How do you map over a Subject?

mapping-subject.js

// Create a single subject for all emitting of data
var subject = new Rx.Subject();

// Create something just for mapping
var obs = subject.map(function(message) { 
    return message + ' dawg.';
  });

// Subscribe to that
obs.subscribe(function(message) {console.log(message);});

// Send data to the subject
subject.onNext('next!');