borisyankov/DefinitelyTyped — pull request #3286 by kompa3 (2014-12-09 11:24:47+00:00)
Title: SocketIOClient.Socket.off() arguments are optional

The arguments are actually optional. Socket.io-client package mixes into itself the implementation of component-emitter package that declares off() method with two optional arguments.
You can see method declaration on lines 90-105 here:
https://github.com/component/emitter/blob/master/index.js