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The `!` operator is called **logical negation (NOT)**.
It **inverts** (flips) the logical value of an expression.

---

### Rule:

```javascript
!true  → false
!false → true
```

> If a value is **truthy** - it becomes `false`.
> If it is **falsy** - it becomes `true`.

---

## Examples of using `!`

```javascript
!true            // false
!false           // true
!0               // true   (0 - a falsy value)
!1               // false  (1 - truthy)
!""              // true   (an empty string - falsy)
!"hello"         // false  (a non-empty string - truthy)
!undefined       // true
!null            // true
!NaN             // true
![]              // false  (an array - truthy)
!{}              // false  (an object - truthy)
```

---

## Double negation `!!`

A double operator is very common in JavaScript:

```javascript
!!value
```

It is used to **coerce any value to the** `boolean` **type**.

### Example:

```javascript
!!'hello'   // true
!!''        // false
!!0         // false
!!123       // true
!!null      // false
!!{}        // true
!![]        // true
```

> `!!` is a short and clear way to explicitly say:
> "Convert this value to boolean".

---

## In logical expressions

`!` is often used to check conditions:

```javascript
const isLoggedIn = false;

if (!isLoggedIn) {
  console.log('User is not authenticated');
}
```

> Here, `!isLoggedIn` → `!false` → `true`,
> so the `if` block will execute.

---

## How it works internally

1. JS **first coerces the value to a logical type** (`true` / `false`);
2. then it **inverts** it.

Example:

```javascript
!'text'   // step 1: 'text' → true; step 2: !true → false
```

---

## Comparison with other logical operators

| Operator | Name | Example | Result |
|---|---|---|---|
| `!` | Logical **NOT** | `!true` | `false` |
| `&&` | Logical **AND** | `true && false` | `false` |
| `\|\|` | `Logical OR` | `true \|\| false` | `true` |

---

## Summary

| Situation | What `!` does | Example |
|---|---|---|
| Negates a logical value | `!true → false` |  |
| Coerces a value to `boolean` | `!!'js' → true` |  |
| Used in conditions | `if (!user)` → "if user is not set" |  |

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