combine.js

// When either California OR Colorado values are emitted from their queries,
// combine both observables 
var californiaOrColorado$ = Rx.Observable.combineLatest(california$, colorado$).switchMap((cities) => {
    // Destructure the values to combine a single array.
    var [californiaCities, coloradoCities] = cities;
    var combined = [
        //spread the arrays out to combine as one array
        ...californiaCities,
        ...coloradoCities
    ]

    // Return as a new Observable that contains the combined list.
    return Rx.Observable.of(combined);
})