jquery没有更新下拉选项 - Ajax

Question

我有两个下拉列表

一个是

<select data-placeholder="Menu" name="menu" required data-rel="chosen" required id="menu">
    <c:if test="${not empty menuList}">
        <optgroup label="Choose one from below">
            <c:forEach var="thisMenu" items="${menuList}">
                <option value="${thisMenu.id}" <c:if test="${menu.id == thisMenu.id}">selected</c:if>>${thisMenu.name}</option>
            </c:forEach>
        </optgroup>
    </c:if>
</select>

，第二个是

<div class="controls">
    <select data-placeholder="Category" name="category" id="category" required data-rel="chosen" required>
        <c:if test="${not empty menu1.categories}">
            <optgroup label="Choose one from below">
                <c:forEach var="thisCategory" items="${menu1.categories}">
                    <option value="${thisCategory.id}" <c:if test="${categoryId == thisCategory.id}">selected</c:if>>${thisCategory.name}</option>
                </c:forEach>
            </optgroup>
        </c:if>
    </select>
</div>

我的ajax电话是

$(document).ready(function() {
        $('#menu').on('change', function() {
            var elt = document.getElementById("menu");
            var menuId = elt.options[elt.selectedIndex].value;
            alert("menuId : " + menuId);
            $.ajax({
                type: "GET",
                url: "/superadmin/product/ajaxupdatecategories",
                data: ({menu:menuId}),
                dataType: "json",
                success: function(response){
                    alert("response : " + response.categoryList);
                    $("#category").get(0).options.length = 0;
                    $.each(response.categoryList, function(index, item) {
                        alert(index + " category : " + item.name);
                        $("#category").get(0).options[$("#category").get(0).options.length] = new Option(item.id, item.name);
                    });
                },
                error: function() {
                    $("#category").get(0).options.length = 0;
                }
            });
        });
    })

问题是我从ajax获取新类别但类别下拉选项没有改变。 请纠正我的错误。