variable passed to function result in objectHTMLInputElement

[albert kao]

I like to pass a global variable to a function but
<script type="text/javascript">
var sortorder = true;
</script>

<input type="checkbox" name="choice" value="20" onchange="submitField(document.alertform.choice, 20, sortorder);">

This function is defined a file:
function submitField(field, id, sortorder) {
alert('sortorder ' + sortorder);

}

The alert box "sortorder [objectHTMLInputElement]" appear instead of "sortorder true".
Please help.