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The **"key-value" pair (key-value)** is the foundation of data storage in JavaScript objects (and in many other structures).
The **key** is the property name,
the **value** is the data that corresponds to that name.

---

### Detailed explanation

In JavaScript, an object stores data as a set of such pairs:

```javascript
const user = {
  name: 'Tim',     // key → value
  age: 25,
  city: 'Berlin'
};
```

Here:

- `name`, `age`, `city` are the **keys** (or "property names"),
- `'Tim'`, `25`, `'Berlin'` are the **values**.

Each property is a **key-value pair** that describes a piece of data.

---

### How keys work

- Keys are always **unique** within a single object.
- They are usually **strings** (or `Symbol`s), even if you write a number:

```javascript
  const obj = { 1: 'a', 2: 'b' };
  console.log(Object.keys(obj)); // ["1", "2"]
  ```

---

### How to get a value by key

```javascript
console.log(user.name);      // "Tim"
console.log(user['city']);   // "Berlin"
```

Access is possible:

- via **dot notation**, if the name is known in advance;
- via **brackets**, if the key is stored in a variable.

```javascript
const key = 'age';
console.log(user[key]); // 25
```

---

### What key-value pairs consist of

You can see them as an array:

```javascript
console.log(Object.entries(user));
```

Output:

```javascript
[
  ['name', 'Tim'],
  ['age', 25],
  ['city', 'Berlin']
]
```

Each pair is `[key, value]`.

---

### Where else the "key-value" principle appears

| Structure | Example | What it stores |
|---|---|---|
| **Object** | `{ name: 'Tim' }` | Keys → strings |
| **Map** | `map.set('role', 'admin')` | Keys → any type |
| **URLSearchParams** | `?id=1&name=Tim` | Keys and values from a URL string |
| **FormData** | `form.append('email', 'tim@example.com')` | Form fields |

---

### SUMMARY

| Element | Description |
|---|---|
| **Key** | The property name used to access the data |
| **Value** | The data associated with that key |
| **Example** | `{ "key": "value" }` |
| **Typical storage format** | `{ name: 'Tim', age: 25 }` |

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