RxJS: what is bufferSize in shareReplay?

Question

I don't understand what bufferSize parameter means and what does it effect.

What are the differences between the following?

var published = source
    .shareReplay();

var published = source
    .shareReplay(0)

var published = source
    .shareReplay(1);

var published = source
    .shareReplay(10);

Answer 1

BufferSize :

• BufferSize means the number of items cached and replayed
• On subscribe, It will replays a specific number of emissions
• Items stayed Cached, even after there is no subscribers

Coming to Question :

var published = source
    .shareReplay();

any subscriber to this will get all the items/stream of data emitted by source

var published = source
    .shareReplay(0)

It will cache last emitted value

var published = source
    .shareReplay(1);

It will cache last emitted value, same like above

var published = source
    .shareReplay(10);

It will cache the last 10 items emitted by source.

More Information : I'm going to explain this concept with one example.

 let source$ = interval(1000)
  .pipe(
    take(5),
    shareReplay(3)
  )
source$.subscribe(res => console.log('1st time=>', res))

setTimeout(() => {
  source$.subscribe(res => console.log('2nd time=>', res))
}, 5000)

Note: Here first subscription just mean to start emitting the values. It will emit values five times as I m using take operator to limit the emission interval It will lead to output :

1st time=> 0
1st time=> 1
1st time=> 2
1st time=> 3 
1st time=> 4

Now Just focus on second observable : as we can see the bufferSize value is set to 3, so It will log last three emitted values

2nd time=> 2
2nd time=> 3
2nd time=> 4

Answer 2

source     --1--2--3--4--5--6--7
subscriber -----------S---------

with source.shareReplay(2) subscriber will get [2, 3, 4, 5,...]