javascript - 无法读取未定义 jquery 的属性 'length'

Question

我使用 ajax 从 C# Web 方法获取一个列表(下面的代码)，该列表返回正常，但在成功方法完成后，它给我一个错误 - (无法读取未定义的属性“长度”) jquery(下面的屏幕截图)

我是不是漏掉了什么？

function getMainParentMenus() {

    $.ajax({
        type: "POST",
        url: "/Mainpage.aspx/getLeftMainNavParentMenus",

        data: "{}",
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: function (msg) {
            alert(msg.d);
            parentMenuss = msg.d;
        }, //It goes to the screenshot below after this bracket
        error: function (error) {
            alert("An error has occured while fetching the Left Nav Parent Menus");
        }
    });

};

上面的方法被下面的方法调用。

    var parentMenuss;
    var listOfSubMenus;
    function bindLeftNavMenu() {
//    var parentMenus = getMainParentMenus();


getMainParentMenus();



    var html = "<div id='accordian'> ddd";

    $.each(parentMenuss, function () {

        html += " <h3 class='accordianHeader' href='" + this['URL'] + "'>" + this['Title'] + "</h3>  ";
        alert("okK");
        $.each(listOfSubMenus, function () {
            html += "<div class='accordianContent'><a href='" + this['URL']  + "'>" + this['Title'] + "</a>";
        });

    });
    html += "</div>";
    $("#leftNavigationMenu").append(html);
 };

编辑:

上面第一段代码中警报中的数据显示如下

在 Chrome 调试器中:

Answer 1

因为 getMainParentMenus 使用 AJAX，所以它是异步的。调用 getMainParentMenus 后的下一行代码将在 AJAX 调用的 .success 部分之前执行，因此它将被执行在填充 parentMenuss 之前。

有几种方法可以解决这个问题，一种方法是将回调函数传递给 getMainParentMenus，如下所示:

function getMainParentMenus(myCallback) {
    $.ajax({
        type: "POST",
        url: "/Mainpage.aspx/getLeftMainNavParentMenus",

        data: "{}",
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: function (msg) {
            alert(msg.d);
            parentMenuss = msg.d;
            if (callback && typeof(callback)==="function") {
                callback();
            }
        }, //It goes to the screenshot below after this bracket
        error: function (error) {
            alert("An error has occured while fetching the Left Nav Parent Menus");
        }
    });

};

现在你可以这样调用它:

var html = "<div id='accordian'> ddd";

getMainParentMenus(function() { 
    $.each(parentMenuss, function () {
        html += " <h3 class='accordianHeader' href='" + this['URL'] + "'>" + this['Title'] + "</h3>  ";
        alert("okK");
        $.each(listOfSubMenus, function () {
            html += "<div class='accordianContent'><a href='" + this['URL']  + "'>" + this['Title'] + "</a>";
        });
    });
});