Why withLatestFrom RxJS method not static?

Question

The RxJS withLatestFrom is an instance method. An instance of an Observable has to be newed up/created to call the method.

The merge method is a static method. There is no need of an instance needed.

Why the RxJS withLatestFrom is not a static method but an instance method?

Answer 1

I think a good example is the combineLatest operator that exists as both static and instance method. The order of source Observables to combineLatest doesn't matter. Its internals work the same way and the output is going to be the same (just the order of values in the resulting array is going to be different which is irrelevant). The same applies to concat, merge, zip, forkJoin and so on.

However with the withLatestFrom operator it's different and the order of Observables matters. There's one source Observable that controls when the operator emits. If you changed the order it'd produces different results. For example if you had the following:

Observable.withLatestFrom(o1, o2, o3)

This means you could also use the following:

Observable.withLatestFrom(...observables)

Now you can't know which Observable is the source and it's going to be hard to debug what's going on.

So the reason why there's no Observable.withLatestFrom static method is because it doesn't make much sense. The order of Observables matters and it'd make things just more obfuscated.

For the same reason other operators such as buffer or window don't have their static forms even though there's no technical limitation. It'd just allow you to write more imperative code instead of simply chaining operators.

Answer 2

withLatestFrom is different from merge, because it has the concept of a canonical "source" observable, which is augmented by the latest value from another observable. merge is not "hierarchical" like this, observables are just combined as equals into a single stream.