function checkLogin() {
        var user = document.getElementById("username").value;
        var password = document.getElementById("password").value;
        var data = "userName=" + user + "&password=" + password;

        var request = new XMLHttpRequest();

        request.open("POST", "check.php", false);
        request.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
        request.send(data);

        if (request.status === 200) {
            window.open("test.html");
        } else {
            var messageSpan = document.getElementById("response");

            var responseJson = JSON.parse(request.responseText);
            messageSpan.innerHTML = "Your password of " + responseJson["password"] + " was incorrect.";
        }
    }