Res.send() not sending anything

Res/send() returns nothing back to the browser when called.

server:

router.post('/createUser',
    function (req, res){
        var response;
        console.log('adding user');
        userDB.createUser(req.body.email, req.body.username, req.body.password,        req.body.zipcode, req.body.birthYear,
        function (err, addingResult){
            if(err){

            } else {
                db.checkForUser(req.body.email, req.body.password, 
                    function (err, result){
                        console.log('hit login');
                        if (!result) {
                            console.log('invalid user');
                            res.send(false);
                        }
                        else {
                            console.log('successful login');
                            console.log(result.userID);
                            console.log(result.username);

                            req.session.userID = result.userID;
                            req.session.username = result.username;
                            response = 'true';
                            console.log('response should be: ' + response);
                            res.send(response);
                        }
                    }
                );
            }
        }
    );
 }
 );

client:

    $('#signUp').on('submit', function (){
    var payload = {
        email: $('#Email').val(),
        username: $('#username').val(),
        password: $('#passWord').val(),
        birthYear: $('#birthYear').val(),
        zipcode: $('#zip').val()
    }
    console.log(payload);
    $.ajax({
        url: "signUp/createUser",
        type: "POST",
        contentType: "application/json",
        processData: false,
        data: JSON.stringify(payload),
        complete: function (data) {
            console.log(data.responseText);
            if (data.responseText == 'true'){
                window.location.replace('/feed');
            }
        }
        });
        });

The response is logged and is correct on the console but nothing is logged or sent back to the console. Nothing else like res.render or res.redirect works either. Does it have to do with the nested functions?