# Socket.IO Quick Reference

*Events, rooms, namespaces, middleware, real-time patterns*

> Source: Socket.IO Documentation (socket.io/docs) · MIT

## Setup

### Server Setup (Node.js)

```
import { Server } from "socket.io";
const io = new Server(3000, {
  cors: { origin: "http://localhost:5173" }
});
```

### Client Setup

```
import { io } from "socket.io-client";
const socket = io("http://localhost:3000");
```

### With Express

```
import express from "express";
import { createServer } from "http";
import { Server } from "socket.io";
const app = express();
const server = createServer(app);
const io = new Server(server);
```

### Server Options

| Command | Description |
|---------|-------------|
| `cors` | CORS configuration for cross-origin |
| `path` | Custom path (default: /socket.io) |
| `pingInterval` | Heartbeat interval (ms, default 25000) |
| `pingTimeout` | Timeout before disconnect (default 20000) |
| `maxHttpBufferSize` | Max message size in bytes (default 1MB) |

## Events

### Built-in Events (Server)

| Command | Description |
|---------|-------------|
| `connection` | Client connects |
| `disconnect` | Client disconnects |
| `disconnecting` | Client is disconnecting (still in rooms) |
| `error` | Error event |

### Built-in Events (Client)

| Command | Description |
|---------|-------------|
| `connect` | Connected to server |
| `disconnect` | Disconnected from server |
| `connect_error` | Connection failed |
| `reconnect` | Successfully reconnected |
| `reconnect_attempt` | Attempting to reconnect |

### Connection Lifecycle

```
io.on("connection", (socket) => {
  console.log(`connected: ${socket.id}`);
  socket.on("disconnect", (reason) => {
    console.log(`disconnected: ${reason}`);
  });
});
```

## Emitting

### Server Emit

```
socket.emit("hello", { msg: "world" });
socket.emit("data", arg1, arg2);
io.emit("broadcast", data);
```

### Client Emit

```
socket.emit("chat:message", { text });
socket.emit("update", data, (res) => {
  console.log("ack:", res);
});
```

### Emit Patterns

| Command | Description |
|---------|-------------|
| `socket.emit(ev, data)` | Send to this socket only |
| `io.emit(ev, data)` | Send to all connected clients |
| `socket.broadcast.emit()` | All clients except sender |
| `io.to(room).emit()` | All clients in room |
| `socket.to(room).emit()` | Room members except sender |

## Broadcasting

### Broadcast Methods

```
io.emit("msg", data);
socket.broadcast.emit("msg", data);
io.to("room1").emit("msg", data);
io.except("room2").emit("msg", data);
```

### Volatile & Compressed

| Command | Description |
|---------|-------------|
| `socket.volatile.emit()` | Drop if client not ready (no buffering) |
| `socket.compress(true).emit()` | Enable per-message compression |
| `io.local.emit()` | Broadcast to local server only (multi-node) |
| `socket.timeout(5000).emit()` | Emit with timeout for acknowledgement |

## Rooms

### Room Operations

```
socket.join("room-1");
socket.join(["room-1", "room-2"]);
socket.leave("room-1");
io.to("room-1").emit("msg", data);
```

### Room Properties

| Command | Description |
|---------|-------------|
| `socket.rooms` | Set of rooms this socket is in |
| `socket.id` | Each socket auto-joins its own ID room |
| `io.sockets.adapter.rooms` | Map of all rooms and their members |

### Room Patterns

```
socket.on("join:room", (room) => {
  socket.join(room);
  io.to(room).emit("user:joined", socket.id);
});
socket.on("disconnecting", () => {
  for (const room of socket.rooms) {
    socket.to(room).emit("user:left", socket.id);
  }
});
```

## Namespaces

### Creating Namespaces

```
const chat = io.of("/chat");
const admin = io.of("/admin");
chat.on("connection", (socket) => {
  chat.emit("user:online", socket.id);
});
```

### Client Connecting to Namespace

```
const chat = io("http://localhost:3000/chat");
const admin = io("http://localhost:3000/admin");
```

### Dynamic Namespaces

```
io.of(/^\/project-\d+$/).on("connection",
  (socket) => {
    const ns = socket.nsp.name;
    console.log(`joined namespace: ${ns}`);
  }
);
```

## Middleware

### Server Middleware

```
io.use((socket, next) => {
  const token = socket.handshake.auth.token;
  if (isValid(token)) return next();
  next(new Error("authentication failed"));
});
```

### Namespace Middleware

```
const admin = io.of("/admin");
admin.use((socket, next) => {
  if (socket.handshake.auth.role === "admin")
    return next();
  next(new Error("not authorized"));
});
```

### Middleware Properties

| Command | Description |
|---------|-------------|
| `socket.handshake.auth` | Auth data sent from client |
| `socket.handshake.headers` | HTTP headers from initial request |
| `socket.handshake.query` | Query parameters from connection URL |
| `socket.data` | Arbitrary data attached in middleware |

## Error Handling

### Server-Side Errors

```
socket.on("action", (data, callback) => {
  try {
    const result = process(data);
    callback({ status: "ok", data: result });
  } catch (err) {
    callback({ status: "error", msg: err.message });
  }
});
```

### Client-Side Errors

```
socket.on("connect_error", (err) => {
  console.log("connection error:", err.message);
});
socket.io.on("reconnect_failed", () => {
  console.log("reconnection failed");
});
```

### Client Reconnection Options

| Command | Description |
|---------|-------------|
| `reconnection` | Enable auto-reconnect (default true) |
| `reconnectionAttempts` | Max attempts (default Infinity) |
| `reconnectionDelay` | Initial delay ms (default 1000) |
| `reconnectionDelayMax` | Max delay ms (default 5000) |

## Acknowledgements

### Client Sends, Server Acks

```
// client
socket.emit("save", data, (response) => {
  console.log("server ack:", response);
});
// server
socket.on("save", (data, callback) => {
  callback({ saved: true, id: 42 });
});
```

### Server Sends, Client Acks

```
// server
socket.emit("ping", (response) => {
  console.log("client ack:", response);
});
// client
socket.on("ping", (callback) => {
  callback("pong");
});
```

### With Timeout

```
socket.timeout(5000).emit("save", data,
  (err, response) => {
    if (err) console.log("timeout!");
    else console.log("ack:", response);
  }
);
```

## Common Patterns

### Chat Room

```
io.on("connection", (socket) => {
  socket.on("chat:join", (room) => {
    socket.join(room);
    socket.to(room).emit("chat:joined",
      socket.id);
  });
  socket.on("chat:message", ({ room, text }) => {
    io.to(room).emit("chat:message", {
      from: socket.id, text
    });
  });
});
```

### Online Presence

```
const users = new Map();
io.on("connection", (socket) => {
  users.set(socket.id, socket.handshake.auth);
  io.emit("users:list", [...users.values()]);
  socket.on("disconnect", () => {
    users.delete(socket.id);
    io.emit("users:list", [...users.values()]);
  });
});
```

### Rate Limiting

```
io.use((socket, next) => {
  const ip = socket.handshake.address;
  if (rateLimiter.consume(ip)) return next();
  next(new Error("rate limit exceeded"));
});
```