index.html

<!DOCTYPE html>
<html>
    <head>
        <title>Home</title>
        <script src = "https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
    </head>
    <body>
        <p> Click the button to authenticate a fitbit device:</p>
        <button id = "btn">Click me</button>
        <div id = "action">
        </div>
    </body>

    <script>
        $(document).ready(function(){
            $('#btn').click(function() {
                console.log("Button Click");
                $.get('/registerDev');
            });
        });

        $(window).on('load', function(e){
            if (window.location.hash == '#_=_') {
                window.location.hash = ''; // for older browsers, leaves a # behind
                history.pushState('', document.title, window.location.pathname); // nice and clean
                e.preventDefault(); // no page reload
            }
        });
    </script>
</html>