I have an observable that emits values every now and then. At the consumer, I get the latest value but I’d like to get said value as a part of (and including) the previously emitted ones. I get that I need to use
switchMap to cancel the current observable and return a new one that contains all the historical emissions, adhering the latest one.
Starting with the identity mapping below, what operator should I use? I have tried with a bunch of different ones but didn’t really see a methodical way to narrow down the set of available choices.
const source = interval(3000); const transform = source.pipe(switchMap(_ => of(_))); const subscribe = transform.subscribe(val => console.log("unchanged: " + val) );
At the moment, the emitted values result a sequence 0, 1, 2, 3, 4, … but I’d like it to conserve the previously emitted values and build up an array amending the latest emission to it. So the final result would become , , [0,1], [0,1,2], [0,1,2,3], [0,1,2,3,4], ….
Which RxJs operator is the proper one to use in
switchMap(...)? Is there a better choice than
switchMap(...)? Is the approach with
pipe(...) appropriate at all?
I thought I found a pre-existing answer but that’s not really what I’m helped by.
Based on the answers, I sense that
scanMap might be an appropriate choice. I didn’t realize it when I was checking the docs for it, which might be due to my confusion and lack of certainty.
I have the impression that creating a new observable for each new emitted value is to be preferred but I can’t tell how that would affect the performance etc. as I can’t see pros and cons of using
Sounds like you could use
switchScan which is a new operator since RxJS 7 (partly implemented by me :)) but it really depends on what exactly you want to do.
const source = interval(500); const transform = source.pipe( switchScan((acc, num) => of([...acc, num]), ), );
For most cases, even
mergeScan does the job and even
scan if there’s no requirement to return an observable. At the moment, only RxJs 6 is publicly documented making the plain and merge versions more recognizable.
const source = interval(500); const transform = source.pipe( mergeScan((acc, num) => of([...acc, num]), ), );
Live demo for
switchScan. Original author’s demo (for