property virtualKeyCode

This property returns the virtual key code (this is the wParam of WM_KEYDOWN).

<box>
    <reaction event="keyPress" virtualKeyCode="65" action="onKeyPress();"/>
</box>

function onKeyPress()
{
    var eventObj = shell.currentEvent;
    if (eventObj)
    {
        var virtualKeyCode = eventObj.virtualKeyCode;
        // virtualKeycode == 65 (0x41), which maps to VK_A.
        // you can look in MSDN for all the VK value.
    }
}