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`tap()` in RxJS is an operator that lets you **run side effects** **without changing the stream itself**.

---

### A simple example:

```ts
import { of } from 'rxjs';
import { tap, map } from 'rxjs/operators';

of(1, 2, 3)
  .pipe(
    tap(val => console.log('Received:', val)),
    map(val => val * 10)
  )
  .subscribe(result => console.log('Result:', result));
```

Outputs:

```javascript
Received: 1
Result: 10
Received: 2
Result: 20
Received: 3
Result: 30
```

---

### Why you need `tap()`:

- For **logging and debugging**
- For **calling methods** (for example, showing/hiding a loading indicator)
- For **saving to localStorage, sending analytics**, and so on
- It doesn't change the stream's values, it just observes

---

### Where it's used:

```ts
this.http.get('/api/data')
  .pipe(
    tap(() => this.isLoading = true),
    tap(data => console.log('Data arrived:', data)),
    tap(() => this.isLoading = false)
  )
  .subscribe();
```

---

### Conclusion:

`tap()` is like a **mirror beside the stream**:
you see what's happening, you can do something,
but **you don't touch the data itself**.

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