The Socket instance (server-side)

A Socket is the fundamental class for interacting with the client. It inherits all the methods of the Node.js EventEmitter, like emit, on, once or removeListener.

Besides:

• emitting and listening to events
• broadcasting events
• joining and leaving rooms

The Socket instance has a few attributes that may be of use in your application:

Socket#id

Each new connection is assigned a random 20-characters identifier.

This identifier is synced with the value on the client-side.

// server-side
io.on("connection", (socket) => {
  console.log(socket.id); // ojIckSD2jqNzOqIrAGzL
});

// client-side
socket.on("connect", () => {
  console.log(socket.id); // ojIckSD2jqNzOqIrAGzL
});

caution

Please note that, unless connection state recovery is enabled, the id attribute is an ephemeral ID that is not meant to be used in your application (or only for debugging purposes) because:

• this ID is regenerated after each reconnection (for example when the WebSocket connection is severed, or when the user refreshes the page)
• two different browser tabs will have two different IDs
• there is no message queue stored for a given ID on the server (i.e. if the client is disconnected, the messages sent from the server to this ID are lost)

Please use a regular session ID instead (either sent in a cookie, or stored in the localStorage and sent in the auth payload).

See also:

• Part II of our private message guide
• How to deal with cookies

Note: you can't overwrite this identifier, as it is used in several parts of the Socket.IO codebase.

Socket#handshake

This object contains some details about the handshake that happens at the beginning of the Socket.IO session.

{
  headers: /* the headers of the initial request */
  query: /* the query params of the initial request */
  auth: /* the authentication payload */
  time: /* the date of creation (as string) */
  issued: /* the date of creation (unix timestamp) */
  url: /* the request URL string */
  address: /* the ip of the client */
  xdomain: /* whether the connection is cross-domain */
  secure: /* whether the connection is secure */
}

Example:

{
  "headers": {
    "user-agent": "xxxx",
    "accept": "*/*",
    "host": "example.com",
    "connection": "close"
  },
  "query": {
    "EIO": "4",
    "transport": "polling",
    "t": "NNjNltH"
  },
  "auth": {
    "token": "123"
  },
  "time": "Sun Nov 22 2020 01:33:46 GMT+0100 (Central European Standard Time)",
  "issued": 1606005226969,
  "url": "/socket.io/?EIO=4&transport=polling&t=NNjNltH",
  "address": "::ffff:1.2.3.4",
  "xdomain": false,
  "secure": true
}

Socket#rooms

This is a reference to the rooms the Socket is currently in.

io.on("connection", (socket) => {
  console.log(socket.rooms); // Set { <socket.id> }
  socket.join("room1");
  console.log(socket.rooms); // Set { <socket.id>, "room1" }
});

Socket#data

An arbitrary object that can be used in conjunction with the fetchSockets() utility method:

// server A
io.on("connection", (socket) => {
  socket.data.username = "alice";
});

// server B
const sockets = await io.fetchSockets();
console.log(sockets[0].data.username); // "alice"

More information here.

caution

Do not rely on reference equality when using properties of data.

// BAD
const activeUsers = new Set();
io.on("connection", (socket) => {
  socket.data.user = { name: "alice" };
  activeUsers.add(socket.data.user);

  socket.on("disconnect", () => {
    // Socket.IO makes no guarantee data.user will reference the same object...
    activeUsers.delete(socket.data.user);
  });
});

// GOOD
const activeUsers = Map();
io.on("connection", (socket) => {
  const newUser = { name: "alice" };
  socket.data.user = newUser;
  activeUsers.set(socket.data.user.name, newUser);

  socket.on("disconnect", () => {
    // ... however primitive values will be preserved
    activeUsers.delete(socket.data.user.name);
  });
});

Socket#conn

A reference to the underlying Engine.IO socket (see here).

io.on("connection", (socket) => {
  console.log("initial transport", socket.conn.transport.name); // prints "polling"

  socket.conn.once("upgrade", () => {
    // called when the transport is upgraded (i.e. from HTTP long-polling to WebSocket)
    console.log("upgraded transport", socket.conn.transport.name); // prints "websocket"
  });

  socket.conn.on("packet", ({ type, data }) => {
    // called for each packet received
  });

  socket.conn.on("packetCreate", ({ type, data }) => {
    // called for each packet sent
  });

  socket.conn.on("drain", () => {
    // called when the write buffer is drained
  });

  socket.conn.on("close", (reason) => {
    // called when the underlying connection is closed
  });
});

Additional attributes

As long as you do not overwrite any existing attribute, you can attach any attribute to the Socket instance and use it later:

// in a middleware
io.use(async (socket, next) => {
  try {
    const user = await fetchUser(socket);
    socket.user = user;
  } catch (e) {
    next(new Error("unknown user"));
  }
});

io.on("connection", (socket) => {
  console.log(socket.user);

  // in a listener
  socket.on("set username", (username) => {
    socket.username = username;
  });
});

Socket middlewares

Those middlewares look a lot like the usual middlewares, except that they are called for each incoming packet:

socket.use(([event, ...args], next) => {
  // do something with the packet (logging, authorization, rate limiting...)
  // do not forget to call next() at the end
  next();
});

The next method can also be called with an error object. In that case, the event will not reach the registered event handlers and an error event will be emitted instead:

io.on("connection", (socket) => {
  socket.use(([event, ...args], next) => {
    if (isUnauthorized(event)) {
      return next(new Error("unauthorized event"));
    }
    next();
  });

  socket.on("error", (err) => {
    if (err && err.message === "unauthorized event") {
      socket.disconnect();
    }
  });
});

Note: this feature only exists on the server-side. For the client-side, you might be interested in catch-all listeners.

Events

On the server-side, the Socket instance emits two special events:

• disconnect
• disconnecting

disconnect

This event is fired by the Socket instance upon disconnection.

io.on("connection", (socket) => {
  socket.on("disconnect", (reason) => {
    // ...
  });
});

Here is the list of possible reasons:

Reason	Description
server namespace disconnect	The socket was forcefully disconnected with socket.disconnect().
client namespace disconnect	The client has manually disconnected the socket using socket.disconnect().
server shutting down	The server is, well, shutting down.
ping timeout	The client did not send a PONG packet in the pingTimeout delay.
transport close	The connection was closed (example: the user has lost connection, or the network was changed from WiFi to 4G).
transport error	The connection has encountered an error.
parse error	The server has received an invalid packet from the client.
forced close	The server has received an invalid packet from the client.
forced server close	The client did not join a namespace in time (see the connectTimeout option) and was forcefully closed.

disconnecting

This event is similar to disconnect but is fired a bit earlier, when the Socket#rooms set is not empty yet

io.on("connection", (socket) => {
  socket.on("disconnecting", (reason) => {
    for (const room of socket.rooms) {
      if (room !== socket.id) {
        socket.to(room).emit("user has left", socket.id);
      }
    }
  });
});

Note: those events, along with connect, connect_error, newListener and removeListener, are special events that shouldn't be used in your application:

// BAD, will throw an error
socket.emit("disconnect");

Complete API

The complete API exposed by the Socket instance can be found here.