PhiLho PhiLho - 1 month ago 12x
Javascript Question

RxJS: concat three promises, distinguish results

I have three promises, Rest requests returning lists of data.

The third one has references (ids) to the first two lists, so I want to map these ids to corresponding names when I have all the data.

The mapping isn't an issue, I just use Lodash for that.

But the issue is to wait for the three promises to resolve before starting to computing this mapping.

I figured out to use


Rx.Observable.concat(p1, p2, p3).subscribe(
function onNext(list)
// Assign the list to the corresponding variable in the scope
function onError(e)
// Notify of error
function onCompleted()
// Do the mapping

My problem is that
will be called in random order. Ie. I don't know which list I receive at some point, and it is hard to tell from the data it contains.

Is there a way to track which promises produced which list? A kind of zip? concatMap? concatMapObserver? I admit I haven't fully understood the usage of the last two...


If these are promises we are talking about, I think you can have a look at the forkJoin operator. Cf.,

You could then have something like :

Rx.Observable.forkJoin(p1, p2, p3, function(p1,p2,p3){
/* your combining code here */

In short, forkJoin has semantics similar to that of RSVP.all if you know the promises library RSVP.