js如何获取<s:radio>的值

<s:form action="userAction_save.action" theme="simple">

<!--    省略部分代码-->

<s:radio list="{'男','女'}" name="sex" ></s:radio>

<input type="image"  οnclick="return checkData();" />

</s:form>

<script type="text/javascript"> 
  functioncheckData(){ 
     if (!document.getElementsByName('sex')[0].checked) {
alert("必须选择性别");
return false;

}else {

return true;

}

  } 
</script>