Blog post: https://socket.io/blog/socket-io-4-release/ Migration guide: https://socket.io/docs/v3/migrating-from-3-x-to-4-0/

Bug Fixes

• make io.to(...) immutable (ac9e8ca)

Features

• add some utility methods (b25495c)
• add support for typed events (#3822) (0107510)
• allow to exclude specific rooms when broadcasting (#3789) (7de2e87)
• allow to pass an array to io.to(...) (085d1de)

BREAKING CHANGES

• io.to(...) now returns an immutable operator

Previously, broadcasting to a given room (by calling io.to()) would mutate the io instance, which could lead to surprising behaviors, like:

io.to("room1");
io.to("room2").emit(/* ... */); // also sent to room1

// or with async/await
io.to("room3").emit("details", await fetchDetails()); // random behavior: maybe in room3, maybe to all clients

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

Links:

• Diff: https://github.com/socketio/socket.io/compare/3.1.2...4.0.0
• Client release: 4.0.0
• engine.io version: ~5.0.0
• ws version: ~7.4.2