can you catch the rxjs filter like catching an either left

June 27, 2022

I would like to know if it’s possible to catch when the condition of the rxjs filter condition isn’t true.

this is what I have:

    of(1)
    .pipe(
      map((d) => d + 1),
      filter((d) => d === 0),
      map((d) => d + 1), // this will go in because of the filter
    )
    .toPromise()
    .then((d) => console.log(d)) // display indefined
    .catch(() => console.log("ERRRRRROOOOOOR")) // do not display :(

In this case, I would like to return a specific data (or at least throw) when the filter condition isn’t true.

I tried to add parameters to the filter

filter(filter((d) => d === 0), "john doe")

But this doesn’t display the string,

I guess the equivalent in fp-ts is to catch the either left but I don’t know if you can do functional programing with rxjs or if you can only work with observable.
The fact that you can using pipe and multiple operator enable the use of this library in my project so it will be good to avoid changing the library

Thanks a lot !

>Solution :

You need to throw an error to be able to catch it !

of(1)
  .pipe(
    map((d) => d + 1),
    switchMap((d) => {
      if (d === 0) {
        return of(d + 1);
      }
      throwError(() => new Error('Erroooooor'));
    })
  )
  .toPromise()
  .then((d) => console.log(d)) // display indefined
  .catch(() => console.log('ERRRRRROOOOOOR')); // thrown if d !=== 0

It could also be written using the iif operator

of(1)
  .pipe(
    map((d) => d + 1),
    switchMap((d) =>
      iif(
        () => d === 0,
        of(d + 1),
        throwError(() => new Error('Erroooooo'))
      )
    )
  )
  .toPromise()
  .then((d) => console.log(d)) // display indefined
  .catch(() => console.log('ERRRRRROOOOOOR')); // thrown if d !=== 0