const xhr = new XMLHttpRequest();
const url = '/stats.php';

const clientData = {
    referrer: document.referrer
};

const jsonData = JSON.stringify({
    client_data: clientData
});

xhr.open('POST', url, true);
xhr.setRequestHeader('Content-Type', 'application/json');
xhr.send(jsonData);

console.log('server data has been sent');