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`Object.freeze()` is a built-in JavaScript method that **"freezes" an object**,
making it **fully immutable**:
properties cannot be changed, removed, or added.

---

## In short

Syntax:

```javascript
Object.freeze(obj)
```

| Argument | Description |
|---|---|
| `obj` | The object to "freeze" |

Returns **the same object**, but with **frozen properties**.

---

## Example - freezing an object

```javascript
const user = {
  name: 'Bohdan',
  age: 25
};

Object.freeze(user);

user.name = 'Oleh';   // will not change
user.city = 'Kyiv';   // cannot add a new one
delete user.age;      // cannot delete

console.log(user); // { name: 'Bohdan', age: 25 }
```

After `Object.freeze()` the object becomes **immutable**:

- properties cannot be overwritten;
- new ones cannot be deleted or added;
- `configurable` or `writable` cannot be changed.

---

## Checking whether an object is frozen

```javascript
console.log(Object.isFrozen(user)); // true
```

---

## Important: the freeze is **shallow**

If there are **nested objects** inside, they **remain mutable**:

```javascript
const user = {
  name: 'Bohdan',
  address: {
    city: 'Kyiv'
  }
};

Object.freeze(user);

user.address.city = 'Lviv'; // works!
console.log(user.address.city); // "Lviv"
```

So `Object.freeze()` protects **only the top level**.
For a "deep" freeze you need to freeze **recursively**.

---

## Example of a deep freeze

```javascript
function deepFreeze(obj) {
  Object.freeze(obj);

for (const key in obj) {
    if (typeof obj[key] === 'object' && obj[key] !== null && !Object.isFrozen(obj[key])) {
      deepFreeze(obj[key]);
    }
  }

return obj;
}

const user = {
  name: 'Bohdan',
  address: { city: 'Kyiv' }
};

deepFreeze(user);

user.address.city = 'Lviv'; // will not change
console.log(user.address.city); // "Kyiv"
```

Now the object and all its content are fully "frozen".

---

## Difference from other methods

| Method | What it does | Can change values | Can add/remove properties |
|---|---|---|---|
| `Object.freeze()` | Fully freezes | no | no |
| `Object.seal()` | Forbids adding/removing, but allows changing values | yes | no |
| `Object.preventExtensions()` | Forbids adding new properties | yes | yes (existing ones can be removed) |

---

## Example: `freeze` vs `seal`

```javascript
const user = { name: 'Bohdan' };

Object.seal(user);
user.name = 'Oleh'; // allowed
delete user.name;   // not allowed

Object.freeze(user);
user.name = 'Max'; // not allowed
```

---

## SUMMARY

| Property | Behavior |
|---|---|
| `Object.freeze()` | makes the object fully immutable |
| not allowed | adding, removing, or changing properties |
| works shallowly | nested objects remain mutable |
| check | `Object.isFrozen(obj)` |

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