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## 1. What `Record<K, T>` does

> `Record<K, T>` creates an object type
> whose **keys** have type `K` and whose **values** have type `T`.

---

### Syntax

```javascript
Record<KeyType, ValueType>
```

---

### Example

```javascript
type Roles = Record<string, number>;

const userRoles: Roles = {
  admin: 1,
  user: 2,
  guest: 3,
};
```

> Here:
>
> - all keys are `string`;
> - all values are `number`.

---

## 2. Example with specific keys

```javascript
type Status = 'pending' | 'success' | 'error';

type StatusMessages = Record<Status, string>;

const messages: StatusMessages = {
  pending: 'Loading...',
  success: 'Done',
  error: 'Error',
};
```

> TypeScript strictly checks that all keys are present (`pending`, `success`, `error`)
> and the type of their values (`string`).

---

## 3. How `Record<K, T>` is built internally

```javascript
type Record<K extends keyof any, T> = {
  [P in K]: T;
};
```

Breakdown:

- `K` is the set of allowed keys (`string | number | symbol`);
- `T` is the value type;
- `[P in K]` creates an object with properties `P` from the set `K`;
- `: T` specifies that every property has type `T`.

---

## 4. Usage with `union` types

`Record` automatically expands every variant of `K` into a key.

```javascript
type Page = 'home' | 'about' | 'contact';

type PageInfo = Record<Page, { title: string }>;

const pages: PageInfo = {
  home: { title: 'Home' },
  about: { title: 'About' },
  contact: { title: 'Contact' },
};
```

> TypeScript checks that you did not skip any page and did not add an extra one.

---

## 5. Comparing `Record` with a plain object

| Feature | Plain object | `Record<K, T>` |
|---|---|---|
| Key typing | not strict | strict (via union) |
| Checking all fields | no | yes |
| Convenience for dictionaries | moderate | ideal |
| Autocomplete support | limited | full |

---

## 6. Practical examples

### Example 1. A settings map

```javascript
type SettingKey = 'theme' | 'language' | 'timezone';

type Settings = Record<SettingKey, string>;

const settings: Settings = {
  theme: 'dark',
  language: 'uk',
  timezone: 'Europe/Kyiv',
};
```

---

### Example 2. A counter by id

```javascript
const scores: Record<number, number> = {
  1: 10,
  2: 25,
  3: 40,
};
```

---

### Example 3. A translation dictionary

```javascript
type Locale = 'en' | 'de';
type Translations = Record<Locale, string>;

const greeting: Translations = {
  en: 'Hello',
  de: 'Hallo',
};
```

---

### Example 4. A typed object by enum

```javascript
enum Role {
  Admin = 'admin',
  User = 'user',
}

type RolePermissions = Record<Role, string[]>;

const permissions: RolePermissions = {
  [Role.Admin]: ['create', 'edit', 'delete'],
  [Role.User]: ['view'],
};
```

---

## 7. Combining with other Utility Types

You can combine `Record` with `Partial`, `Readonly`, and so on.

```javascript
type PartialConfig = Partial<Record<'host' | 'port' | 'ssl', string>>;

const config: PartialConfig = {
  host: 'localhost',
  ssl: 'true',
};
```

> Now not all properties have to be specified.

---

## 8. Errors with invalid keys

TypeScript will not allow adding a key that does not exist:

```javascript
type Roles = 'admin' | 'user';
type RoleLabels = Record<Roles, string>;

const labels: RoleLabels = {
  admin: 'Administrator',
  user: 'User',
  guest: 'Guest', // Error - "guest" is not part of Roles
};
```

---

## Summary

| Question | Answer |
|---|---|
| What does `Record<K, T>` do | Creates an object type where keys have type `K` and values have type `T` |
| How to choose keys | Via a `union` or an enumeration (`'key1' |
| Does it check all fields | Yes, TypeScript requires all keys to be present |
| Can it be combined | Yes, with `Partial`, `Readonly`, `Pick`, and others |

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