Rxjs nested subscribe with multiple inner subscriptions

Original promise based code I’m trying to rewrite:

parentPromise
    .then((parentResult) => {
        childPromise1
            .then(child1Result => child1Handler(parentResult, child1Result));
        childPromise2
            .then(child1Result => child2Handler(parentResult, child2Result));
        childPromise3
            .then(child1Result => child3Handler(parentResult, child3Result));
    });   

I’m trying to figure a way how to avoid the nested subscriptions anti-pattern in the following scenario:

parent$
    .pipe(takeUntil(onDestroy$))
    .subscribe((parentResult) => {
        child1$
            .pipe(takeUntil(onDestroy$))
            .subscribe(child1Result => child1Handler(parentResult, child1Result));
        child2$
            .pipe(takeUntil(onDestroy$))
            .subscribe(child2Result => child2Handler(parentResult, child2Result));
        child3$
            .pipe(takeUntil(onDestroy$))
            .subscribe(child3Result => child3Handler(parentResult, child3Result));
    });

What would be the correct ‘RxJS way’ to do this?

Answer

That seems pretty strange to me. You’re creating new subscription for each child every time parentResult arrives. Even though those eventually indeed will be destroyed (assuming onDestroy$ implementation is correct), seems wrong.

You probably want withLatestFrom(parent$) and three separate pipes for each child.

It might look something like: child1$.pipe(takeUntil(globalDeath$), withLatestFrom(parent$)).subscribe(([childResult, parentResult]) => ...). Not sure if my JS is correct, can’t test it at the moment; but the point is: you’re getting the latest result from the parent$ every time child1$ fires. Note that you can reverse the direction if necessary (withLatestFrom(child1$)).