Websocket连接失败，意外响应代码：404

Question

我无法在Express服务器上创建Websocket连接。我在这个问题上进行了很多搜索，但找不到任何解决方案。我的代码如下：

const express = require('express');
const cors = require('cors');
const http = require('http');
const mongoose = require('mongoose');
const ws = require('ws');

const app = express();

// CORS
app.use(cors({ origin: true, credentials: true }));
app.options("*", cors({ origin: true, credentials: true }));

const httpServer = http.createServer(app);
const websocketServer = new ws.Server({ server: httpServer })

websocketServer.on('connection', (websocket, req) => {
  console.log('connected');
})

const server = app.listen(3000, () => {
  mongoose.connect(`mongodb://localhost:27017/quizzer`,
    { useUnifiedTopology: true, useNewUrlParser: true }, () => {
      console.log(`Serverstarted on port ${server.address().port}`);

    });
});

服务器已启动并且正在侦听端口3000。但是仍然出现以下错误：

WebSocket connection to 'ws://localhost:3000/' failed: Error during WebSocket handshake: Unexpected response code: 404