helper.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Helper</title>
    <script type="text/javascript">

        let callbacks = {
            setUserId: () => {
                if (localStorage.getItem('userId') === null) {
                    localStorage.setItem('userId', Math.random().toString().substr(2));
                }
            },
            getUserId: () => {
                window.parent.postMessage(localStorage.getItem('userId'),  '*')
            }
        }

        window.addEventListener('message', () => {
            callbacks[event.data]();
        });

        // Set User Id by default when frame loaded
        // It can be triggered from parent frame also
        callbacks.setUserId();

    </script>
</head>
<body></body>
</html>