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Version: 3.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().


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


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