<aui:select name="testSelect" id="testSelect" label ="Test Select">
            <aui:option selected="true" value="ALL">All</aui:option>
            <aui:option  value="YES">Yes</aui:option>
            <aui:option  value="NO">No</aui:option>
</aui:select>
<aui:script>
AUI().use('aui-base', function(A){
A.one("#<portlet:namespace/>testSelect").on('change',function(){
var selectedValue = A.one('#<portlet:namespace/>testSelect').get('value');
 alert(selectedValue);
});
});
</aui:script>