How to skip/consume/swallow a value after tapped into it? - rxjs

Question

I am trying to achieve something like below
from([1,2,3,4]).pipe(
Filter(v=> v % 2 === 0),
Tap( () => call server for even number only),
Swallow it, don't emit
).subscribe(()=>{log my odd numbers})
I know I can use filter( () => false) but was wondering if there is a better solution to it? Thanks

Answer 1

What do you mean? Swallow what? Based on what? Filter accepts/rejects values based on a predicate, if you want a better solution, what criteria would you prefer?
If you want to swallow everything, that's not functionally much different from terminating your Observable:
of([1,2,3,4]).pipe(
filter(v=> v % 2 === 0)
).subscribe(() => call server here);
Though I would expect an error as you can't use % on arrays. Though this should work:
from([1,2,3,4]).pipe(
filter(v=> v % 2 === 0)
).subscribe(() => call server here);
or
of([1,2,3,4]).pipe(
filter(v=> v.length % 2 === 0)
).subscribe(() => call server here);
Higher-order operators
If your server call is an observable-like object (promise, array, observable, ect), then you can use the higher-order operators like mergeMap, switchMap, concatMap, ect. In this case, you're not 'swallowing' a value so much as transforming (mapping) it into your call.
This also means whatever value your call server observable emits still gets passed forward. This really is a stream transformation rather than filtering/swallowing values.
from([1,2,3,4]).pipe(
filter(v=> v % 2 === 0),
concatMap(v => call server observable)
);
Update
Helper Function
/****
* Pipeable Operator:
* Takes arrays emitted by the source and spaces out their
* values by the given interval time in milliseconds
****/
function intervalArray<T>(intervalTime = 1000): OperatorFunction<T[], T> {
return pipe(
concatMap((v: T[]) =>
concat(
...v.map((value: T) =>
EMPTY.pipe(
delay(intervalTime),
startWith(value)
)
)
)
)
);
}
Mocking a hot observerable emitting values overtime:
const source = of([1,2,3,4,5]).pipe(
intervalArray(250),
shareReplay(1)
);
Solution
Here is how I might implement your question for such an observable. The simplest thing to do is to just subscribe to your observable twice:
source.pipe(
filter(v => v % 2 === 0),
concatMap(evenNumber => this.serviceCall(evenNumber))
).subscribe(/* serviceCallResult => Do Nothing */);
source.pipe(
filter(v => v % 2 !== 0)
).subscribe(oddNumber => console.log(
"I got an odd Number: ",
oddNumber
));