on method
Used to listen to TalkJS session events
@remarks
A "message" event is fired every time a message is sent or received by the
current user (even if no TalkJS UI is visible). Your handler
function is passed
a {@link Message} object with some information about each message and its conversation.
For an example, see {@link https://gist.github.com/eteeselink/607e585eb40be76f2ed150d4090e5261}
Related method: {@link Session.off | Session.off}
Implementation
external void on(String eventType, Function handler);