"ReferenceError: WebSocket is not defined" when using RxJs WebSocketSubject and Angular Universal

All it takes is this:

// set WebSocket on global object
(global as any).WebSocket = require('ws');

And of course, we need the ws dependency as well in the package.json:

npm i ws -s

The RxJS webSocket function can receive a WebSocket Constructor as an argument. This can be used to run webSocket in a non-browser/universal environment like this.

const WebSocketConstructor = WebSocket || require('ws')

const socket: WebSocketSubject<any> = webSocket({
    url: 'wss://echo.websocket.org', 
    WebSocketCtor: WebSocketConstructor,
});