What is subject and what types of subjects exist in RxJS?

What is Subject in RxJS?

Subject is special type of Observable that can multicast data: i.e. send values to multiple subscribers at once.

It combines properties of Observable (emits values) and Observer (can pass values using .next()).

Features

You initiate value passing through .next()
Subscribers receive same data
Allows centralized event stream management

Regular Subject Usage Example

typescript

import { Subject } from 'rxjs';

const subject = new Subject<string>();

subject.subscribe(val => console.log('Subscriber A:', val));
subject.subscribe(val => console.log('Subscriber B:', val));

subject.next('Hello'); 
// → Subscriber A: Hello
// → Subscriber B: Hello

Subject Types

RxJS provides several Subject types, which differ in behavior and value storage:

Type	Stores value?	What new subscriber gets
`Subject`	No	Only new values after subscription
`BehaviorSubject`	Yes (only last)	Immediately gets last value
`ReplaySubject`	Yes (all or part)	Gets all/several past values
`AsyncSubject`	Yes (only final)	Gets only last when `complete()`

BehaviorSubject

Used when you need subscribers to receive current value immediately on subscription.

typescript

import { BehaviorSubject } from 'rxjs';

const behavior$ = new BehaviorSubject<number>(0);

behavior$.subscribe(val => console.log('A:', val)); // A: 0
behavior$.next(1); // A: 1
behavior$.subscribe(val => console.log('B:', val)); // B: 1

Ideal for storing state (e.g., current user, application theme, etc.)

ReplaySubject

Allows "replaying" past values for new subscribers.

typescript

import { ReplaySubject } from 'rxjs';

const replay$ = new ReplaySubject<number>(2); // stores 2 last values

replay$.next(1);
replay$.next(2);
replay$.next(3);

replay$.subscribe(val => console.log('Subscriber:', val));
// → Subscriber: 2
// → Subscriber: 3

AsyncSubject

Emits only last value — only when stream is completed (complete())

typescript

import { AsyncSubject } from 'rxjs';

const async$ = new AsyncSubject<number>();

async$.subscribe(val => console.log('Result:', val));

async$.next(1);
async$.next(2);
async$.next(3);

async$.complete(); // → Result: 3

Used for deferred values, for example, when making one-time data request

When to Use Which?

Scenario	Subject
Event stream (e.g., buttons)	`Subject`
State storage	`BehaviorSubject`
Message or log history	`ReplaySubject`
Only result after operation completion	`AsyncSubject`
Tip:

In Angular BehaviorSubject is most often used for managing component and service state, as it guarantees current value on subscription.

Content

What is Subject in RxJS?FeaturesRegular Subject Usage ExampleSubject TypesBehaviorSubjectReplaySubjectAsyncSubjectWhen to Use Which?

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