What is BehaviorSubject?

AngularMiddle~1 min read

Briefly (as in an interview)

BehaviorSubject is a special type of Subject in RxJS that stores the latest value and immediately gives it to new subscribers. Unlike a regular Subject, it requires an initial value.

It's often used in Angular for simple state management and passing data between components.

In detail

BehaviorSubject is at the same time:

an Observable (you can subscribe to it)
an Observer (you can send values to it via next())

The main difference from Subject is having a current value.

Example:

import { BehaviorSubject } from 'rxjs';

const subject = new BehaviorSubject<number>(0);

subject.subscribe(v => console.log('A:', v));

subject.next(1);
subject.next(2);

subject.subscribe(v => console.log('B:', v));

Output:

A: 0
A: 1
A: 2
B: 2

The second subscriber immediately received the latest value (2).

Used in Angular for

Very often, for storing state in a service:

@Injectable()
export class UserService {
  private userSubject = new BehaviorSubject<User | null>(null);
  user$ = this.userSubject.asObservable();

  setUser(user: User) {
    this.userSubject.next(user);
  }
}

Component:

this.userService.user$.subscribe(user => {
  console.log(user);
});

Difference from Subject

Type	Stores a value	Initial value	New subscriber gets the latest
Subject	No	No	No
BehaviorSubject	Yes	Yes	Yes

When BehaviorSubject is used

Usually when you need:

current state
global state
shared data
a light-version reactive store

For example:

the current user
the application theme
loading state
filters

A typical follow-up interview question

They usually ask:

BehaviorSubject vs ReplaySubject?

In short:

BehaviorSubject → stores 1 latest value
ReplaySubject → stores N values

Short Answer

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What is BehaviorSubject?

#Briefly (as in an interview)

#In detail

#Used in Angular for

#Difference from Subject

#When BehaviorSubject is used

#A typical follow-up interview question