javascript - 尝试迭代时，嵌套 JSON 数组抛出“未定义的 "Uncaught TypeError: Cannot read property ' 长度”

Question

我试图找出为什么我的代码抛出此错误:未捕获的类型错误:无法读取未定义的属性“长度”。

此错误消息引用此行:“for (var j = 0; j

我不知道为什么会发生这个错误，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

您收到该错误是因为并非所有 pluginObj 都定义了 installedMimes。

您可以通过两种方式解决此问题:

a) 将 "installedMimes": [] 添加到 json 中存在的所有插件中。

b) 更新您的 JS 代码以检查 installedMimes 是否存在。

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