How to get "Observable.of([]);" to work?

Since RxJS 6 you should import all "creation" observables directly from 'rxjs' (assuming you have path maps set when bundling your app).

More detailed explanation: https://github.com/ReactiveX/rxjs/blob/master/MIGRATION.md#import-paths

import { of } from 'rxjs';

of(1).subscribe(console.log);

See demo: https://stackblitz.com/edit/typescript-e3lxkb?file=index.ts