Error: : fromEvent is not declared writable or has no setter

You need to spy on a property of rxjs. Using spyOnProperty will solve the error. Try this

 import * as rxjs from 'rxjs'
 import { of, fromEvent } from 'rxjs';  

spyOnProperty(rxjs, 'fromEvent').and.returnValue(of({}))

you can also add to getter/setters using this like

spyOnProperty(rxjs, 'fromEvent', 'get').and.returnValue(false)

Hope this helps