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`Object.preventExtensions()` is a built-in JavaScript method
that **forbids adding new properties to an object**,
but **allows changing and deleting the ones that already exist**.

This is the "softest" form of object protection (unlike `seal()` and `freeze()`).

---

## Short summary

Syntax:

```javascript
Object.preventExtensions(obj)
```

| Argument | Description |
|---|---|
| `obj` | The object that **new properties cannot be added to** |

Returns the object itself (now "closed" for extension).

---

## Example

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

Object.preventExtensions(user);

user.city = 'Kyiv';  // will not be added
delete user.age;     // can be deleted
user.name = 'Oleh';  // can be changed

console.log(user); // { name: 'Oleh' }
```

After `Object.preventExtensions()`:

- new properties **cannot be added**;
- old properties **can be deleted**;
- existing values **can be changed**.

---

## Checking whether an object can be extended

```javascript
console.log(Object.isExtensible(user)); // false
```

`Object.isExtensible(obj)` returns `false`
if new properties cannot be added to the object.

---

## What happens "under the hood"

`Object.preventExtensions(obj)` does the following:

- forbids adding new properties (including via `Object.defineProperty()`);
- existing properties **remain changeable and deletable**;
- `configurable`, `writable`, `enumerable` are left untouched.

---

## Example: attempting to add a new property

```javascript
const obj = {};
Object.preventExtensions(obj);

try {
  Object.defineProperty(obj, 'x', { value: 1 });
} catch (err) {
  console.log('Error:', err.message);
}
```

Throws an error (in strict mode):

```javascript
Error: Cannot define property x, object is not extensible
```

---

## Differences from `seal()` and `freeze()`

| Method | Can add | Can delete | Can change | `configurable` descriptors |
|---|---|---|---|---|
| `preventExtensions()` | No | Yes | Yes | Unchanged |
| `seal()` | No | No | Yes | All `configurable: false` |
| `freeze()` | No | No | No | All `configurable: false`, `writable: false` |

So `preventExtensions()` is the **softest protection option**.
It just "closes the entrance", but does not interfere with working on what already exists.

---

## Example - checking against prototypes

```javascript
const parent = { role: 'user' };
const child = Object.create(parent);

Object.preventExtensions(child);

child.name = 'Tim'; // will not be added
console.log(child.name); // undefined
console.log(child.role); // "user" (inherited from the prototype)
```

`preventExtensions()` does not affect **inherited properties** -
they remain accessible through the prototype.

---

## SUMMARY

| Behavior | Allowed? |
|---|---|
| Adding new properties | No |
| Deleting existing ones | Yes |
| Changing values | Yes |
| Changing descriptors | Yes |
| Check | `Object.isExtensible(obj)` |

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