Subject vs BehaviorSubject vs ReplaySubject vs AsyncSubject in Angular: when do you use which?

MediumIntermediateAngular

By FrontendAtlas Team · Updated Feb 26, 2026

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Quick Answer

All four are RxJS Subject variants, but they differ in what late subscribers receive and when values are emitted. Interviewers expect you to choose the right one based on initial value needs, replay history, completion behavior, and memory tradeoffs.

Answer

Core idea

These are all multicasting primitives, but they answer different product questions:
• Do I need an initial value right now?
• Should late subscribers see old values?
• Do I only care about the final value after completion?

Pick the wrong one and your UI will either miss state, replay too much history, or never emit when you expect.

Type	Initial value required?	What a late subscriber gets	Best fit
`Subject`	No	Only future emissions	Fire-and-forget events (click intents, toast triggers)
`BehaviorSubject`	Yes	Latest value immediately	Current UI state (selected tab, auth status, filter value)
`ReplaySubject(n)`	No	Last `n` values	History/buffer use-cases (chat stream, recent actions)
`AsyncSubject`	No	Final value only, and only after completion	One-shot workflows where only the final result matters

The interview-grade difference in one table

TYPESCRIPT

import { Subject, BehaviorSubject, ReplaySubject, AsyncSubject } from 'rxjs';

// 1) Subject: event bus for "do something now" signals
const refreshClick$ = new Subject<void>();

// 2) BehaviorSubject: always has a current state value
const selectedTheme$ = new BehaviorSubject<'light' | 'dark'>('light');

// 3) ReplaySubject: keep last N values for late subscribers
const recentSearches$ = new ReplaySubject<string>(3);

// 4) AsyncSubject: emit only final value when complete() is called
const finalReport$ = new AsyncSubject<string>();
finalReport$.next('draft');
finalReport$.next('final');
finalReport$.complete(); // subscribers receive only 'final'

Scenario	Pick	Why
Button click intent stream	`Subject`	No initial or replayed value needed
Store current search/filter state	`BehaviorSubject`	New subscribers need immediate current value
Keep a short in-memory event history	`ReplaySubject(2-5)`	Late subscribers need recent context
Only publish final batch/job result	`AsyncSubject`	Intermediate values are noise

Fast decision guide for real Angular code

TYPESCRIPT

import { Injectable } from '@angular/core';
import { BehaviorSubject, Subject } from 'rxjs';

@Injectable({ providedIn: 'root' })
export class SearchUiStore {
  private readonly querySubject = new BehaviorSubject<string>('');
  readonly query$ = this.querySubject.asObservable();

  private readonly refreshSubject = new Subject<void>();
  readonly refresh$ = this.refreshSubject.asObservable();

  setQuery(value: string): void {
    this.querySubject.next(value);
  }

  refresh(): void {
    this.refreshSubject.next();
  }
}

Common pitfall	Why it hurts	Fix
Using `BehaviorSubject` with a fake default like `null`	Consumers must handle meaningless initial states everywhere	Use `ReplaySubject(1)` if no true initial value exists
Using large `ReplaySubject` buffers in long-lived services	Memory grows and old state lingers too long	Keep buffer small and intentional
Choosing `AsyncSubject` for never-ending streams	No value is emitted because completion never happens	Use `BehaviorSubject`/`ReplaySubject` for ongoing streams
Exposing the Subject itself publicly	Any consumer can call `next()` and break state discipline	Keep Subject private and expose only `asObservable()`

Mistakes that show up in production code reviews

Interview summary

Subject = live events only. BehaviorSubject = current value + future updates. ReplaySubject = recent history + future updates. AsyncSubject = only final value on completion. In Angular, default to BehaviorSubject for local UI state, Subject for intent events, and ReplaySubject only when you truly need history.