In order to ease the migration to Socket.IO v3, the v3 server is now able to communicate with v2 clients:

const io = require("socket.io")({
  allowEIO3: true // false by default
});

Note: the allowEIO3 refers to the version 3 of the Engine.IO protocol which is used in Socket.IO v2

Features

• confirm a weak but matching ETag (#3485) (161091d)
• esm: export the Namespace and Socket class (#3699) (233650c)
• add support for Socket.IO v2 clients (9925746)
• add room events (155fa63)

Bug Fixes

• allow integers as event names (1c220dd)

Links:

• Diff: https://github.com/socketio/socket.io/compare/3.0.5...3.1.0
• Client release: 3.1.0
• engine.io version: ~4.1.0
• ws version: ~7.4.2