Understanding rxjs BehaviorSubject, ReplaySubject and AsyncSubject

Luuk Gruijs
May 3, 2018 · 5 min read

Subjects are used for multicasting Observables. This means that Subjects will make sure each subscription gets the exact same value as the Observable execution is shared among the subscribers. You can do this using the Subject class. But rxjs offers different types of Subjects, namely: BehaviorSubject, ReplaySubject and AsyncSubject.

If you think you understand Subjects, read on! Else i would suggest to read my other article about Subjects: Understanding rxjs Subjects.

Photo by Cory Schadt on Unsplash

The BehaviorSubject

One of the variants of the Subject is the BehaviorSubject. The BehaviorSubject has the characteristic that it stores the “current” value. This means that you can always directly get the last emitted value from the BehaviorSubject.

There are two ways to get this last emited value. You can either get the value by accessing the .value property on the BehaviorSubject or you can subscribe to it. If you subscribe to it, the BehaviorSubject will directly emit the current value to the subscriber. Even if the subscriber subscribes much later than the value was stored. See the example below:

There are a few things happening here:

  1. We first create a subject and subscribe to that with Subscriber A. The Subject then emits it’s value and Subscriber A will log the random number.

Last but not least, you can create BehaviorSubjects with a start value. When creating Observables this can be quite hard. With BehaviorSubjects this is as easy as passing along an initial value. See the example below:

The ReplaySubject

The ReplaySubject is comparable to the BehaviorSubject in the way that it can send “old” values to new subscribers. It however has the extra characteristic that it can record a part of the observable execution and therefore store multiple old values and “replay” them to new subscribers.

When creating the ReplaySubject you can specify how much values you want to store and for how long you want to store them. In other words you can specify: “I want to store the last 5 values, that have been executed in the last second prior to a new subscription”. See example code below:

There are a few things happening here:

  1. We create a ReplaySubject and specify that we only want to store the last 2 values

As mentioned before you can also specify for how long you wan to store values in the replay subject. Let’s see an example of that:

Again, there are a few things happening here.

  1. We create the ReplaySubject and specify that we only want to store the last 2 values, but no longer than a 100 ms

The AsyncSubject

While the BehaviorSubject and ReplaySubject both store values, the AsyncSubject works a bit different. The AsyncSubject is aSubject variant where only the last value of the Observable execution is sent to its subscribers, and only when the execution completes. See the example code below:

This time there’s not a lot happening. But let’s go over the steps:

  1. We create the AsyncSubject


The BehaviorSubject, ReplaySubject and AsyncSubject can still be used to multicast just like you would with a normal Subject. They do however have additional characteristics that are very handy in different scenario’s.

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Luuk Gruijs

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Senior front-end developer writing about Angular and Javascript