What is the difference between a Observable and a Subject in rxjs?

Question

i was going through this blog and to read about Observables and i couldnt figure out the difference between the Observable and a Subject

Answer 1

From another perspective, it is good to note that the subscription to an Observable re-execute the Observable function. This can lead performance issue if the data source is a service for instance.

If you want several subscribers to get the same value, you may need a Subject. For this, make sure that your subscription is set before the Subject subscribed to the data source. Otherwise, your process would be stuck.

More details here: https://javascript.tutorialhorizon.com/2017/03/23/rxjs-subject-vs-observable/

Answer 2

In stream programming there are two main interfaces: Observable and Observer.

Observable is for the consumer, it can be transformed and subscribed:

observable.map(x => ...).filter(x => ...).subscribe(x => ...)

Observer is the interface which is used to feed an observable source:

observer.next(newItem)

We can create new Observable with an Observer:

var observable = Observable.create(observer => { 
    observer.next('first'); 
    observer.next('second'); 
    ... 
});
observable.map(x => ...).filter(x => ...).subscribe(x => ...)

Or, we can use a Subject which implements both the Observable and the Observer interfaces:

var source = new Subject();
source.map(x => ...).filter(x => ...).subscribe(x => ...)
source.next('first')
source.next('second')