Ignore switchMap return value - rxjs

Question

I want to resolve an observable but I don't want the return value to replace the previous value in the pipe. Is there any asynchronous tap()? I need an operator like a switchMap but I want to ignore the return.
of(1).pipe(switchMap(() => of(2))).subscribe(console.log); // expected: 1
I could create a custom operator but sure there's something built-in in rxjs.

Answer 1

I ended up with this custom operator. It is like tap but resolves observables (and should be updated to also support promises).
export function switchTap<T, R>(next: (x: T) => Observable<R>): MonoTypeOperatorFunction<T>;
export function switchTap<R>(observable: Observable<R>): MonoTypeOperatorFunction<R>;
export function switchTap<T, R>(
arg: Observable<T> | ((x: T) => Observable<R>)
): MonoTypeOperatorFunction<T> {
const next: (x: any) => Observable<T | R> =
typeof arg === 'function' ? arg : (x: any): Observable<T> => arg;
return switchMap<T, T>(value => next(value).pipe(ignoreElements(), concat(of(value))));
}
Usage:
of(1).pipe(switchTap(of(2))).subscribe(console.log) // 1
or with a function:
of(1)
.pipe(
switchTap(value => {
console.log(value); // value: 1
return of(value + 1);
})
)
.subscribe(console.log); // 1

Answer 2

If you just want to simply ignore the values of the subscribe, then just don't pass in any arguments in the subscribe callback:
of(1).pipe(switchMap(() => of(2))).subscribe(()=>{
console.log('no arguments')
});
If you however want to retain the values of the first observable, things can get tricky. One way is to use Subject to retain the value:
//create a BehaviorSubject
var cache = new BehaviorSubject<any>(0);
of(1).pipe(switchMap((first) => {
cache.next(first);
return of(2);
})).subscribe(() => {
console.log(cache.value) //gives 1
});
Or you can use .map() to alter the values. This is kind of hacky and the code is harder to maintain:
of(1).pipe(switchMap((first) => {
return of(2).map(() => first);
})).subscribe((second) => {
console.log(second) //gives 1 because the values was mapped
});

Answer 3

I do it like so
of(2).pipe(
switchMap( num => this.doSmtg(num), num => num)
).subscribe(num => console.log(num)); // 2
Second param of switchmap receives two value the one passed to this.doSmtg and the value returned by doSmtg(num)'s observable.

Answer 4

For anyone new having the same problem I would advise using the resultSelector parameter supported by switchMap and other RxJS mapping operators.
Example:
switchMap(1 => of(2), (one, two) => one)
For further reading: https://www.learnrxjs.io/operators/transformation/mergemap.html

Answer 5

I think you could use delayWhen operator to achieve a similar functionality.
of(1)
.pipe(
delayWhen(value => {
console.log(value); // value: 1
return of(value + 1);
})
).subscribe(console.log); // 1