Edit in JSFiddle

//extend the A and B object with Backbone.Events methods
var A = _.extend({name:'A'}, Backbone.Events);
var B = _.extend({name:'B'}, Backbone.Events);

var whosListeningToMe = function(){

//Tell B to listen for the whosListeningToMe event triggered on A, and then invoke whosListeningToMe
B.listenTo(A, 'whosListeningToMe', whosListeningToMe);

A.trigger('whosListeningToMe'); //logs B
A has no whosListeningToMe event, but it still tells listeners that it's been triggered