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rxjsobservablesequential

Sequentially execute observables and emit one result

I have an array of observables which I'm executing in parallel using: 

let observables: Observable<any>[]

Observable.forkJoin(observables)

This works perfectly, however, I need to execute the array of observables sequentially, and only emit one result if the last observable has been completed. That's when I tried to use

Observable.concat(observables)

But this returns multiple results, and not only one - combined - result which I get when using forkJoin. So I actually need a combination of the two.

I've tried to use the reduce functionality to execute them sequentially, like this:

return observables.reduce((previous, current) => {
  return previous.flatMap(() => current);
}, Observable.empty());

But with this solution the observables are not executed at all.

Solution

  • Assuming that your observables emit singular values, not arrays, you could rework your current approach to something like:

    return Observable.concat(...observables).reduce((acc, current) => [...acc, current], []);

    or even shorter:

    return Observable.concat(...observables).toArray();

    In the case that they emit array values, you could do the following:

    const source = Observable.concat(...observables).flatMap(list => list).toArray();