Node.js: websocket and express server shows unresolved server Problem

I am trying to install a new websocket server with an existing express server like this:

const http = require('http');
const express = require('express');
const app = express();
const port = 3000;
const bodyparser = require('body-parser');

const server = http.createServer(app);

const WebSocket = require('ws');
const wss = new WebSocket.Server({server:server});

wss.on('connection', function connection(ws) {
    console.log('A new client Connected!');
    ws.send('Welcome New Client!');

    ws.on('message', function incoming(message) {
        console.log('received: %s', message);

        wss.clients.forEach(function each(client) {
            if (client !== ws && client.readyState === WebSocket.OPEN) {
                client.send(message);
            }
        });

    });
});

//routing
const roomsRoute = require('./Implementierung/Ressourcen/rooms');

app.get("https://stackoverflow.com/", (req, res) => {
    res.send('Hear them');
});

server.listen(port, () => {
    console.log(`listening on port ${port}`);
});

First, I got a problem with this line:

const wss = new WebSocket.Server({server:server});

I am not able to create a new WebSocket Server this way.

At second:

if (client !== ws && client.readyState === WebSocket.OPEN) {
                client.send(message);
            }

WebSocket.OPEN shows an error, maybe because of non-existing WebSocket?