Get value of RadioButtonList

I have a RadioButtonList control that I want to grab the value checked in javascript before running my asp.net code.  Below is what I am using for js and the control but it does not fire the alert when I click a value in the RadioButtonList.  What am I missing?  Thanks.
    <script type="text/javascript">
        function rblpick(radioButtonList) {
            for (var x = 0; x < radioButtonList.length; x++) {
                if (radioButtons[x].checked) {
                    alert('selected a value of ' + radioButtons[x].value);
                }
   &nb ...