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What is the difference between Subject and BehaviorSubject?

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

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

var bSubject= new Rx.BehaviorSubject(0);  // 0 is the initial value

  next: (v) => console.log('observerA: ' + v)  // output initial value, then new values on `next` triggers
});;  // output new value 1 for 'observer A';  // output new value 2 for 'observer A', current value 2 for 'Observer B' on subscription

  next: (v) => console.log('observerB: ' + v)  // output current value 2, then new values on `next` triggers

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();; //Subjects will not output this value

  next: (v) => console.log('observerA: ' + v)
  next: (v) => console.log('observerB: ' + v)

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