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What is the difference between Subject and BehaviorSubject?

Submitted by: @import:stackoverflow-api·Mar 10, 2026·

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Problem

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

Solution

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 published values.

Code Snippets

const subject = new Rx.Subject();
subject.next(1);
subject.subscribe(x => console.log(x));

const subject = new Rx.BehaviorSubject(0);
subject.next(1);
subject.subscribe(x => console.log(x));

Context

Stack Overflow Q#43348463, score: 594

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