What's the use case of Notification in RxJS?

Question

I'm somewhat familiar with basic RxJS concepts like Observables, Observers and Subjects but RxJS Notifications concept is completely new to me.

What is it for? When should I use it?

Answer 1

The documentation you quoted mentions :

This class is particularly useful for operators that manage notifications, like materialize, dematerialize, observeOn, and others. Besides wrapping the actual delivered value, it also annotates it with metadata of, for instance, what type of push message it is (next, error, or complete).

So the question turns out to be about use cases for materialize and the like.

Basically, you use materialize to get meta-information about the dataflow without incurring into the associated side-effects (an error incurring in a stream for example propagates, a stream which completes can lead to the completion of other streams etc.). dematerialize allows to restore the side-effects.

Here are uses case from former SO questions :

• Receiving done notifications from observables built using switch
• RxJs - parse file, group lines by topics, but I miss the end

Answer 2

A use case: as errors or completions are propagated immediately, you can't for example delay them. To do so, you can try this approach:

// sample stream
interval(500).pipe(
  mapTo('normal value'),
  // sometimes value, sometimes throw
  map(v => {
    if (randomInt() > 50) {
     throw new Error('boom!')
    } else return v;
   }),
  materialize(),
  // turns Observable<T> into Notification<Observable<T>>
  // so we can delay or what you want
  delay(500),
  // and we need to do some magic and change Notification of error into
  // Notification of value (error message)
  map(n => n.hasValue? n : new Notification('N', n.error.message, null)),
  // back to normal
  dematerialize()
)
// now it never throw so in console we will have 
// `normal value` or `boom!` but all as... normal values (next() emmision)
// and delay() works as expected
.subscribe(v => console.log(v))