How to Make AJAX Requests With Raw Javascript

XHR.js

var xhr = new XMLHttpRequest();
                xhr.onreadystatechange = isReady(function(xhr) {
                    console.log(xhr.responseText);
                });

                function isReady(callback) {
                    if (xhr.readyState < 4) {
                        return;
                    }

                    if (xhr.status !== 200) {
                        return;
                    }

                    // all is well
                    if (xhr.readyState === 4) {
                        callback(xhr);
                    }
                }

                xhr.open('POST', 'https://android.googleapis.com/gcm/send', true);
                // xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
                xhr.setRequestHeader("Content-type", "application/json");
                xhr.setRequestHeader("Authorization", 'key=AIzaSyCl2AMo2itAacoNs3NZXPKF2_9NSAetkus');
                xhr.send(JSON.stringify(message));
            }