https://issues.apache.org/jira/browse/MYFACES-2649 Ok after talking to Jim Driscoll he gave me clearification the spec meant that the event object either can be an event object null or undefined, so we need to check for its existence, the rest of the params list then is varargs!
myfaces/core/trunk/api/src/main/javascript/META-INF/resources/myfaces/_impl/core/jsf_impl.js