borisyankov/DefinitelyTyped — pull request #1003 by areel (2013-09-05 17:40:46+00:00)
Title: If eventName is an instance of an event map e.g '{change:action}' then 

If eventName is an instance of an event map e.g '{change:action}' then Callback is not required. So callback should be marked as optional. Related to earlier commit (cb1b62852708536407d535aae31af77fef68cdd4)