I've got a radio button with id="po3"

<input type=radio name="product_options[3]" id="po3" value="13" checked>

This JS generates a "document.getElementById(...).0.checked is null or not an object

alert (document.getElementById('po3')[0].checked)

Since the radio name has [3] in it I can't access it via formid.product_options[3][0].checked.

Help!