recursive rxjs switchmap ish

user_1421218 0
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I want a variable states$: an observable stream of objects, with each object containing a member nextState$ of type observable. This property nextState$ sends a unique item corresponding to the next state, and so on...

example:

const states$ = of({ nextState$: createObservableWithNextState$() }).pipe(
switchMap(state => state.nextState$),
switchMap(state => state.nextState$),
switchMap(state => state.nextState$),
switchMap(state => state.nextState$),
...
)

of course it doesn't work, for two reasons at least:

  • I don't want the ... infinite repetition of switchMap in my code
  • I want to subscribe to state$ and receive each object (including the initial one in the of())

Of course I could create my own observable from scractch but before I would like to known if it would be possible with existing rxjsoperators. Any idea ?...

rxjs javascript
1 Answers

RxJS#expand

Expand should do what you're after pretty simply.
I assume at some point you'll reach a state without a nextState$, but you can change that condition easily.

const states$ = of({ 
  nextState$: createObservableWithNextState$() 
}).pipe(
  expand(state => state.nextState$ != null? state.nextState$ : EMPTY)
);

Expand is closer to mergeMap than switchMap. You can set concurrent to 1 to make it work like concatMap. If you're really after a switchMap-like behaviour, this gets a bit more complicated.

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