如何在JavaScript的类回调中读取已修改的类属性？

我看了你的代码，问题核心出在WSS类里connection回调函数的this指向错误！

你在this.wss.on('connection', function connection(ws) { ... })中使用了普通函数作为回调，这个函数里的this并不是指向WSS类的实例，而是指向WebSocket.Server对象（非严格模式下可能是全局对象）。所以你在回调里给this.activeWs赋值时，根本没修改到WSS实例的activeWs属性，导致后续调用sendCommand时，实例的activeWs始终是初始的null。

下面给你三种可行的解决办法：

方法1：使用箭头函数（最推荐） #

箭头函数不会绑定自己的this，它会继承外层作用域的this，也就是WSS类的实例。修改后的connection回调如下：

this.wss.on('connection', (ws) => {
    console.log("Client Connected");
    if (this.activeWs != null) {
        this.activeWs.close(1000, "Server initiated close");
        console.log("Closed old session")
    }
    this.activeWs = ws;
})

方法2：提前保存this的引用 #

在回调外部先把WSS实例的this存到一个变量（比如self）里，然后在回调中用这个变量访问实例属性：

const self = this;
this.wss.on('connection', function connection(ws) {
    console.log("Client Connected");
    if (self.activeWs != null) {
        self.activeWs.close(1000, "Server initiated close");
        console.log("Closed old session")
    }
    self.activeWs = ws;
})

方法3：用bind绑定this #

通过bind方法把回调函数的this强制绑定到WSS实例上：

this.wss.on('connection', function connection(ws) {
    console.log("Client Connected");
    if (this.activeWs != null) {
        this.activeWs.close(1000, "Server initiated close");
        console.log("Closed old session")
    }
    this.activeWs = ws;
}.bind(this))

另外，我注意到你的sendCommand方法只做了打印，没有真正发送数据给客户端，建议补充发送逻辑：

this.sendCommand = (data) => {
    console.log(this.activeWs)
    if (this.activeWs == null) {
        return;
    }
    console.log(data);
    this.activeWs.send(data); // 新增这行实现数据发送
}

最后附上修改后的完整代码（箭头函数版本）：

class WSS {
    constructor(){
        const WebSocket = require('ws');
        this.wss = new WebSocket.Server({ port: 3001});
        this.activeWs = null;

        this.wss.on('connection', (ws) => {
            console.log("Client Connected");
            if (this.activeWs != null) {
                this.activeWs.close(1000, "Server initiated close");
                console.log("Closed old session")
            }
            this.activeWs = ws;
        })

        this.sendCommand = (data) => {
            console.log(this.activeWs)
            if (this.activeWs == null) {
                return;
            }
            console.log(data);
            this.activeWs.send(data);
        }
    }
}

class WS {
    constructor(clientId, onmessage) {
        this.clientId = clientId;
        const WebSocket = require('ws');
        this.ws = new WebSocket('ws://localhost:8080'); // 补充ws://协议前缀，避免连接失败
        this.ws.onmessage = (event) => { onmessage(event)};
    }
}

const serverWSS = new WSS();
const listener = new WS("000", forwardMessage)

function forwardMessage(event) {
    serverWSS.sendCommand(event.data);
}

备注：内容来源于stack exchange，提问作者servvs