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`Object.seal()` is a built-in JavaScript method that **"seals" an object**,
forbidding **adding or removing properties**,
but **allowing the values of existing** properties to be changed.

It is similar to `Object.freeze()`, but slightly softer.

---

## In short

Syntax:

```javascript
Object.seal(obj)
```

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

Returns the same object, but with restrictions.

---

## Example

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

Object.seal(user);

user.age = 30;        // the value can be changed
user.city = 'Berlin'; // cannot add a new one
delete user.name;     // cannot delete

console.log(user); // { name: 'Tim', age: 30 }
```

After `Object.seal()`:

- new properties cannot be added;
- existing ones cannot be deleted;
- but **the values** of existing properties **can be changed**.

---

## Checking whether an object is sealed

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

---

## What happens "under the hood"

`Object.seal(obj)`:

1. **Prevents adding new properties** (`Object.preventExtensions(obj)`);
2. Sets `configurable: false` for all existing properties -
they cannot be deleted or have their descriptor changed;
3. Leaves `writable` unchanged - so the values can still be changed.

---

## Example with a descriptor

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

Object.seal(user);

console.log(Object.getOwnPropertyDescriptor(user, 'name'));
```

Will print:

```javascript
{
  value: 'Tim',
  writable: true,
  enumerable: true,
  configurable: false
}
```

You can see that `configurable: false`, while `writable` stays `true`.

---

## Comparison with `Object.freeze()`

| Method | Can change values | Can add | Can delete | Can change descriptors |
|---|---|---|---|---|
| `Object.preventExtensions()` | Yes | No | Yes | Yes |
| `Object.seal()` | Yes | No | No | No |
| `Object.freeze()` | No | No | No | No |

So `seal()` is an intermediate option between "flexible" and "rigid".

---

## Shallowness (shallow seal)

`Object.seal()` **does not protect nested objects**:

```javascript
const user = {
  name: 'Tim',
  address: { city: 'Berlin' }
};

Object.seal(user);

user.address.city = 'Lviv'; // allowed (the nested object isn't sealed)
user.city = 'Warsaw';       // not allowed (a new property)

console.log(user.address.city); // "Lviv"
```

To "seal" the whole tree, you need to do a **deep seal** recursively.

---

## Example of a deep "seal"

```javascript
function deepSeal(obj) {
  Object.seal(obj);

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

return obj;
}
```

Now everything can be "sealed", including nested structures.

---

## SUMMARY

| What it does | Example | Behavior |
|---|---|---|
| "Seals" an object | `Object.seal(obj)` | Forbids adding and removing properties |
| Can change values | Yes |  |
| Can add new properties | No |  |
| Can delete properties | No |  |
| Check | `Object.isSealed(obj)` |  |
| Depth | Shallow (top level only) |  |

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