RxJs - forkJoin with empty array

Question

I'm currently using forkJoin to wait for an array of Observable(s) to finish before pipe(ing) and tap(ping).

I noticed if the array is empty nothing is emitted and I cannot even tap. How do I solve this kind of problem? Should I just check if the array is empty?

myFirstFunction(...) {    const observables = ...    return forkJoin(observables) }  mySecondFunction(...) {    return myFirstFunction().pipe(tap(() => ...)) } 

Answer 1

That's because forkJoin requires all source Observables to emit at least one item and when there are no source Observables there's nothing to emit. However, forkJoin will still send the complete notification so you can use for example defaultIfEmpty operator to make sure it always emits at least one next.

forkJoin(observables).pipe(   defaultIfEmpty(null), ).subscribe(...); 

Demo: https://stackblitz.com/edit/rxjs-kkd1qa?file=index.ts

Answer 2

Additionally to martin's answer.

I had 2 observables returning arrays and if one of them gives me an empty array, it did not wait for the other observable to finish and completed instantly. You can handle such cases as follows using defaultIfEmpty.

const response1: Observable<any[]> = this.service.getResponse(params1).pipe(defaultIfEmpty([])); const response2: Observable<any[]> = this.service.getResponse(params2).pipe(defaultIfEmpty([]));  Observable.forkJoin(response1, response2).subscribe((response) => {   console.log(response); }, () => {   console.log('handle error here'); }, () => {   console.log('completed'); });