Why is event.ctrlKey returning undef?

I am calling a JavaScript function when an option is selected in a select element like so:

<select id="select-thingy" onchange="foo(event, this); false;">
    <option value="bar">asdf</option>
...
</select>

The function does something like this:

function foo(e, elem) {
    var thingummy = elem.options[elem.selectedIndex].value;

    alert(e.ctrlKey); // for testing only

    if (e.ctrlKey) {
        // do something
    } else {
        // do something else
    }
}

According to the alert, e.ctrlKey is undefined – I thought this was supposed to return either true or false? What am I missing here?

Answer

As per the standard, the attribute ctrlKey is only available on MouseEvents (like click, mouseover, etc.) but not HTMLEvents.