combine-rxfire.js

import { combineLatest } from 'rxjs';

var californiaOrColorado$ = combineLatest(california$, colorado$)
.pipe(
  map((cities) => {
    // Destructure the values to combine a single array.
    var [californiaCities, coloradoCities] = cities;
    return [
        //spread the arrays out to combine as one array
        ...californiaCities,
        ...coloradoCities
    ];

}));

californiaOrColorado$.subscribe(cities => console.log(cities));