javascript如何知道　s:doubleselect 选择的值

javascript怎么知道　s:doubleselect 选择的值
javascript怎么知道　s:doubleselect 选择的值
因为我要判断如果没选择的话，不让表单提交

Java code

<s:doubleselect list="bigClassList" name="bigClassID" listKey="id"                    listValue="name" label="请选择省市" doubleList="bigClassMap.get(top.id)"                    doubleListKey="id" doubleName="smallClassID" doubleListValue="name"                    required="true" />