Sharan Panthi

Socket.IO is a JavaScript library that enables real-time, bidirectional, and event-based communication between web clients and servers. It’s widely used for applications like chat systems, live notifications, and collaborative tools, offering an intuitive API to manage connections, emit events, and organize communication with rooms and namespaces. This cheatsheet provides a concise reference for Socket.IO’s core server-side functionality, covering the io and socket objects, rooms, namespaces, and built-in events, making it a handy guide for developers building real-time applications.

io (Server-Level Broadcaster)

The io object manages all connected clients on the server and enables broadcasting events to them.

Method	Description
`io.emit(event, data)`	Sends an event to all connected clients.
`io.to(room).emit()`	Sends an event to all clients in a specific room.
`io.in(room).emit()`	Alias for `io.to(room).emit()`; targets a room.
`io.of(namespace)`	Targets a namespace (e.g., `/admin`, `/chat`).
`io.sockets.sockets`	Accesses all connected sockets as a Map.
`io.sockets.adapter.rooms`	Lists all active rooms as Sets of socket IDs.
`io.sockets.adapter.sids`	Maps socket IDs to their joined rooms.

socket (Per-Connection Object)

The socket object represents an individual client connection, allowing targeted communication and management.

Method	Description
`socket.emit(event, data)`	Sends an event to this specific client.
`socket.broadcast.emit()`	Sends to all clients except this socket.
`socket.to(room).emit()`	Sends to others in a room, excluding this socket.
`socket.join(room)`	Adds this socket to a room.
`socket.leave(room)`	Removes this socket from a room.
`socket.disconnect()`	Forcefully disconnects the socket.
`socket.id`	Unique ID for this socket connection.
`socket.rooms`	Set of all rooms this socket is part of.
`socket.handshake`	Contains connection details (headers, query, auth).
`socket.on(event, callback)`	Listens for custom or built-in events from this client.
`socket.data`	Stores custom data, e.g., user info, for this socket.

Rooms & Namespaces

Rooms and namespaces organize communication by grouping sockets or creating separate channels.

Concept	Description
Room	Logical group of sockets for targeted communication (e.g., "room123").
Namespace	Separate channel with its own events and logic (e.g., `/chat`, `/admin`).
`io.of("/chat")`	Accesses the `/chat` namespace.
`socket.nsp`	The namespace this socket belongs to.

Events (Built-in)

Socket.IO provides built-in events to handle connection states and errors.

Event Name	Description
`"connection"`	Emitted on server when a new socket connects.
`"disconnect"`	Emitted when a socket disconnects.
`"connect"`	Emitted on client when connected to server.
`"connect_error"`	Emitted on client during connection errors.
`"error"`	Emitted for general errors.
`"reconnect"`	Emitted on client upon successful reconnection.
`"reconnect_attempt"`	Emitted on client when attempting to reconnect.

Conclusion

Socket.IO streamlines real-time communication with a robust and flexible API, making it ideal for building dynamic applications like chat systems or live dashboards. This cheatsheet covers the essential server-side methods and concepts, helping developers quickly reference key functionalities for managing connections, events, rooms, and namespaces. For more details, consult the Socket.IO Documentation.