Question

我试图弄清楚为什么我的代码会抛出此错误：未捕获TypeError：无法读取属性＆＃39; length＆＃39;未定义的。

此错误消息引用此行：＆＃34; for（var j = 0; j＆lt; pluginObj.installedMimes.length; j ++）＆＃34;

我无法解释为什么会发生这种错误，我已经使用JSONLint检查了我的JSON代码，它说没有错误。任何帮助和建议将不胜感激！

var pluginsJson = {
    "installedPlugins": [
        {
            "pluginName": "Chrome PDF Viewer",
            "pluginVersion": "",
            "pluginDescription": "Portable Document Format",
            "pluginFilename": "internal-pdf-viewer",
            "installedMimes": [
                {
                    "mimeType": "application/x-google-chrome-pdf",
                    "mimeDesc": "Portable Document Format",
                    "mimeSuffixes": "pdf"
                },
                {
                    "mimeType": "application/x-nacl",
                    "mimeDesc": "Native Client Executable",
                    "mimeSuffixes": ""
                }
            ]
        },
        {
            "pluginName": "Widevine Content Decryption Module",
            "pluginVersion": "",
            "pluginDescription": "Enables Widevine licenses for playback of HTML audio/video content. (version: 1.4.8.866)",
            "pluginFilename": "widevinecdmadapter.dll"
        },
        {
            "pluginName": "Shockwave Flash",
            "pluginVersion": "",
            "pluginDescription": "Shockwave Flash 21.0 r0",
            "pluginFilename": "pepflashplayer.dll"
        },
        {
            "pluginName": "Native Client",
            "pluginVersion": "",
            "pluginDescription": "",
            "pluginFilename": "internal-nacl-plugin"
        }
    ]
};

var pluginString = "";
var pluginObj;
var mimeObj;

for (var i = 0; i < pluginsJson.installedPlugins.length; i++) {
    pluginObj = pluginsJson.installedPlugins[i];

    pluginString += pluginObj.pluginName;
    pluginString += " | ";
    pluginString += pluginObj.pluginFilename;
    pluginString += " | ";
    pluginString += pluginObj.pluginDescription;
    pluginString += " | ";
    pluginString += pluginObj.pluginVersion;
    pluginString += "<br>";

    for (var j = 0; j < pluginObj.installedMimes.length; j++) {
        mimeObj = pluginObj.installedMimes[j];

        pluginString += mimeObj.mimeType;
        pluginString += " /\ ";
        pluginString += mimeObj.mimeDesc;
        pluginString += " /\ ";
        pluginString += mimeObj.mimeSuffixes;
        pluginString += "<br>";
    }
    pluginString += "<br>";
}

Answer 1

您收到该错误，因为并非所有pluginObj定义installedMimes。

您可以通过两种方式解决此问题：

a）将"installedMimes": []添加到json中的所有插件。

b）更新您的JS代码以检查是否存在installedMimes。

if (pluginObj.installedMimes) {
    for (var j = 0; j < pluginObj.installedMimes.length; j++) {
        ...
    }
}

Answer 2

我发现只有第一个installedMimes中有installedPlugin个数组，而其他数据中没有...

我会在你的内循环之前添加一个测试：

if (typeof pluginObj.installedMimes !== 'undefined') {
    for (var j = 0; j < pluginObj.installedMimes.length; j++) {
        ...
    }
}

嵌套的JSON数组抛出＆＃34;未捕获的TypeError：无法读取属性＆＃39;长度＆＃39;未定义＆＃34;当试图迭代

2 个答案: