// Behavior Subject

​

// a is an initial value. if there is a subscription 

// after this, it would get "a" value immediately

let bSubject = new BehaviorSubject("a"); 

​

bSubject.next("b");

​

bSubject.subscribe(value => {

  console.log("Subscription got", value); // Subscription got b, 

                                          // ^ This would not happen 

                                          // for a generic observable 

                                          // or generic subject by default

});

​

bSubject.next("c"); // Subscription got c

bSubject.next("d"); // Subscription got d

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));

​

BehaviorSubject example:

const subject = new Rx.Subject();

subject.next(1);

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