By the way, your CDN version of this sample does also not work but is throwing this error:
browser console log:
sockjs.js:1606 GET http://localhost:9999/sockjs-node/info?t=1587708040981 net::ERR_CONNECTION_REFUSED
node.js:
[2020-04-24T06:03:20.294Z] “GET /sockjs-node/info?t=1587708199986” Error (404): “Not found”