What is the difference between Subject and BehaviorSubject?

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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 get previously publised values.

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