I made my own test app and was able to figure out what is going on.

If you hit F5 quite fast multiple times, It does temporarily accumulate some extra socket.io connections in Chrome, but within a relatively short time (maybe a few minutes), it recovers and the total count of connected sockets is back to 1.

After further testing, I discovered that this is not a browser issue. This is an issue with how socket.io starts a socket.io connection. If you replace this in the client:

var socket = io();

with this:

var socket = io({transports: ['websocket'], upgrade: false});

which forces socket.io to ONLY use a webSocket and never use HTTP polling, then the problem disappears.

So, the issue is because the default behavior for socket.io is to start with an http polling version of a socket.io connection. After a little data is exchanged, socket.io will then attempt to switch over to a real webSocket. If that real webSocket works, then it will stop using the http polling connection.

But, if you hit an F5 in the middle of this transition between polling and a real webSocket, there is no persistent connection yet for socket.io to know that the web page it was just communicating with is now gone. So, all it can do is to figure out some time later that there is no longer any incoming communication from that web page and thus it should clear up it's socket.io connection (it was in polling mode when you hit F5).

But, if you turn off that initial polling mode with the above client code, then it only ever uses a real webSocket (never uses the simulated polling mode) and the browsers are very good at cleaning up the webSocket when you hit F5 so the server either hasn't finished establishing it's socket.io connection (in which case there's no connection yet to get temporarily orphaned) or it's already converted over to a webSocket (and the browser will cleanly close that on the F5).

So, this is a design limitation of the http polling mode that socket.io starts in. Since there is no continuous connection when in that mode, there is no immediately notification by the browser when that page is replaced with F5 and thus the server has no way of knowing that the client just disappeared. But, if you skip the http polling mode and start with a real webSocket, then there is no such window of time where there's a socket.io connection, but no real webSocket and thus the server is always told immediately by the browser closing the webSocket connection when the page goes away.