There are lots of posts already about how to gracefully shutdown Node http servers:

Graceful shutdown of a node.JS HTTP server

Nodejs/express, shutdown gracefully

How do I shut down my Express server gracefully when its process is killed?

Quitting node.js gracefully

To also gracefully shutdown all open websocket connections, just add this to your shutdown handler:

for (const client of wss.clients) {
    client.close(1001, 'Server shutdown');
}
wss.close();