RxJS Observables nested subscriptions?

Question

What's the way to simplify something like the following code example? I can't find the right operator.. could anyone give a short example?

this.returnsObservable1(...)
  .subscribe(

    success => {

      this.returnsObservable2(...)
        .subscribe(

          success => {

            this.returnsObservable3(...)
              .subscribe(

                success => {
                   ...
                },

Answer 1

As mentioned in comments, you are looking for the flatMap operator.

You can find more details in previous answers :

• How to do the chain sequence in rxjs
• Why do we need to use flatMap?

Your example would read as :

this.returnsObservable1(...)   .flatMap(success => this.returnsObservable2(...))   .flatMap(success => this.returnsObservable3(...))   .subscribe(success => {(...)});  

Answer 2

Previously answered for RxJS 5, I ended up on this page whilst using 6.

In the case you're on 6 as well (and I think you should be by now) you can use flatmap as an operator within pipe.

Modified the example code of @user3743222:

this.returnsObservable1(...)   .pipe(     flatMap(success => this.returnsObservable2(...)),     flatMap(success => this.returnsObservable3(...))   )   .subscribe(success => {(...)});  

Answer 3

The switchMap operator can also be useful. Some examples which describe the usefulness of switchMap compared to nested subscriptions can be found here:

1. situation with nested subscriptions

This codepen gives a demo: https://codepen.io/anon/pen/zdXBvP?editors=1111

Rx.Observable
  .interval(5000)
  .subscribe((val) => {
    console.log("outer:", val);
    Rx.Observable
      .interval(1000)
      .subscribe((ival) => {
        console.log("inner:", val, ival); 
      });
  });

2. situation with switchMap

This codepen gives a demo: https://codepen.io/anon/pen/xLeOZW?editors=1111

Rx.Observable
  .interval(5000)
  .switchMap((val) => {
    console.log("outer:", val);
    return Rx.Observable.interval(1000).map((ival) => [val, ival]);
  })
  .subscribe((val) => {
    console.log("inner:", val[0], val[1]);
  });

Answer 4

You basically need sequencing. With RxJS concat, you can just do this:

import { concat } from 'rxjs';
...

concat(
  this.returnsObservable1(...), 
  this.returnsObservable2(...), 
  this.returnsObservable3(...), 
  ...
)
.subscribe(success => {(...)});

If your subsequent observables need to know the values from the previous ones, you could use the RxJS concatMap operator.

import { concatMap } from 'rxjs/operators';
...

this.returnsObservable1(...)
  .pipe(
    concatMap(result1 => this.returnsObservable2(...)),
    concatMap(result2 => this.returnsObservable3(...)),
    ...
  )
  .subscribe(success => {(...)});

concat: https://www.learnrxjs.io/learn-rxjs/operators/combination/concat

concatMap: https://www.learnrxjs.io/learn-rxjs/operators/transformation/concatmap