Getting status 200 but no response in body

Here is my service call

<script>
function secureClicked() {
    var uname = document.getElementById('uname').value;
    var pwd = document.getElementById('pwd').value;
    var obj = new Object();
    obj.username = uname;
    obj.password = pwd;
    var userObj = JSON.stringify(obj);

    xhr = new XMLHttpRequest();
    var url = "http://127.0.0.1:9923/login";
    xhr.open("POST", url, true);
    xhr.setRequestHeader("Content-Type", "application/json;charset=UTF-8");
    xhr.onreadystatechange = function () {
        if (xhr.readyState == 4 && xhr.status == 200) {
            console.log(xhr.responseText);
            var json = JSON.parse(xhr.responseText);
            document.write(json.accesstoken);
            onComplete(xmlhttp.responseText);
        }
    };

    xhr.send(userObj);
}

Here is my backend code

    module.exports = {

    login: function(req, res) {

        MongoClient.connect(mongoUrl, function (err, db) {

            if (err) {
                console.log('into error of mongo');
                res.status(400).json({
                    message: 'Connection to database failed !!',
                    error: err
                });
            }

            insertDocument(db, req.body, function (result) {
                console.log('into insert doc');
                res.setHeader('Content-Type', 'application/json');
                res.status(200).json({
                    access_token: result._id
                });
            });

        });

        var insertDocument = function (db, data, callback) {
            console.log('into insert doc function');
            console.log(data);
            var string = JSON.stringify(data);
            var objectValue = JSON.parse(string);
            var username = objectValue['username'];
            var password = objectValue['password'];
            console.log(username);
            db.collection('user_list').findOne({$and:[{ 'username':username},{'password':password}]}, function(err, result) {
                if (err) {
                    res.status(500).json({
                        message: 'Failed to add in DB!!'
                    });
                } else {
                    callback(result);
                }
            });
        }
    }
};

I'm able to get response in postman, but unable to get response in browser. Am I doing anything worng here ? Can anyone help me on this? Thanks in advance !!

via Avinash Ravilla