Empty .to([]) sends to everybody instead of nobody

[jyustman]

Describe the bug
io.to([]).emit("hello") sends to everybody instead of nobody

To Reproduce

Socket.IO server version: 4.6.1

Server

import { Server } from "socket.io";

const io = new Server(3000, {});

io.on("connection", () => {
  io.to([]).emit("hello");
});

Socket.IO client version: 4.6.1

Client

import { io } from "socket.io-client";

const socket = io("ws://localhost:3000/", {});

socket.on("hello", () => {
  console.log("hello received");
});

Expected behavior
I expect no message to be sent when an empty array is passed to .to().

Platform:

• OS: macOS 13.2

Additional context
It's easy to work around, but the behavior was unexpected.

[myYang2016]

I have this problem too, and I have to check every time to see if it's an empty array

[darrachequesne]

@jyustman I could indeed reproduce the behavior, thanks. That is indeed surprising, I guess we can fix this in the next major release.

[inimaz]

I can take a look at this one if that's fine?

[Frank3K]

I observed the same. Could it be due to the following condition in socket.io-adapter?

https://github.com/socketio/socket.io-adapter/blob/28f60b880ad04ff4b48ade0cd529f1970643604f/lib/index.ts#L335-L355

[drmgc]

@Frank3K it is a in-memory adapter, and indeed it should be smth like if (rooms.size !== undefined). However, I have encountered same issue with a redis adapter.

So, it must be fixed in multiple places

UPD: Nevermind, the redis adapter still relies on the in-memory one

[darrachequesne]

@drmgc the issue is that there is no distinction between no room and an empty room:

Source:

socket.io/packages/socket.io/lib/broadcast-operator.ts

Lines 20 to 27 in 72d61da

	constructor(
	private readonly adapter: Adapter,
	private readonly rooms: Set<Room> = new Set<Room>(),
	private readonly exceptRooms: Set<Room> = new Set<Room>(),
	private readonly flags: BroadcastFlags & {
	expectSingleResponse?: boolean;
	} = {},
	) {}

[drmgc]

@darrachequesne yep, I found out already that no rooms is treated as no specific rooms.

I thought it would be a simple one-line fix, but properly introducing a "broadcast to [] rooms" requires changes across quite a few places of this codebase, which I'm not familiar with tbh