Ignore switchMap return value

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
});