foo.js

/* jshint esnext: true */

// `getStream` is some third party thing
var stream = getStream();
var events = Rx.fromEvent(stream, 'meta');

stream.write('foo');
// meta event fired by `stream`
stream.write(null);

// When subscribing to the observable, RxJS *then* starts listening for the
// `meta` event – but it's too late because the event has already been fired!
events.subscribe(x => console.log(x));