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Version: 3.x

Migrating from 3.x to 4.0

The 4.0.0 release adds quite a lot of new features, which are detailed below, but it also contains a few API breaking changes (hence the major bump).

Please note that these breaking changes only impact the API on the server side. The Socket.IO protocol itself was not updated, so a v3 client will be able to reach a v4 server and vice-versa. Besides, the compatibility mode (allowEIO3: true) is still available between a Socket.IO v2 client and a Socket.IO v4 server.

Here is the complete list of changes:

Breaking changes# is now immutable#

Previously, broadcasting to a given room (by calling would mutate the io instance, which could lead to surprising behaviors, like:"room1");"room2").emit(/* ... */); // also sent to room1
// or with async/"room3").emit("details", await fetchDetails()); // random behavior: maybe in room3, maybe to all clients

Calling (or any other broadcast modifier) will now return an immutable instance.


const operator1 ="room1");const operator2 ="room2");const operator3 = socket.broadcast;const operator4 ="room3").to("room4");
operator1.emit(/* ... */); // only to clients in "room1"operator2.emit(/* ... */); // to clients in "room1" or in "room2"operator3.emit(/* ... */); // to all clients but the senderoperator4.emit(/* ... */); // to clients in "room3" or in "room4" but the sender

wsEngine option#

The format of the wsEngine option was updated in order to get rid of the following error:

Critical dependency: the request of a dependency is an expression

when bundling the server with webpack.


const io = require("")(httpServer, {  wsEngine: "eiows"});


const io = require("")(httpServer, {  wsEngine: require("eiows").Server});


Ensure compatibility with Swift v15 clients#

Before version 16.0.0, the Swift client would not include the EIO query parameter in the HTTP requests, and the Socket.IO v3 server would infer EIO=4 by default.

That's why a Swift client v15 was not able to connect to the server, even when the compatibility mode was enabled (allowEIO3: true), unless you explicitly specified the query param:

let manager = SocketManager(socketURL: URL(string: "http://localhost:8080")!, config: [  .log(true),  .connectParams(["EIO": "3"])])let socket = manager.defaultSocket

The Socket.IO v4 server will now infer EIO=3 if the EIO query param is not included.

The default value of pingTimeout was increased#

The default value of pingTimeout (used in the heartbeat mechanism) value was updated from 60000 to 5000 in (March 2018).

The reasoning back then:

Some users experienced long delays between disconnection on the server-side and on the client-side. The "disconnect" event would take a long time to fire in the browser, probably due to a timer being delayed. Hence the change.

That being said, the current value (5s) caused unexpected disconnections when a big payload was sent over a slow network, because it prevents the ping-pong packets from being exchanged between the client and the server. This can also happen when a synchronous task blocks the server for more than 5 seconds.

The new value (20s) thus seems like a good balance between quick disconnection detection and tolerance to various delays.

New features#

Allow excluding specific rooms when broadcasting#

Thanks to the awesome work of Sebastiaan Marynissen, you can now exclude a specific room when broadcasting:

io.except("room1").emit(/* ... */); // to all clients except the ones in "room1""room2").except("room3").emit(/* ... */); // to all clients in "room2" except the ones in "room3"
socket.broadcast.except("room1").emit(/* ... */); // to all clients except the ones in "room1" and the sendersocket.except("room1").emit(/* ... */); // same as"room4").except("room5").emit(/* ... */); // to all clients in "room4" except the ones in "room5" and the sender

Allow to pass an array to

The to() method now accepts an array of rooms.


const rooms = ["room1", "room2", "room3"];for (const room of rooms) {;}// broadcast to clients in "room1", "room2" or "room3"// WARNING !!! this does not work anymore in v4, see the breaking change aboveio.emit(/* ... */);

After:["room1", "room2", "room3"]).emit(/* ... */);["room1", "room2", "room3"]).emit(/* ... */);

Additional utility methods#

Some (long-awaited) methods were added:

  • socketsJoin: makes the matching socket instances join the specified rooms
// make all Socket instances join the "room1" roomio.socketsJoin("room1");
// make all Socket instances of the "admin" namespace in the "room1" room join the "room2" roomio.of("/admin").in("room1").socketsJoin("room2");
  • socketsLeave: makes the matching socket instances leave the specified rooms
// make all Socket instances leave the "room1" roomio.socketsLeave("room1");
// make all Socket instances of the "admin" namespace in the "room1" room leave the "room2" roomio.of("/admin").in("room1").socketsLeave("room2");
  • disconnectSockets: makes the matching socket instances disconnect
// make all Socket instances disconnectio.disconnectSockets();
// make all Socket instances of the "admin" namespace in the "room1" room disconnectio.of("/admin").in("room1").disconnectSockets();
// this also works with a single socket IDio.of("/admin").in(theSocketId).disconnectSockets();
  • fetchSockets: returns the matching socket instances
// return all Socket instancesconst sockets = await io.fetchSockets();
// return all Socket instances of the "admin" namespace in the "room1" roomconst sockets = await io.of("/admin").in("room1").fetchSockets();
// this also works with a single socket IDconst sockets = await;

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(;  console.log(socket.handshake);  console.log(socket.rooms);  socket.emit(/* ... */);  socket.join(/* ... */);  socket.leave(/* ... */);  socket.disconnect(/* ... */);}

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


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)


Typed events#

Thanks to the awesome work of Maxime Kjaer, TypeScript users can now type the events sent between the client and the server.

First, you declare the signature of each event:

interface ClientToServerEvents {  noArg: () => void;  basicEmit: (a: number, b: string, c: number[]) => void;}
interface ServerToClientEvents {  withAck: (d: string, cb: (e: number) => void) => void;}

And you can now use them on the client side:

import { io, Socket } from "";
const socket: Socket<ServerToClientEvents, ClientToServerEvents> = io();
socket.emit("basicEmit", 1, "2", [3]);
socket.on("withAck", (d, cb) => {    cb(4);});

Your IDE should now properly infer the type of each argument:

Typed events

Similarly on the server side (the ServerToClientEvents and ClientToServerEvents are reversed):

import { Server } from "";
const io = new Server<ClientToServerEvents, ServerToClientEvents>(3000);
io.on("connect", (socket) => {    socket.on("noArg", () => {      // ...    });
    socket.on("basicEmit", (a, b, c) => {      // ...    });
    socket.emit("withAck", "42", (e) => {        console.log(e);    });});

By default, the events are untyped and the arguments will be inferred as any.

autoUnref option#

And finally, thanks to the awesome work of KC Erb, the autoUnref option was added.

With autoUnref set to true (default: false), the Socket.IO client will allow the program to exit if there is no other active timer/socket in the event system:

const socket = io({  autoUnref: true});

Note: this option only applies to Node.js clients.

Known migration issues#

  • cannot get emit of undefined

The following expression:"room1").broadcast.emit(/* ... */);

was working in Socket.IO v3 but is now considered invalid, as the broadcast flag is useless because the to("room1") method already puts the Socket instance in broadcasting mode.

// VALIDsocket.broadcast.emit(/* ... */); // to all clients but the"room1").emit(/* ... */); // to clients in "room1" but the sender
// VALID (but useless 'broadcast' flag)"room1").emit(/* ... */);
//"room1").broadcast.emit(/* ... */);