【JS】js获得下拉列表选中项的值和id

function tijiao(){
        var elem = document.getElementById("dish_sort");
        var index=elem.selectedIndex;         
        alert(elem.options[index].value);
        alert(elem.options[index].id);
    }

<td>菜品分类：</td><td><select name="dish_sort" id="dish_sort">
            <c:forEach var="a" items="${list}">    
                <option id="${a.id}" value="${a.sort}">${a.sort}</option>
            </c:forEach>
            </select></td>