How to get current value of RxJS Subject or Observable?

Solution 1:

A Subject or Observable doesn't have a current value. When a value is emitted, it is passed to subscribers and the Observable is done with it.

If you want to have a current value, use BehaviorSubject which is designed for exactly that purpose. BehaviorSubject keeps the last emitted value and emits it immediately to new subscribers.

It also has a method getValue() to get the current value.

Solution 2:

The only way you should be getting values "out of" an Observable/Subject is with subscribe!

If you're using getValue() you're doing something imperative in declarative paradigm. It's there as an escape hatch, but 99.9% of the time you should NOT use getValue(). There are a few interesting things that getValue() will do: It will throw an error if the subject has been unsubscribed, it will prevent you from getting a value if the subject is dead because it's errored, etc. But, again, it's there as an escape hatch for rare circumstances.

There are several ways of getting the latest value from a Subject or Observable in a "Rx-y" way:

1. Using BehaviorSubject: But actually subscribing to it. When you first subscribe to BehaviorSubject it will synchronously send the previous value it received or was initialized with.
2. Using a ReplaySubject(N): This will cache N values and replay them to new subscribers.
3. A.withLatestFrom(B): Use this operator to get the most recent value from observable B when observable A emits. Will give you both values in an array [a, b].
4. A.combineLatest(B): Use this operator to get the most recent values from A and B every time either A or B emits. Will give you both values in an array.
5. shareReplay(): Makes an Observable multicast through a ReplaySubject, but allows you to retry the observable on error. (Basically it gives you that promise-y caching behavior).
6. publishReplay(), publishBehavior(initialValue), multicast(subject: BehaviorSubject | ReplaySubject), etc: Other operators that leverage BehaviorSubject and ReplaySubject. Different flavors of the same thing, they basically multicast the source observable by funneling all notifications through a subject. You need to call connect() to subscribe to the source with the subject.

Solution 3:

I had similar situation where late subscribers subscribe to the Subject after its value arrived.

I found ReplaySubject which is similar to BehaviorSubject works like a charm in this case. And here is a link to better explanation: http://reactivex.io/rxjs/manual/overview.html#replaysubject