[RxJS] AsyncSubject and ReplaySubject - Learn the Differences

We can use Subject as Observable and Observer:

// Subject should be only used to emit value for private
// Using subject as an Observer
const subject = new Subject();

subject.next(1);
subject.next(2);
subject.next(3);

// Using subject as an Observable
const source$ = subject.asObservable();

source$.subscribe(console.log);

 

AsyncSubject:

AsyncSubject only subscribe the latest value on subject complete.

const as = new AsyncSubject();

as.next(1);
as.next(2);
as.next(3);
as.complete(); // it is necessary to complete it in order to get last value

const source$ = as.asObservable();

source$.subscribe(console.log); // 3

If we don't call 'as.complete()', we won't get any value in the console.

 

Different from AsyncSubject, we have ReplySubject:

It doesn't need to call complete() in order to emit value. And it emit all the values from every beginning.

const rs = new ReplaySubject();

rs.next(1);
rs.next(2);
rs.next(3);

const source$ = as.asObservable();

source$.subscribe(console.log); // 1,2,3