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## 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:

```ts
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:

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

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

Component:

```ts
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**

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