如何在JavaScript中获取WebSocket的错误代码及错误信息

问题背景 #

创建WebSocket连接时触发错误，控制台提示："Websocket connection to ...ip... failed: Error during WebSocket handshake: Unexpected response code: 499"，推测是杀毒软件拦截连接导致。当前代码中try/catch无法捕获该错误，onclose事件触发但无法从事件对象中提取错误详情。

相关代码 #

let errMsg = "";
try {
    socket = new WebSocket(server_ip.value, "chat");    // (*)
}
catch(err) {
    // js doesn't come to this branch
    Websocketerror = true;
    errMsg = "Websocket creation error. Code = " + err.code.toString() +
                "  Description = _" + err.message + "_";
}

if (websocketError) {
    appendMessageText(errMsg, "msg_err");
}
else {
    socket.onopen = ws_OnConnectionCreated;
    socket.onclose = ws_onConnectionClose;
    socket.onerror = ws_OnConnectionError;
}

function ws_onConnectionClose(event) {
    let status = event.wasClean ? "ok." : "(aborted).";
    appendMessageText("Websocket is closed" + status, "msg_log");
    console.log("------> ws_onConnectionClose(event): event =", event);
};

解决方案 #

1. 实现onerror事件处理函数 #

WebSocket连接错误属于异步错误，不会被同步try/catch捕获，必须通过onerror事件获取错误详情：

function ws_OnConnectionError(event) {
    console.error("WebSocket错误详情:", event);
    // 部分浏览器会在event.message中返回错误描述
    errMsg = "WebSocket连接失败: " + (event.message || "未知错误");
    appendMessageText(errMsg, "msg_err");
    websocketError = true;
}

2. 增强onclose事件的信息提取 #

在onclose事件中，可通过event.code和event.reason获取关闭的错误码与原因（非标准错误码可能因浏览器存在差异）：

function ws_onConnectionClose(event) {
    let status = event.wasClean ? "正常关闭" : "异常中断";
    let closeInfo = `WebSocket已关闭: ${status}，错误码: ${event.code}，原因: ${event.reason || "无"}`;
    appendMessageText(closeInfo, "msg_log");
    console.log("WebSocket关闭详情:", event);
}

3. 修正变量名大小写错误 #

代码中Websocketerror与websocketError大小写不一致，会导致错误状态判断失效，需统一变量名：

// catch块中修正为小写开头
websocketError = true;

关键说明 #

• WebSocket连接错误为异步触发，同步try/catch无法捕获，必须依赖onerror事件
• 结合onerror和onclose事件可完整获取连接失败的错误信息
• 非标准错误码（如499）可能需要通过onerror事件对象或浏览器控制台日志进一步确认

内容的提问来源于stack exchange，提问作者Tobic