What is the difference between Subject and BehaviorSubject?

Question

I'm not clear on the difference between a Subject and a BehaviorSubject. Is it just that a BehaviorSubject has the getValue() function?

Answer 1

A BehaviorSubject holds one value. When it is subscribed it emits the value immediately. A Subject doesn't hold a value.

Subject example (with RxJS 5 API):

const subject = new Rx.Subject(); subject.next(1); subject.subscribe(x => console.log(x)); 

Console output will be empty

BehaviorSubject example:

const subject = new Rx.BehaviorSubject(0); subject.next(1); subject.subscribe(x => console.log(x)); 

Console output: 1

In addition:

• BehaviorSubject should be created with an initial value: new Rx.BehaviorSubject(1)
• Consider ReplaySubject if you want the subject to hold more than one value

Answer 2

BehaviourSubject

BehaviourSubject will return the initial value or the current value on Subscription

var bSubject= new Rx.BehaviorSubject(0);  // 0 is the initial value  bSubject.subscribe({   next: (v) => console.log('observerA: ' + v)  // output initial value, then new values on `next` triggers });  bSubject.next(1);  // output new value 1 for 'observer A' bSubject.next(2);  // output new value 2 for 'observer A', current value 2 for 'Observer B' on subscription  bSubject.subscribe({   next: (v) => console.log('observerB: ' + v)  // output current value 2, then new values on `next` triggers });  bSubject.next(3); 

With output:

observerA: 0 observerA: 1 observerA: 2 observerB: 2 observerA: 3 observerB: 3 

Subject

Subject does not return the current value on Subscription. It triggers only on .next(value) call and return/output the value

var subject = new Rx.Subject();  subject.next(1); //Subjects will not output this value  subject.subscribe({   next: (v) => console.log('observerA: ' + v) }); subject.subscribe({   next: (v) => console.log('observerB: ' + v) });  subject.next(2); subject.next(3); 

With the following output on the console:

observerA: 2 observerB: 2 observerA: 3 observerB: 3