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---

## Briefly (as in an interview)

**ReplaySubject** is a type of `Subject` in RxJS that **remembers several of the latest values and replays them to new subscribers**.

The number of stored values is set in the constructor.

---

## In detail

`ReplaySubject` is an extension of `Subject` that **buffers previous values** and **replays them** on a new subscription.

Example:

```ts
import { ReplaySubject } from 'rxjs';

const subject = new ReplaySubject<number>(2);

subject.next(1);
subject.next(2);
subject.next(3);

subject.subscribe(v => console.log(v));
```

Result:

```
2
3
```

Because the buffer is set to **2 values**.

---

### Constructor

```ts
new ReplaySubject<T>(bufferSize?, windowTime?)
```

Parameters:

- `bufferSize` - how many values to store
- `windowTime` - how long to store values

Example with time:

```ts
new ReplaySubject(3, 5000)
```

Stores:

- a maximum of 3 values
- no older than 5 seconds

---

## Difference from BehaviorSubject

| Type | Stores values | Initial value | New subscriber gets |
|---|---|---|---|
| Subject | No | No | nothing |
| BehaviorSubject | 1 | required | the latest value |
| ReplaySubject | N | not required | the last N |

The main difference:

- BehaviorSubject = **current state**
- ReplaySubject = **history**

---

## When ReplaySubject is used

Less often than BehaviorSubject. For example:

- caching events
- event history
- logging streams
- state rehydration
- event bus

Example of an event system:

```ts
events$ = new ReplaySubject<AppEvent>(10);
```

---

## An important point (often asked)

ReplaySubject **can lead to memory leaks** if:

- the buffer is large
- the stream is long-lived
- the values are heavy

That's why the buffer is almost always limited.

---

## A typical follow-up question

In an interview you might be asked:

**Why is BehaviorSubject used more often in Angular?**

Answer:
Because Angular state is usually a **current value**, not a history.

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