multiScan RxJS operator

multiscan.js

/**
Run a scan operation across multiple observables

    Rx.EXT.multiScan((state$) => [
        Rx.Observable
            .just({ initial: false })
            .withLatestFrom(state$, (x, state) => ({ ...state, ...x })),

        Rx.Observable
            .interval(1000)
            .withLatestFrom(state$, (count, state) => ({ ...state, count })),

        actions.match(A.GET_FOO)
            .concat(
                Rx.Observable.just({ loading: true }),
                Rx.EXT.fetch('/path/to/foo', {
                    method: 'POST',
                    // Sneaky peak!
                    body: tojson({ value: state$.value }),
                })
                    .flatMap(getBody)
                    .flatMap((data) => ({
                        loading: false,
                        data,
                    })),
            ),
    ], { initial: true, count: null });
**/
/**
Run a scan operation across multiple observables

    const count$ = Rx.EXT.multiScan((state$) => [
        Rx.interval(1000).map((count) => ({ count })),
    ], { count: 0 });
**/
Rx.EXT.multiScan = (fn, seed, ...args) => {
    const state$ = new Rx.BehaviorSubject(seed);

    const ret = fn(state$, ...args);
    if (ret === undefined) return Rx.Observable.emtpy();

    let obs = Rx.Observable.merge(...[].concat(ret));
    if (seed !== undefined) {
        obs = obs.startWith(seed);
    }

    return obs
        .distinctUntilChanged(identity, (a, b) => a === b)
        .tap((newState) => state$.onNext(newState));
};