create-observables.js

// Query for California
var californiaRef = citiesRef.where("state", "==", "CA");

// Query for Colorado
var coloradoRef = citiesRef.where("state", "==", "CO");

// Create Observables.
var california$ = new Rx.Subject();
var colorado$ = new Rx.Subject();

// Hook values from callback to the observable
californiaRef.onSnapshot((querySnapshot) => {
    var data = querySnapshot.docs.map(d => d.data());
    california$.next(data);
});

coloradoRef.onSnapshot((querySnapshot) => {
    var data = querySnapshot.docs.map(d => d.data());
    colorado$.next(data);
});