rxJs 中的 flatMap 与 concatMap 的挣扎

Question

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);
                            })
                        })
                   })
               })
             })

Answer 1

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 themerge 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.

Answer 2

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