Allowed Var Names In For Use With Forms

I am trying to adapt a script for use validating a drop-down menu in a form.

The script works fine in my lttle demo. In the head of the page I've got...


Code:
<script type="text/javascript">
function checkList() {
var testForm = document.testForm;
if (testForm.demoSelect[testForm.demoSelect.selectedIndex].value == "") {
alert("Please make a valid selection from the list.");
return false;
}
}
</script>
And then in the body...


HTML Code:
<form action="demo.php" method="post" name="testForm" id="testForm" onSubmit="return checkList();">
<p>
demo select menu
<select name="demoSelect" id="demoSelect">
<option value="" selected>Please Select</option>
<option value="1">Attrib1</option>
<option value="2">Attrib2</option>
<option value="3">Attrib3</option>
</select>
</p>
<p>Submit
<input type="submit" name="Submit" value="Submit">
</p>
</form>
No problem, works fine. The problem is that I am trying to apply this code to a PHP page in osCommerce where the name of the drop-down menu (i.e. demoSelect) is generated dynamically. It is only for use in one particular situation where I know the name of the drop-down menu is generated as id[1] but it wont work when I replace the demoSelect for id[1].

I'm only really beginner with JS but I guess it's because JavaScript identifies id[1] as an array? Is this right and any ideas how I can correctly identify my drop-down to JS?