const Observable = Rx.Observable;

const Subject = Rx.Subject;

​

const sources = {

  'a': Observable.of(1),

  'b': Observable.of(42),

  'c': Observable.of(3),

};

​

const keys = Object.keys(sources);

​

console.log(keys);

​

Observable.forkJoin(Object.values(sources))

  .mergeMap(results => Observable.zip(results, keys))

  .subscribe();

​

​