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Version: 4.x

The Server instance

The Server instance (often called io in the code examples) has a few attributes that may be of use in your application.

It also inherits all the methods of the main namespace, like namespace.use() (see here) or namespace.allSockets().

Server#engine​

A reference to the underlying Engine.IO server.

It can be used to fetch the number of currently connected clients:

const count = io.engine.clientsCount;
// may or may not be similar to the count of Socket instances in the main namespace, depending on your usage
const count2 = io.of("/").sockets.size;

Or to generate a custom session ID (the sid query parameter):

const uuid = require("uuid");

io.engine.generateId = (req) => {
return uuid.v4(); // must be unique across all Socket.IO servers
}

As of socket.io@4.1.0, the Engine.IO server emits three special events:

  • initial_headers: will be emitted just before writing the response headers of the first HTTP request of the session (the handshake), allowing you to customize them.
io.engine.on("initial_headers", (headers, req) => {
headers["test"] = "123";
headers["set-cookie"] = "mycookie=456";
});
  • headers: will be emitted just before writing the response headers of each HTTP request of the session (including the WebSocket upgrade), allowing you to customize them.
io.engine.on("headers", (headers, req) => {
headers["test"] = "789";
});
  • connection_error: will be emitted when a connection is abnormally closed
io.engine.on("connection_error", (err) => {
console.log(err.req); // the request object
console.log(err.code); // the error code, for example 1
console.log(err.message); // the error message, for example "Session ID unknown"
console.log(err.context); // some additional error context
});

Here is the list of possible error codes:

CodeMessage
0"Transport unknown"
1"Session ID unknown"
2"Bad handshake method"
3"Bad request"
4"Forbidden"
5"Unsupported protocol version"

Utility methods​

Some utility methods were added in Socket.IO v4.0.0 to manage the Socket instances and their rooms:

The serverSideEmit method was added in Socket.IO v4.1.0.

Those methods share the same semantics as broadcasting, and the same filters apply:

io.of("/admin").in("room1").except("room2").local.disconnectSockets();

Which makes all Socket instances of the "admin" namespace

  • in the "room1" room (in("room1") or to("room1"))
  • except the ones in "room2" (except("room2"))
  • and only on the current Socket.IO server (local)

disconnect.

Please note that they are also compatible with the Redis adapter (starting with socket.io-redis@6.1.0), which means that they will work across Socket.IO servers.

socketsJoin​

This method makes the matching Socket instances join the specified rooms:

// make all Socket instances join the "room1" room
io.socketsJoin("room1");

// make all Socket instances in the "room1" room join the "room2" and "room3" rooms
io.in("room1").socketsJoin(["room2", "room3"]);

// make all Socket instances in the "room1" room of the "admin" namespace join the "room2" room
io.of("/admin").in("room1").socketsJoin("room2");

// this also works with a single socket ID
io.in(theSocketId).socketsJoin("room1");

socketsLeave​

This method makes the matching Socket instances leave the specified rooms:

// make all Socket instances leave the "room1" room
io.socketsLeave("room1");

// make all Socket instances in the "room1" room leave the "room2" and "room3" rooms
io.in("room1").socketsLeave(["room2", "room3"]);

// make all Socket instances in the "room1" room of the "admin" namespace leave the "room2" room
io.of("/admin").in("room1").socketsLeave("room2");

// this also works with a single socket ID
io.in(theSocketId).socketsLeave("room1");

disconnectSockets​

This method makes the matching Socket instances disconnect:

// make all Socket instances disconnect
io.disconnectSockets();

// make all Socket instances in the "room1" room disconnect (and discard the low-level connection)
io.in("room1").disconnectSockets(true);

// make all Socket instances in the "room1" room of the "admin" namespace disconnect
io.of("/admin").in("room1").disconnectSockets();

// this also works with a single socket ID
io.of("/admin").in(theSocketId).disconnectSockets();

fetchSockets​

This method returns the matching Socket instances:

// return all Socket instances of the main namespace
const sockets = await io.fetchSockets();

// return all Socket instances in the "room1" room of the main namespace
const sockets = await io.in("room1").fetchSockets();

// return all Socket instances in the "room1" room of the "admin" namespace
const sockets = await io.of("/admin").in("room1").fetchSockets();

// this also works with a single socket ID
const sockets = await io.in(theSocketId).fetchSockets();

The sockets variable in the example above is an array of objects exposing a subset of the usual Socket class:

for (const socket of sockets) {
console.log(socket.id);
console.log(socket.handshake);
console.log(socket.rooms);
console.log(socket.data);
socket.emit(/* ... */);
socket.join(/* ... */);
socket.leave(/* ... */);
socket.disconnect(/* ... */);
}

The data attribute is an arbitrary object that can be used to share information between Socket.IO servers:

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

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

serverSideEmit​

This method allows to emit events to the other Socket.IO servers of the cluster, in a multi-server setup.

Syntax:

io.serverSideEmit("hello", "world");

And on the receiving side:

io.on("hello", (arg1) => {
console.log(arg1); // prints "world"
});

Acknowledgements are supported too:

// server A
io.serverSideEmit("ping", (err, responses) => {
console.log(responses[0]); // prints "pong"
});

// server B
io.on("ping", (cb) => {
cb("pong");
});

Notes:

  • the connection, connect and new_namespace strings are reserved and cannot be used in your application.

  • you can send any number of arguments, but binary structures are currently not supported (the array of arguments will be JSON.stringify-ed)

Example:

io.serverSideEmit("hello", "world", 1, "2", { 3: "4" });
  • the acknowledgement callback might be called with an error, if the other Socket.IO servers do not respond after a given delay
io.serverSideEmit("ping", (err, responses) => {
if (err) {
// at least one Socket.IO server has not responded
// the 'responses' array contains all the responses already received though
} else {
// success! the 'responses' array contains one object per other Socket.IO server in the cluster
}
});

Events​

The Server instance emits one single event (well, technically two, but connect is an alias for connection):

connection​

This event is fired upon a new connection. The first argument is a Socket instance.

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

Complete API​

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