[A-Za-z0-9_]*$/)) {
            errors.push('Username is invalid');
            finishedCheck();
        } else {
            db.users.findOne({uppercase: username}, function(err, val) {
                if(val) {
                    errors.push('Username Taken');
                }
                finishedCheck();
            });
        }

        //check if the is a valid email address if so then check to see if its already registered
        if(!email.match(/^[-a-z0-9~!$%^&*_=+}{\'?]+(\.[-a-z0-9~!$%^&*_=+}{\'?]+)*@([a-z0-9_][-a-z0-9_]*(\.[-a-z0-9_]+)*\.(aero|arpa|biz|com|coop|edu|gov|info|int|mil|museum|name|net|org|pro|travel|mobi|[a-z][a-z])|([0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}))(:[0-9]{1,5})?$/i)) {
            errors.push('Email is invalid');
            finishedCheck();
        } else {
            db.users.findOne({email: email}, function(err, val) {
                if(val) {
                    errors.push('Email is already in the database');
                } 
                finishedCheck();
            });
        }


    };

    check(req.body.username.toUpperCase(), req.body.email.toLowerCase());

});