Socket.IO Cheatsheet: Essential Server-Side Methods and Events

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.