What operator is used to get several values from observable - rxjs

Question

return this.usersTableService.fetchRequestedPageUsersIds(request).pipe(
switchMap((idsToFetch) => {
requestedIds = idsToFetch;
return [this.usersTableService.getNewIdsToFetch(requestedIds, entities), of(idsToFetch)];
}),
//.......?(([newIds, idsToFetch]) => {
return this._fetchNewUsersFromAPI(requestedIds, request, newIds, entities);
}),
catchError((err) => of(loadPageFail(err)))
);
what operator should I use in order to get the value of the return tuple before ?

Answer 1

You can use forkJoin for this
return this.usersTableService.fetchRequestedPageUsersIds(request).pipe(
switchMap((idsToFetch) => {
return forkJoin([this.usersTableService.getNewIdsToFetch(requestedIds, entities), of(idsToFetch)]);
}),
mergeMap(([newIds, idsToFetch]) => {
return this._fetchNewUsersFromAPI(requestedIds, request, newIds, entities);
}),
catchError((err) => of(loadPageFail(err)))
)

Answer 2

You would normally use the map operator(https://stackblitz.com/edit/so-tuple-map?file=index.ts):
const obs$ = of(1).pipe(map(y => ['abc', 'def']), map(([str1, str2]) => str1 + str2))
But if you try that you will encounter other issues with your code ie:
its not good practice to store a local variable inside a switchMap then return it using of
_fetchNewUsersFromAPI needs to be inside a switchMap
Ultimately you'll still be faced with the fundamental problem of how to pass parameters down the observable chain, which I suspect is how you've ended up in this situation to begin with.
There is currently a bountied question asking about the same problem here: How to pass results between chained observables
IMO the best solution from that question is to use nested pipes ie:
const newUsers$ = requestsSubject.pipe(
switchMap(request =>
this.usersTableService.fetchRequestedPageUsersIds(request).pipe(
switchMap(idsToFetch =>
this.usersTableService.getNewIdsToFetch(idsToFetch).pipe(
switchMap(newIds =>
this._fetchNewUsersFromAPI(idsToFetch, request, newIds, entities)
)
)
)
)
)
);
An alternative way using await and toPromise:
function getUsers(request){
const idsToFetch = await this.usersTableService.fetchRequestedPageUsersIds(request).toPromise();
const newIds = await this.usersTableService.getNewIdsToFetch(idsToFetch, entities).toPromise();
const newUsers = await this._fetchNewUsersFromAPI(idsToFetch, request, newIds, entities).toPromise();
return newUsers;
}