The documentation about IIS WebSocket and Socket.IO might be out-of-date

[lextm]

In the current article, there is an extra section

Disable WebSocket when using socket.io on node.js

If you are using the WebSocket support in socket.io on node.js in your site, you will need to disable the default IIS WebSockets module by adding the below snippet to your web.config or applicationHost.config. If this is not done, the IIS WebSockets module will attempt to handle the WebSocket communication rather than letting this task fall through to node.js (and hence your application). This will result in unexpected errors when you attempt to access your site.

But based on my testing, if the Node.js web app is hosted on IIS via HttpPlatformHandler, then IIS WebSocket support might be enabled for the connections to pass. Wonder if it is the time to remove this section.