Develop Reference

Can this rxjs merge logic be simplified?

I have an observable (onAuthStateChanged) from the Firebase client that:
emits null immediately if the user is not signed in, and
emits null and then a user object a few moments later if the user is signed in.
const observable = new Observable((obs) => {
return app.auth().onAuthStateChanged(
obs.next,
obs.error,
obs.complete
)
})
What I want is to:
ignore any emitted null values for the first 1000ms of the app lifecycle (null coming after 1000ms is accepted)
always emit user object regardless of what time it comes
if no user object comes in the first 1000ms, then emit null at the 1000ms mark
Here is what I've done (and it seems to work). However, I'm reluctant to use this code as it doesn't seem that concise:
const o1 = observable.pipe(skipUntil(timer(1000)))
const o2 = observable.pipe(
takeUntil(o1),
filter((user) => user !== null)
)
const o3 = timer(1000).pipe(takeUntil(o2), mapTo(null))
merge(o1, o2, o3).subscribe({
next: setUser,
error: console.log,
complete: () => console.log("error: obs completed, this shouldn't happen"),
})
Is there a way to do this without merge? I tried going through the docs but I'm quite lost.
Thanks for your help!

You could use concat instead of merge. Think of it as using the first source until it completes, then use the second source.
const nonNullUser = firebaseUser.pipe(
filter(user => user !== null),
takeUntil(timer(1000))
);
const user = concat(nonNullUser, firebaseUser);
user.subscribe(...);
I just realized that this solution will not explicitly perform step #3 "emit null at the 1000ms mark". I was thinking subscribing to firebaseUser would emit the latest value. But, I'm not sure if that's true for your scenario.
If not, we could easily achieve this by adding shareReplay like this:
const firebaseUser = observable.pipe(shareReplay(1));

While I liked the answer from #BizzyBob I was genuinely intrigued by these requirements that I wanted to see what other options were available. Here's what I produced:
const auth$ = observable.pipe(
startWith(null)
)
const null$ = timer(1000).pipe(
switchMap(_=>auth$)
)
const valid$ = auth$.pipe(
filter(user=>!!user)
)
const user$ = race(null$, valid$);
We have our source auth$ observable which gets your Firebase data. However, startWith() will immediately emit null before any values coming from Firebase.
I declared two observables for null and non-null cases, null$ and valid$.
The null$ observable will subscribe to auth$ after 1000ms. When this happens it immediately emits null thanks to the startWith() operator.
The valid$ observable subscribes to auth$ immediately but only emits valid user data thanks to filter(). It won't emit startWith(null) because it is caught by the filter.
Last, we declare user$ by using the race() operator. This operator accepts a list of observables as its parameters. The first observable to emit a value wins and is the resulting subscription.
So in our race, valid$ has 1000ms to emit a valid user. If it doesn't, race() will subscribe to null$ resulting in the immediate null, and all future values coming from Firebase.

Struggling with flatMap vs concatMap in rxJs

I am struggling to understand the difference between the flatMap and concatMap in rxJs.
The most clear answer that I could understand was that here difference-between-concatmap-and-flatmap
So I went and tried things out by my self.
import "./styles.css";
import { switchMap, flatMap, concatMap } from "rxjs/operators";
import { fromFetch } from "rxjs/fetch";
import { Observable } from "rxjs";
function createObs1() {
return new Observable<number>((subscriber) => {
setTimeout(() => {
subscriber.next(1);
subscriber.complete();
}, 900);
});
}
function createObs2() {
return new Observable<number>((subscriber) => {
setTimeout(() => {
subscriber.next(2);
//subscriber.next(22);
//subscriber.next(222);
subscriber.complete();
}, 800);
});
}
function createObs3() {
return new Observable<number>((subscriber) => {
setTimeout(() => {
subscriber.next(3);
//subscriber.next(33);
//subscriber.next(333);
subscriber.complete();
}, 700);
});
}
function createObs4() {
return new Observable<number>((subscriber) => {
setTimeout(() => {
subscriber.next(4);
subscriber.complete();
}, 600);
});
}
function createObs5() {
return new Observable<number>((subscriber) => {
setTimeout(() => {
subscriber.next(5);
subscriber.complete();
}, 500);
});
}
createObs1()
.pipe(
flatMap((resp) => {
console.log(resp);
return createObs2();
}),
flatMap((resp) => {
console.log(resp);
return createObs3();
}),
flatMap((resp) => {
console.log(resp);
return createObs4();
}),
flatMap((resp) => {
console.log(resp);
return createObs5();
})
)
.subscribe((resp) => console.log(resp));
console.log("hellooo");
I have used that playground here playground example
Questions
1)
From my understanding the use of flatMap should mix the outputs so that the console logs are like (1,3,2,4,5). I have tried more than 30 times and always come on the same row (1, 2, 3, 4, 5)
What am I doing wrong or have undestood wrong?
2)
If on createObs2() and createObs3() you remove the comments and include the code with multiple emitted events then things get messy. Even if you change to concatMap it messes things and results come mixed. Multiple numbers that I expect only once come multiple times. The result can be (1, 2, 33, 3, 2, 22, 3, 33, 4, 5, 4, 3, 4, 5) Why this happens?
How I test the example on playground. I just remove only 1 letter from the last console.log("hello"). Only one change for example console.log("heloo") and is then observed and project is compiled again and output printed in console.
Edit: The reason I have gone to flatMap and concatMap was to find a replacement for nested subscriptions in angular using the http library.
createObs1().subscribe( (resp1) => {
console.log(resp1);
createObs2().subscribe( (resp2) => {
console.log(resp2);
createObs3().subscribe( (resp3) => {
console.log(resp3);
createObs4().subscribe( (resp4) => {
console.log(resp4);
createObs5().subscribe( (resp5) => {
console.log(resp5);
})
})
})
})
})

Your test scenario is not really sufficient to see the differences between these two operators. In your test case, each observable only emits 1 time. If an observable only emits a single value, there is really no different between concatMap and flatMap (aka mergeMap). The differences can only be seen when there are multiple emissions.
So, let's use a different scenario. Let's have a source$ observable that simply emits an incrementing integer every 1 second. Then, within our "Higher Order Mapping Operator" (concatMap & mergeMap), we will return an observable that emits a variable number of times every 1 second, then completes.
// emit number every second
const source$ = interval(1000).pipe(map(n => n+1));
// helper to return observable that emits the provided number of times
function inner$(max: number, description: string): Observable<string> {
return interval(1000).pipe(
map(n => `[${description}: inner source ${max}] ${n+1}/${max}`),
take(max),
);
}
Then let's define two separate observables based on the source$ and the inner$; one using concatMap and one using flatMap and observe the output.
const flatMap$ = source$.pipe(
flatMap(n => inner$(n, 'flatMap$'))
);
const concatMap$ = source$.pipe(
concatMap(n => inner$(n, 'concatMap$'))
);
Before looking the differences in the output, let's talk about what these operators have in common. They both:
subscribe to the observable returned by the passed in function
emit emissions from this "inner observable"
unsubscribe from the inner observable(s)
What's different, is how they create and manage inner subscriptions:
concatMap - only allows a single inner subscription at a time. As it receives emissions, it will only subscribe to one inner observable at a time. So it will initially subscribe to the observable created by "emission 1", and only after it completes, will it subscribe to the observable created by "emission 2". This is consistent with how the concat static method behaves.
flatMap (aka mergeMap) - allows many inner subscriptions. So, it will subscribe to the inner observables as new emissions are received. This means that emissions will not be in any particular order as it will emit whenever any of its inner observables emit. This is consistent with how the merge static method behaves (which is why I personally prefer the name "mergeMap").
Here's a StackBlitz that shows the output for the above observables concatMap$ and mergeMap$:
Hopefully, the above explanation helps to clear up your questions!
#1 - "use of flatMap should mix the outputs"
The reason this wasn't working as you expected was because only one emission was going through the flatMap, which means you only ever had a single "inner observable" emitting values. As demonstrated in the above example, once flatMap receives multiple emissions, it can have multiple inner observables that emit independently.
#2 - "...and include the code with multiple emitted events then things get messy."
The "things get messy" is due to having multiple inner subscription that emit values.
For the part you mention about using concatMap and still getting "mixed" output, I would not expect that. I have seen weird behavior in StackBlitz with observable emissions when "auto save" is enabled (seems like sometimes it doesn't completely refresh and old subscriptions seem to survive the auto refresh, which gives very messy console output). Maybe code sandbox has a similar problem.
#3 - "The reason I have gone to flatMap and concatMap was to find a replacement for nested subscriptions in angular using the http library"
This makes sense. You don't want to mess around with nested subscriptions, because there isn't a great way to guarantee the inner subscriptions will be cleaned up.
In most cases with http calls, I find that switchMap is the ideal choice because it will drop emissions from inner observables you no longer care about. Imagine you have a component that reads an id from a route param. It uses this id to make an http call to fetch data.
itemId$ = this.activeRoute.params.pipe(
map(params => params['id']),
distinctUntilChanged()
);
item$ = this.itemId$.pipe(
switchMap(id => http.get(`${serverUrl}/items/${id}`)),
map(response => response.data)
);
We want item$ to emit only the "current item" (corresponds to the id in the url). Say our UI has a button the user can click to navigate to the next item by id and your app finds itself with a click-happy user who keeps smashing that button, which changes the url param even faster than the http call can return the data.
If we chose mergeMap, we would end up with many inner observables that would emit the results of all of those http calls. At best, the screen will flicker as all those different calls come back. At worst (if the calls came back out of order) the UI would be left displaying data that isn't in sync with the id in the url :-(
If we chose concatMap, the user would be forced to wait for all the http calls to be completed in series, even though we only care about that most recent one.
But, with switchMap, whenever a new emission (itemId) is received, it will unsubscribe from the previous inner observable and subscribe to the new one. This means it will not ever emit the results from the old http calls that are no longer relevant. :-)
One thing to note is that since http observables only emit once, the choice between the various operators (switchMap, mergeMap, concatMap) may not seem to make a difference, since they all perform the "inner observable handling" for us. However, it's best to future-proof your code and choose the one that truly gives you the behavior you would want, should you start receiving more than a single emission.

Every time the first observable emits, a second observable is created in the flatMap and starts emitting. However, the value from the first observable is not passed along any further.
Every time that second observable emits, the next flatMap creates a third observable, and so on. Again, the original value coming into the flatMap is not passed along any further.
createObs1()
.pipe(
flatMap(() => createObs2()), // Merge this stream every time prev observable emits
flatMap(() => createObs3()), // Merge this stream every time prev observable emits
flatMap(() => createObs4()), // Merge this stream every time prev observable emits
flatMap(() => createObs5()), // Merge this stream every time prev observable emits
)
.subscribe((resp) => console.log(resp));
// OUTPUT:
// 5
So, it's only the values emitted from createObs5() that actually get emitted to the observer. The values emitted from the previous observables have just been triggering the creation of new observables.
If you were to use merge, then you would get what you may have been expecting:
createObs1()
.pipe(
merge(createObs2()),
merge(createObs3()),
merge(createObs4()),
merge(createObs5()),
)
.subscribe((resp) => console.log(resp));
// OUTPUT:
// 5
// 4
// 3
// 2
// 1

Why does the rxjs share operator not work as expected in this setTimeout() example?

I don't understand why the rxjs share operator does not work with setTimeout().
I'm trying to understand this blogpost. In this example, the concept of "shared subscription" does not seem to work as expected.
const observable1 = Observable.create(observer => {
observer.next(`I am alive.`);
setTimeout(() => {
observer.next(`I am alive again.`);
}, 1000);
}).pipe(share());
observable1.subscribe(x => console.log(x));
observable1.subscribe(x => console.log(x));
Expected:
I am alive.
I am alive again.
Actual:
I am alive.
I am alive again.
I am alive again.
Reproducable stackblitz.

That is the expected output.
From official docs on share() operator:
Returns a new Observable that multicasts (shares) the original Observable. As long as there is at least one Subscriber this Observable will be subscribed and emitting data.
That means as soon as an observer subscribes, the observable starts emitting data.
So when the first subscribe statement observable1.subscribe(x => console.log(x)); executes, an observer subscribes and data is emitted by observer.next('I am alive.); statement.
When second subscribe statement executes, another observer subscribes and it receives only the data emitted from that point of time. This is the data emitted by observer.next('I am alive again.'); in setTimeout() method.
We can see this clearly in this StackBlitz demo where we are logging Observer1 and Observer2 text along with the received data.
I think the point of confusion is seeing two I am alive again. statements. It is logged twice because we are logging it in each subscriber. Move these log statements to the observable and they will only be logged once. This makes it more evident that the observable is executed only once.

This is the supposed behaviour of share(). It monitores and shares only one action. Here is an example taken from learnrxjs.com. As you can see only the tap()-operator is monitored. The mapTo()-operator is ignored.
// RxJS v6+
import { timer } from 'rxjs';
import { tap, mapTo, share } from 'rxjs/operators';
//emit value in 1s
const source = timer(1000);
//log side effect, emit result
const example = source.pipe(
tap(() => console.log('***SIDE EFFECT***')),
mapTo('***RESULT***')
);
/*
***NOT SHARED, SIDE EFFECT WILL BE EXECUTED
TWICE***
output:
"***SIDE EFFECT***"
"***RESULT***"
"***SIDE EFFECT***"
"***RESULT***"
*/
const subscribe = example.subscribe(val => console.log(val));
const subscribeTwo = example.subscribe(val => console.log(val));
//share observable among subscribers
const sharedExample = example.pipe(share());
/*
***SHARED, SIDE EFFECT EXECUTED ONCE***
output:
"***SIDE EFFECT***"
"***RESULT***"
"***RESULT***"
*/
const subscribeThree = sharedExample.subscribe(val => console.log(val));
const subscribeFour = sharedExample.subscribe(val => console.log(val));

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?

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$)).

You can: 1) pass parent$ through share, and 2) use flatMap three times, something like:
const sharedParent$ = parent$.pipe(share());
sharedParent$.pipe(
flatMap(parentResult => forkJoin(of(parentResult), child1$)),
takeUntil(onDestroy$)),
.subscribe((results) => child1Handler(...results)); // repeat for all children
(If there's more than 2 children, extracting that into a function with child stream and handler as parameters is a good idea).
That's following the original behavior of waiting with subscribing children until parent$ emits. If you don't need that, you can skip flatMap and just forkJoin sharedParent$ and children.

How about using higher order observables? Something like this:
const parentReplay$ = parent$.pipe(shareReplay(1));
of(
[child1$, child1Handler],
[child2$, child2Handler],
[child3$, child3Handler]
).pipe(
mergeMap([child$, handler] => parentReplay$.pipe(
mergeMap(parentResult => child$.pipe(
tap(childResult => handler(parentResult, childResult))
)
)
).subscribe();

If you were using Promises then the corresponding Observables emit only once and then complete.
If this is the case, you can use forkJoin to execute in parallel the child Observables.
So the code could look like
parent$.pipe(
takeUntil(onDestroy$),
// wait for parent$ to emit and then move on
// the following forkJoin executes the child observables in parallel and emit when all children complete - the value emitted is an array with the 3 notifications coming from the child observables
concatMap(parentResult => forkJoin(child1$, child2$, child3$)).pipe(
// map returns both the parent and the children notificiations
map(childrenResults => ({parentResult, childrenResults})
)
).subscribe(
({parentResult, childrenResults}) => {
child1Handler(parentResult, childrenResults[0]);
child1Handler(parentResult, childrenResults[1]);
child1Handler(parentResult, childrenResults[2]);
}
)

RxJS share parent observable among partitioned child observables

I'm coding a game in which the character can fire their weapon.
I want different things to happen when the player tries to fire, depending on whether they have ammo.
I reduced my issue down to the following code (btw I'm not sure why SO's snippet feature does not work, so I made CodePen where you can try out my code).
const { from, merge } = rxjs;
const { partition, share, tap } = rxjs.operators;
let hasAmmo = true;
const [ fire$, noAmmo$ ] = from([true]).pipe(
share(),
partition(() => hasAmmo),
);
merge(
fire$.pipe(
tap(() => {
hasAmmo = false;
console.log('boom');
}),
),
noAmmo$.pipe(
tap(() => {
console.log('bam');
}),
)
).subscribe({
next: val => console.log('next', val),
error: val => console.log('error', val),
complete: val => console.log('complete', val),
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/6.3.3/rxjs.umd.js"></script>
When I run this code I get the following:
"boom"
"next" true
"bam"
"next" true
"complete" undefined
I don't understand why I get a "bam".
The first emission goes to fire$ (I get a "boom"), which makes sense because hasAmmo is true. But as a side-effect of fire$ emitting is that the result of the partition condition changes, which I guess is causing me to get a "bam".
Am I not supposed to cause side-effects that affect partition()?
Or maybe is there an issue with the way I share() my parent observable? I may be wrong but I would intuitively think the fire$ and noAmmo$ internally subscribe to the parent in order to split it, in which case share() should work?

It actually works correctly. The confusion comes from the partition operator which is basically just two filter operators.
If you rewrite it without partition it looks like this:
const fire$ = from([true]).pipe(
share(),
filter(() => hasAmmo),
);
const noAmmo$ = from([true]).pipe(
share(),
filter(() => !hasAmmo),
);
Be aware that changing hasAmmo has no effect on partition itself. partition acts only when it receives a value from its source Observable.
When you later use merge() it makes two separate subscriptions to two completely different chains with two different from([true])s. This means that true is passed to both fire$ and noAmmo$.
So share() has no effect here. If you want to share it you'll have to wrap from before using it on fire$ and noAmmo$. If the source Observable is just from it's unfortunately going to be even more confusing because the initial emission will arrive only to the first subscriber which is fire$ later when used in merge:
const shared$ = from([true]).pipe(
share(),
);
const fire$ = shared$.pipe(...);
const noAmmo$ = shared$.pipe(...);
The last thing why you're receiving both messages is that partition doesn't modify the value that goes through. It only decides which one of the returned Observable will reemit it.
Btw, rather avoid partition completely because it's probably going to be deprecated and just use filter which is more obvious:
https://github.com/ReactiveX/rxjs/issues/3797
https://github.com/ReactiveX/rxjs/issues/3807