Socket.io

First off a little background. With a default configuration, when a socket.io connection starts, it first does 2-5 http connections and then once it has established the "logical" connection, it tries to establish a connection using the webSocket transport. If that is successful, then it keeps that webSocket connection as a long lasting connection and sends socket.io packets over it.

If the client refreshes in the middle of the transition to a webSocket connection, it creates a period of unknown state on the server where the server isn't sure if the user is just still in the middle of the transition to a lasting webSocket connection, if the user is gone entirely already, if the user is having some sort of connection issues or if the user is doing some refresh thing. You can easily end up with a situation where the server thinks there are multiple connections all from the same user in the process of being confirmed. It can be a bit messy if your server is sensitive to that kind of thing.

The quickest thing you can do is to force the connection process to go immediately to the webSocket transport. You can do that in the client by adding an options to your connection code:

let socket = io(yourURL, {transports: ["websocket"]});

You can also configure the server to only accept webSocket connections if you're try to protect against any other types of connections besides just from your own web pages.

This will then go through the usual webSocket connection which starts with a single http request that is then "upgraded" to the webSocket protocol. Once connection, one socket. The server will know right away, either the user is or isn't connected. And, once they've switched over to the webSocket protocol, the server will known immediately if the user hits refresh because the browser will close the webSocket immediately.

The "start with http first" feature in socket.io is largely present because in the early days of webSockets, there were some browsers that didn't yet support them and some network infrastructure (like corporate proxies) that didn't always support webSocket connections. The browser issue is completely gone now. All browsers in use support webSocket connections. I don't personally have any data on the corporate proxies issues, but I don't ever hear about any issues with people using webSockets these days so I don't think that's much of an issue any more either.

So, the above change will get you a quick, confirmed connection and get rid of the confusion around whether a user is or isn't connected early in the connection process.

Now, if you still have users who are messing things up by rapid refresh, you probably need to just implement some protection on your server for that. If you cookie each user that arrives on your server, you could create some middleware that would keep track of how many page requests in some recent time interval have come from the browser with this cookie and just return them an error page that explains they can't make requests that quickly. I would probably implement this at the web page level, not the webSocket level as that will give users better feedback to stop hitting refresh. If it's really a refresh you're trying to protect against and not general navigation on your site, then you can keep a record of a combination cookie and URL and if you see even two of those within a few seconds, then return the error page instead of the expected content. If you redirect to an error page, it forces a more conscious action to go back to the right page again before they can get to the content.