[rxjs] What is the difference between Subject and BehaviorSubject?

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

I just created a project which explain what is the difference between all subjects:
https://github.com/piecioshka/rxjs-subject-vs-behavior-vs-replay-vs-async

BehaviorSubject keeps in memory the last value that was emitted by the observable. A regular Subject doesn't.

BehaviorSubject is like ReplaySubject with a buffer size of 1.

It might help you to understand.

import * as Rx from 'rxjs';

const subject1 = new Rx.Subject();
subject1.next(1);
subject1.subscribe(x => console.log(x)); // will print nothing -> because we subscribed after the emission and it does not hold the value.

const subject2 = new Rx.Subject();
subject2.subscribe(x => console.log(x)); // print 1 -> because the emission happend after the subscription.
subject2.next(1);

const behavSubject1 = new Rx.BehaviorSubject(1);
behavSubject1.next(2);
behavSubject1.subscribe(x => console.log(x)); // print 2 -> because it holds the value.

const behavSubject2 = new Rx.BehaviorSubject(1);
behavSubject2.subscribe(x => console.log('val:', x)); // print 1 -> default value
behavSubject2.next(2) // just because of next emission will print 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