Untitled

<!DOCTYPE html>
<html>

<head>
    <meta charset="UTF-8">
    <title>title</title>

    <script>

        var callbackWhenDataGetsBack = function (data) {
            console.log(JSON.parse(data));
        }

        function loadData(httpMethod, url, callback) {
            console.log("preparing request");

            var xhr = new XMLHttpRequest();

            xhr.onreadystatechange = function () {
                if (xhr.readyState === 4) {
                    console.log("data received");
                    callback(xhr.response);
                }
            }

            xhr.open(httpMethod, url, true);
            xhr.send('');
            console.log("sent");
        }


        function loadUsers(callbackForReceivingUsers) {
            loadData('GET', "https://kartoteka-project.firebaseio.com/users.json", callbackForReceivingUsers)
        }


        function loadExercises(callbackForReceivingExercises) {
            loadData('GET', "https://kartoteka-project.firebaseio.com/exercises.json", callbackForReceivingExercises)
        }


        window.onload = function () {
            loadUsers(callbackWhenDataGetsBack);
        };
    </script>
</head>

<body>
    <script src="https://code.jquery.com/jquery-3.4.1.min.js" integrity="sha256-CSXorXvZcTkaix6Yvo6HppcZGetbYMGWSFlBw8HfCJo="
        crossorigin="anonymous"></script>
</body>

</html>