how to invoke addeventlistener from a hta file when user clicks on internet menu item

I have written a JavaScript to capture a screen shot of browser using canvas element and I am calling this script from a internet explorer menu item (add-on) click. But I am unable to invoke the event.

Below is the code sample.

        canvas.addEventListener('mousedown', function (event) {
                alert('8');
                event.preventDefault();
                var point = getPoint(event, event.view);