index.html

<!DOCTYPE html>
<html>

    <head>

        <style>
            canvas {
                border: 1px solid #000;
            }
        </style>
    </head>
    <body>
        <button id="startStop">Start</button>
        <button id="addCreature">Add Creature</button>
        <script src="./js/creature.js"></script>
        <script src="./js/app.js"></script>
        <script type="text/javascript">
            (function() {
                document.getElementById('startStop').addEventListener('click', function() {
                    if(this.innerHTML == 'Start') {
                        this.innerHTML = 'Stop';
                        window.app.start();
                        return;
                    }

                    this.innerHTML = 'Start';
                    window.app.stop();
                });

                document.getElementById('addCreature').addEventListener('click', function() {
                    window.app.addCreature();
                });
            })();
        </script>
    </body>

</html>