Subject and BehaviorSubject differences - Angular

What is the difference between Subject and BehaviorSubject?

BehaviourSubject will return the initial value or the current value on Subscription. 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();

subject.next(1);

subject.subscribe(x => console.log(x));

Console output: 1

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);

Output wiil be:

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. only values emitted after its subscription can be captured.

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);

Output will be:

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

subject-vs-behavior-vs-replay-vs-async

Each next subscribers receive...
Subject	...only upcoming values
BehaviorSubject	...one previous value and upcoming values
ReplaySubject	...all previous values and upcoming values
AsyncSubject	...the latest value when the stream will close