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What is the difference between Subject, BehaviorSubject, and ReplaySubject?

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01

Understand the problem

Compare RxJS subjects.

rxjssubject
02

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03

Study the solution

A Subject multicasts to subscribers but emits nothing on subscribe. A BehaviorSubject requires an initial value and gives new subscribers the latest value. A ReplaySubject replays a buffered number of past emissions to new subscribers.

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04

Read the code

Late subscriber behaviour
import { Subject, BehaviorSubject, ReplaySubject } from 'rxjs';

const s = new Subject<number>();
const b = new BehaviorSubject<number>(0);   // initial value
const r = new ReplaySubject<number>(2);     // buffer last 2

s.next(1); b.next(1); r.next(1); r.next(2);

s.subscribe(v => console.log('subject:', v));   // (nothing — missed 1)
b.subscribe(v => console.log('behavior:', v));  // 1  (current value)
r.subscribe(v => console.log('replay:', v));    // 1, 2  (last two)
05

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