What does Record<K, T> do?

TypeScriptJunior~1 min read

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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