Why does publishReplay(1).refCount() not replay the last value for late subscribers?

a = new Rx.Subject(); 
b = a.publishReplay(1).refCount(); 

a.subscribe(function(x){console.log('timely subscriber:',x)});
a.next(1); 
b.subscribe(function(x){console.log('late subscriber:',x)});

<script src=".js"></script>

Why does publishReplay(1).refCount() not replay the last value for late subscribers?

a = new Rx.Subject(); 
b = a.publishReplay(1).refCount(); 

a.subscribe(function(x){console.log('timely subscriber:',x)});
a.next(1); 
b.subscribe(function(x){console.log('late subscriber:',x)});

<script src="http://reactivex.io/rxjs/user/script/0-Rx.js"></script>

Expected output:

timely subscribe: 1
late subscriber: 1

Actual output

timely subscriber: 1

• javascript
• rxjs
• rxjs5

2 Answers 2

This happens because at the time you call a.next(1) the publishReplay(1) hasn't subscribed to its source Observable (Subject a in this case) and therefore the internal ReplaySubject won't receive the value 1.

In RxJS 5 the actual subscription between operators happens when you subscribe at the end of the chain which is b.subscribe(...) in this example. See:

• https://github./ReactiveX/rxjs/blob/master/src/operator/multicast.ts#L63
• https://github./ReactiveX/rxjs/blob/master/src/Observable.ts#L96

Until you call subscribe() the operators are chained thanks to lift() method that just takes an instance of operator and assigns it to the new Observable. The operator.call() method as you can see in the two links above is called later when subscribing. See:

• https://github./ReactiveX/rxjs/blob/master/src/Observable.ts#L67

Your first subscriber subscribes to a, since refCount first activates the stream when there is at least 1 subscriber (of which there is none, because not b but a is subscribed), it is not active until your last loc.

a = new Rx.Subject(); 
b = a.publishReplay(1).refCount(); 

b.subscribe(function(x){console.log('timely subscriber:',x)});
a.next(1); 
b.subscribe(function(x){console.log('late subscriber:',x)});

<script src="https://unpkg./rxjs/bundles/Rx.min.js"></script>