handler.spec.js

import { marbles } from "rxjs-marbles/jest";
import { map, startWith, scan } from "rxjs/operators";
import { Subject, merge, of } from "rxjs";

describe("basic", () => {
    it(
        "should update state by calling the handler as expected",
        marbles(m => {
            const createHandler = () => {
                const stream$ = new Subject();
                return {
                    stream$,
                    handler: val => stream$.next(val)
                };
            };

            const {
                stream$: numberStream$,
                handler: increment
            } = createHandler();

            const numberState$ = numberStream$.pipe(
                map(value => state => {
                    return {
                        ...state,
                        counter: value
                    };
                })
            );

            const initialState = {
                counter: 0
            };

            const source = numberState$.pipe(
                startWith(initialState),
                scan((state, fn) => fn(state))
            );

            // call the handler
            increment(42);

            const updatedState = { counter: 42 };
            const expected$ = m.cold("i-u", {
                i: initialState, // pases 
                u: updatedState // is not present
            });

            m.expect(source).toBeObservable(expected$);
        })
    );
});