I found the answer for anyone who gets immensely confused by the horrible lack of documentation of socket.io. You cannot source /socket-lib/socket.io.js, you must source http://yourwebsite.com:12345/socket.io/socket.io.js. The server automatically does the rest for you.
The error is pretty clear, you need to specify an absolute (instead of relative) path and/or set root in the config object for res.sendFile(). Examples: or specify a root (which is used as the base path for the first argument to res.sendFile(): Specifying the root path is more useful when you’re passing a user-generated file path which could potentially contain malformed/malicious … Read more
Its advantages are that it simplifies the usage of WebSockets as you described in #2, and probably more importantly it provides fail-overs to other protocols in the event that WebSockets are not supported on the browser or server. I would avoid using WebSockets directly unless you are very familiar with what environments they don’t work … Read more