@app.route("/check", methods=["GET"])
def check():
    """Return true if username available, else false, in JSON format"""

    register_username = request.args.get("username")
    # register_username from register.html form
    check_username = db.execute("SELECT username FROM users WHERE username = :register_username", register_username=register_username)
    check = check_username
    return jsonify(check)


        <script>
            document.querySelector('form').onsubmit = function(event)
            {
                var username = $("input#username").val();
                $.get("/check", username, function(check)
                {
                    if ("/check" == username)
                    {
                        event.preventDefault();
                        alert("Username is taken");
                    }
                    else
                    {
                        document.getElementById("form").submit();
                    }
                });
            };
        </script>