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`Object.defineProperty()` is a **powerful built-in JavaScript method**
that lets you **create or modify object properties with fine-grained control** -
for example, making them **read-only, hidden, computed**, and so on.

---

## Syntax

```javascript
Object.defineProperty(obj, propName, descriptor);
```

| Argument | Description |
|---|---|
| `obj` | The object in which the property is created or changed |
| `propName` | The property name (a string or a `Symbol`) |
| `descriptor` | A descriptor object - specifies **how the property behaves** |

---

## Example - a regular property

```javascript
const user = {};

Object.defineProperty(user, 'name', {
  value: 'Tim',
  writable: true,      // can be changed
  enumerable: true,    // visible when iterated (for...in / Object.keys)
  configurable: true   // can be deleted or changed
});

console.log(user.name); // "Tim"
```

This is the same as if you wrote:

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

But with `defineProperty` you can **precisely control the property's behavior**.

---

## 1. `writable` - whether it can be changed

```javascript
const user = {};
Object.defineProperty(user, 'name', {
  value: 'Tim',
  writable: false
});

user.name = 'Oleh'; // will not change
console.log(user.name); // "Tim"
```

`writable: false` makes the property **read-only**.

---

## 2. `enumerable` - whether the property is visible when iterated

```javascript
const user = {};
Object.defineProperty(user, 'secret', {
  value: '12345',
  enumerable: false
});

console.log(Object.keys(user)); // [] not visible
console.log(user.secret);       // can be read directly
```

Useful for "internal" data.

---

## 3. `configurable` - whether it can be deleted or redefined

```javascript
const user = {};
Object.defineProperty(user, 'name', {
  value: 'Tim',
  configurable: false
});

delete user.name; // will not work
Object.defineProperty(user, 'name', { value: 'Oleh' }); // error
```

`configurable: false` makes the property **permanently "locked"**.

---

## 4. Getter and setter via a descriptor

`Object.defineProperty()` can be used to create **computed properties**:

```javascript
const user = { firstName: 'Tim', lastName: 'Cooper' };

Object.defineProperty(user, 'fullName', {
  get() {
    return `${this.firstName} ${this.lastName}`;
  },
  set(value) {
    const [first, last] = value.split(' ');
    this.firstName = first;
    this.lastName = last;
  }
});

console.log(user.fullName); // "Tim Cooper"
user.fullName = 'Oleh Shevchenko';
console.log(user.firstName); // "Oleh"
```

If `get` or `set` are specified, `value` and `writable` cannot be used.

---

## 5. Inspect the current property settings

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

Example output:

```javascript
{
  get: [Function: get],
  set: [Function: set],
  enumerable: false,
  configurable: false
}
```

---

## 6. Create several properties at once

```javascript
Object.defineProperties(user, {
  age: { value: 25, writable: true },
  city: { value: 'Kyiv', enumerable: true }
});
```

`defineProperties()` is analogous, but for a group of properties.

---

## Summary

| Attribute | Default | What it does |
|---|---|---|
| `value` | `undefined` | The property's value |
| `writable` | `false` | Whether the value can be changed |
| `enumerable` | `false` | Whether the property is visible when iterated |
| `configurable` | `false` | Whether the descriptor can be deleted / changed |
| `get` | `undefined` | The getter function |
| `set` | `undefined` | The setter function |

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