Socket

A Socket is the fundamental class for interacting with browser clients. A Socket belongs to a certain Namespace (by default /) and uses an underlying Client to communicate.

It should be noted the Socket doesn’t relate directly to the actual underlying TCP/IP socket and it is only the name of the class.

Within each Namespace, you can also define arbitrary channels (called room) that the Socket can join and leave. That provides a convenient way to broadcast to a group of Sockets (see Socket#to below).

The Socket class inherits from EventEmitter. The Socket class overrides the emit method, and does not modify any other EventEmitter method. All methods documented here which also appear as EventEmitter methods (apart from emit) are implemented by EventEmitter, and documentation for EventEmitter applies.

More information can be found here.

socket.id

• (String)

A unique identifier for the session, that comes from the underlying Client.

socket.rooms

• (Set)

A Set of strings identifying the rooms this client is in.

io.on("connection", (socket) => {

  console.log(socket.rooms); // Set { <socket.id> }

  socket.join("room1");

  console.log(socket.rooms); // Set { <socket.id>, "room1" }

});

socket.client

• (Client)

A reference to the underlying Client object.

socket.conn

• (engine.Socket)

A reference to the underlying Client transport connection (engine.io Socket object). This allows access to the IO transport layer, which still (mostly) abstracts the actual TCP/IP socket.

socket.request

• (Request)

A getter proxy that returns the reference to the request that originated the underlying engine.io Client. Useful for accessing request headers such as Cookie or User-Agent.

const cookie = require("cookie");

io.on("connection", (socket) => {
  const cookies = cookie.parse(socket.request.headers.cookie || "");
});

socket.handshake

• (Object)

The handshake details:

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

Usage:

io.use((socket, next) => {
  let handshake = socket.handshake;
  // ...
});

io.on("connection", (socket) => {
  let handshake = socket.handshake;
  // ...
});

socket.use(fn)

History

• fn (Function)

Registers a middleware, which is a function that gets executed for every incoming Packet and receives as parameter the packet and a function to optionally defer execution to the next registered middleware.

Errors passed to the middleware callback are then emitted as error events on the server-side:

io.on("connection", (socket) => {
  socket.use(([event, ...args], next) => {
    if (isUnauthorized(event)) {
      return next(new Error("unauthorized event"));
    }
    // do not forget to call next
    next();
  });

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

socket.send([…args][, ack])

• args
• ack (Function)
• Returns Socket

Sends a message event. See socket.emit(eventName[, …args][, ack]).

socket.emit(eventName[, …args][, ack])

(overrides EventEmitter.emit)

• eventName (String)
• args
• ack (Function)
• Returns true

Emits an event to the socket identified by the string name. Any other parameters can be included. All serializable datastructures are supported, including Buffer.

socket.emit("hello", "world");
socket.emit("with-binary", 1, "2", { 3: "4", 5: Buffer.from([6]) });

The ack argument is optional and will be called with the client’s answer.

io.on("connection", (socket) => {
  socket.emit("an event", { some: "data" });

  socket.emit("ferret", "tobi", (data) => {
    console.log(data); // data will be "woot"
  });

  // the client code
  // client.on("ferret", (name, fn) => {
  //   fn("woot");
  // });

});

socket.on(eventName, callback)

(inherited from EventEmitter)

• eventName (String)
• callback (Function)
• Returns Socket

Register a new handler for the given event.

socket.on("news", (data) => {
  console.log(data);
});
// with several arguments
socket.on("news", (arg1, arg2, arg3) => {
  // ...
});
// or with acknowledgement
socket.on("news", (data, callback) => {
  callback(0);
});

socket.once(eventName, listener)

socket.removeListener(eventName, listener)

socket.removeAllListeners([eventName])

socket.eventNames()

Inherited from EventEmitter (along with other methods not mentioned here). See the Node.js documentation for the events module.

socket.onAny(callback)

• callback (Function)

Register a new catch-all listener.

socket.onAny((event, ...args) => {
  console.log(`got ${event}`);
});

socket.prependAny(callback)

• callback (Function)

Register a new catch-all listener. The listener is added to the beginning of the listeners array.

socket.prependAny((event, ...args) => {
  console.log(`got ${event}`);
});

socket.offAny([listener])

• listener (Function)

Removes the previously registered listener. If no listener is provided, all catch-all listeners are removed.

const myListener = () => { /* ... */ };

socket.onAny(myListener);

// then, later
socket.offAny(myListener);

socket.offAny();

socket.listenersAny()

• Returns Function[]

Returns the list of registered catch-all listeners.

const listeners = socket.listenersAny();

socket.join(room)

• room (string) | (string[])
• Returns void | Promise

Adds the socket to the given room or to the list of rooms.

io.on("connection", (socket) => {
  socket.join("room 237");
  
  console.log(socket.rooms); // Set { <socket.id>, "room 237" }

  socket.join(["room 237", "room 238"]);

  io.to("room 237").emit("a new user has joined the room"); // broadcast to everyone in the room
});

The mechanics of joining rooms are handled by the Adapter that has been configured (see Server#adapter above), defaulting to socket.io-adapter.

For your convenience, each socket automatically joins a room identified by its id (see Socket#id). This makes it easy to broadcast messages to other sockets:

io.on("connection", (socket) => {
  socket.on("say to someone", (id, msg) => {
    // send a private message to the socket with the given id
    socket.to(id).emit("my message", msg);
  });
});

socket.leave(room)

• room (String)
• Returns void | Promise

Removes the socket from the given room.

io.on("connection", (socket) => {
  socket.leave("room 237");

  io.to("room 237").emit(`user ${socket.id} has left the room`);
});

Rooms are left automatically upon disconnection.

socket.to(room)

• room (String)
• Returns Socket for chaining

Sets a modifier for a subsequent event emission that the event will only be broadcasted to clients that have joined the given room (the socket itself being excluded).

To emit to multiple rooms, you can call to several times.

io.on("connection", (socket) => {

  // to one room
  socket.to("others").emit("an event", { some: "data" });

  // to multiple rooms
  socket.to("room1").to("room2").emit("hello");

  // a private message to another socket
  socket.to(/* another socket id */).emit("hey");

  // WARNING: `socket.to(socket.id).emit()` will NOT work, as it will send to everyone in the room
  // named `socket.id` but the sender. Please use the classic `socket.emit()` instead.
});

Note: acknowledgements are not supported when broadcasting.

socket.in(room)

Synonym of socket.to(room).

socket.compress(value)

• value (Boolean) whether to following packet will be compressed
• Returns Socket for chaining

Sets a modifier for a subsequent event emission that the event data will only be compressed if the value is true. Defaults to true when you don’t call the method.

io.on("connection", (socket) => {
  socket.compress(false).emit("uncompressed", "that's rough");
});

socket.disconnect(close)

• close (Boolean) whether to close the underlying connection
• Returns Socket

Disconnects this socket. If value of close is true, closes the underlying connection. Otherwise, it just disconnects the namespace.

io.on("connection", (socket) => {
  setTimeout(() => socket.disconnect(true), 5000);
});

Flag: ‘broadcast’

Sets a modifier for a subsequent event emission that the event data will only be broadcast to every sockets but the sender.

io.on("connection", (socket) => {
  socket.broadcast.emit("an event", { some: "data" }); // everyone gets it but the sender
});

Flag: ‘volatile’

Sets a modifier for a subsequent event emission that the event data may be lost if the client is not ready to receive messages (because of network slowness or other issues, or because they’re connected through long polling and is in the middle of a request-response cycle).

io.on("connection", (socket) => {
  socket.volatile.emit("an event", { some: "data" }); // the client may or may not receive it
});

Event: ‘disconnect’

• reason (String) the reason of the disconnection (either client or server-side)

Fired upon disconnection.

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

Possible reasons:

Event: ‘disconnecting’

• reason (String) the reason of the disconnection (either client or server-side)

Fired when the client is going to be disconnected (but hasn’t left its rooms yet).

io.on("connection", (socket) => {
  socket.on("disconnecting", (reason) => {
    console.log(socket.rooms); // Set { ... }
  });
});

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");