[Proposed Post]: RxJS: BehaviorSubject vs ReplaySubject

Proposed topic title:

RxJS: BehaviorSubject vs ReplaySubject

Topic description:

There are different ways of persisting values in RxJS's subjects. Which is the right to use?

Link to existing draft:

No response

Part of series:

No response

Resources

// BehaviorSubject caches the last value emitted
const subject = new BehaviorSubject(0); // 0 is the initial value

subject.next(1);

// A new subscriber will immediately receive the cached value (1)

subject.subscribe(x => console.log(x));

// ReplaySubject caches all values emitted until its size is reached

const rSubject = new ReplaySubject(3); // buffer 3 values for new subscribers

rSubject.next(1);
rSubject.next(2);
rSubject.next(3);
rSubject.next(4);

// A new subscriber will the last 3 values (2, 3, 4) because of the buffer
rSubject.subscribe(x => console.log(x));

// A ReplaySubject with a cache of 1 acts like a BehaviorSubject

Other Notes

No response

Start with the proposed topic and the BehaviorSubject and ReplaySubject TypeScript examples in issue #884. Research the differences and use them to produce a clear comparison explaining when each subject is appropriate; done means the proposed post is written and ready for review.