socketio/socket.io-client ? reverse-engineered prompt

Build me a small Socket.IO client library that lets an app connect to a realtime server, send messages, and listen for updates in both the browser and Node.js.

I want it to feel simple to use, with a clean way to create a connection, handle connect and disconnect events, send custom events, and receive responses without a lot of setup. It should work well for common realtime use cases like chat, live notifications, or syncing data.

Please make the code easy to understand, add a basic example showing how someone would connect and use it, and include enough tests or checks so I can trust it works. If you need current docs or best practices, look them up online first.