Repeat a request inside mergeMap, until its return value meets a certain condition

Question

I make several sequential requests and the result of anotherRequest is used in the second mergeMap-clause. This basically works fine.

My question is: How can I repeat anotherRequest until some condition is met and only pass the approved return-value to the second mergeMap-clause?

someRequest().pipe(
  mergeMap((info) => {
    this.appService.petitionId$.next(Number(lastInfo.petitionId));
    return anotherRequest();
  }),
  mergeMap(
    ...
  ),
);

Answer 1

You could use expand in combination with filter and take(1). By using the expand-operator the call will be repeated until a given condition is met. filter will make sure that an emission will only be made if the condition was met. And take(1) will trigger an unsubscribe after the first successful emission.

Here you can find my stackblitz example

What's more, you can find the crucial part of my code below:

this.someRequest()
  .pipe(
    mergeMap((info) => {
      return this.anotherRequest().pipe(
        expand((data) =>
          this.isConditionMet(data) ? of(data) : this.anotherRequest()
        ),
        filter((data) => this.isConditionMet(data)),
        take(1)
      );
    }),
    mergeMap((res) => {
      ...
    })
  )