npm / socket.io

socket.io

What changed in socket.io 2 to 4, from its official release notes, upgrade guides and advisories. Sources checked 2026-10-23.

18 breaking1 deprecations8 new APIs and options1 performance2 behavior and defaults9 fixes and other

See which of these apply to your code, ranked, in your agent:

npx @unstale/cli init

socket.io 414 changes

Stateful regular expressions are rejected for dynamic namespaces.
4.8.4release notes
Calling io.to(...) or another broadcast modifier now returns an immutable instance, preventing later broadcasts from being affected by earlier calls.
4.0.0release notes

url.parse() is deprecated and has been replaced by new URL().
4.8.2release notes

The types allow emitWithAck() for events with void callbacks.
4.8.4release notes
The http module is imported correctly in the types.
4.8.4release notes
Pending acknowledgements are cleaned up on timeout to prevent a memory leak.
4.8.4release notes
Prevents uWebSockets.js from serving missing client files.
4.8.4release notes
Calling io.close() on a stopped server no longer throws.
4.8.3release notes
adapter.init() is now called when creating each namespace.
4.8.2release notes
The io.close() function was improved.
4.8.2release notes

Clients with no namespace are now closed.
3.0.0release notes
The connected map was renamed to sockets.
3.0.0release notes
Connecting to a different namespace no longer implicitly connects to the default namespace.
3.0.0release notes
The Engine.IO id is no longer reused.
3.0.0release notes
Middleware errors now emit an Error object.
3.0.0release notes
Namespace#connected is now a Map instead of an object.
3.0.0release notes
The origins option was removed; configure CORS with cors and use allowRequest for validation.
3.0.0release notes
Using reserved event names now throws.
3.0.0release notes
The backward-compatibility Server#set() method was removed.
3.0.0release notes
Socket#binary() was removed; this use case is covered by providing a custom parser.
3.0.0release notes
Socket#join() is synchronous and no longer accepts a callback; custom adapters can use await.
3.0.0release notes
Socket#leave() is synchronous and no longer accepts a callback argument; custom adapters can use await.
3.0.0release notes
Socket#rooms is now a Set instead of an object.
3.0.0release notes
The Socket#use() method was removed.
3.0.0release notes

null is no longer accepted as an argument.
2.0.0release notes
A breaking change to utf-8 encoding in engine.io-parser makes this release not backward-compatible.
2.0.0release notes

Applications can provide a custom parser to meet their needs.
2.0.0release notes

Engine.IO and Socket.IO handshake packets were merged, reducing the roundtrips needed to establish a connection.
2.0.0release notes

The join method now allows joining several rooms at once.
2.0.0release notes
path.resolve is used by default, with require.resolve as a fallback.
2.0.0release notes

Connections are now properly closed on error.
2.0.0release notes
The socket id on the client now matches the id on the server-side.
2.0.0release notes