typescript-types

--- name: typescript-types description: Apply, review, and explain TypeScript type conventions. Use automatically for tasks involving inference, readonly data, type-safe constants, template literal types, any and unknown, assertions, type errors, type definitions, arrays, type imports, or generated service types. ---

# Types

Apply the TypeScript Style Guide's type conventions in the context of the current task.

## Workflow

1. Inspect the consuming repository's conventions and configuration. 2. Let explicit repository conventions take precedence over this opinionated guidance. 3. Apply, review, or explain only the guidance relevant to the task. 4. State important tradeoffs when the appropriate model depends on context or judgment.

## Boundaries

- Keep TypeScript and ESLint responsible for checks they can enforce automatically. - Do not introduce unrelated TypeScript Style Guide conventions merely because this skill is active.

<!-- BEGIN CANONICAL GUIDE CONTENT -->

## Types

When creating types, consider how they would best **describe our code**. Being expressive and keeping types as **narrow as possible** offers several benefits to the codebase:

- Increased Type Safety - Catch errors at compile time, as narrowed types provide more specific information about the shape and behavior of your data. - Improved Code Clarity - Reduces cognitive load by providing clearer boundaries and constraints on your data, making your code easier for other developers to understand. - Easier Refactoring - With narrower types, making changes to your code becomes less risky, allowing you to refactor with confidence.

### Type Inference

As a rule of thumb, explicitly declare types only when it helps to narrow them.

<Note> Just because you don't need to add types doesn't mean you shouldn't. In some cases, explicitly declaring types can improve code readability and clarify intent. </Note>

Explicitly declare types when doing so helps to narrow them:

```ts // ❌ Avoid const employees = new Map(); // Inferred as wide type 'Map<any, any>' employees.set('Lea', 17); type UserRole = 'admin' | 'guest'; const [userRole, setUserRole] = useState('admin'); // Inferred as 'string', not the desired narrowed literal type

// ✅ Use explicit type declarations to narrow the types. const employees = new Map<string, number>(); // Narrowed to 'Map<string, number>' employees.set('Gabriel', 32); type UserRole = 'admin' | 'guest'; const [userRole, setUserRole] = useState<UserRole>('admin'); // Explicit type 'UserRole' ```

Avoid explicitly declaring types when they can be inferred:

```ts // ❌ Avoid const userRole: string = 'admin'; // Inferred as wide type 'string' const employees = new Map<string, number>([['Gabriel', 32]]); // Redundant type declaration const [isActive, setIsActive] = useState<boolean>(false); // Redundant, inferred as 'boolean'

// ✅ Use type inference. const USER_ROLE = 'admin'; // Inferred as narrowed string literal type 'admin' const employees = new Map([['Gabriel', 32]]); // Inferred as 'Map<string, number>' const [isActive, setIsActive] = useState(false); // Inferred as 'boolean' ```

### Data Immutability

Immutability should be a key principle. Wherever possible, prevent unintended mutations with types like `Readonly` and `ReadonlyArray`.

- Readonly types help prevent accidental mutations and bugs caused by unintended side effects. This protection exists during type checking and does not freeze values at runtime. - When performing data processing, always return new arrays, objects, or other reference-based data structures. To minimize cognitive load for future developers, strive to keep data objects flat and concise. - Use mutations sparingly, only in cases where they are truly necessary, such as when dealing with complex objects or optimizing for performance.

```ts // ❌ Avoid data mutations const removeFirstUser = (users: Array<User>) => { if (users.length === 0) { return users; } return users.splice(1); };

// ✅ Use readonly type to prevent accidental mutations const removeFirstUser = (users: ReadonlyArray<User>) => { if (users.length === 0) { return users; } return users.slice(1); // Using arr.splice(1) errors - Function 'splice' does not exist on 'users' }; ```

### Type-Safe Constants With Satisfies

The `as const satisfies` syntax combines narrow, readonly inference with type validation. It is useful when a constant should retain its exact values while conforming to a broader type.

Key benefits:

- Readonly values with `as const` - Ensures the constant is treated as readonly. - Narrows the types of values to their literals, preventing accidental modifications. - Validation with `satisfies` - Ensures the object conforms to a broader type without widening its inferred type. - Helps catch type mismatches at compile time while preserving narrowed inferred types.

Array constants:

```ts type UserRole = 'admin' | 'editor' | 'moderator' | 'viewer' | 'guest';

// ❌ Avoid constant of wide type const DASHBOARD_ACCESS_ROLES: ReadonlyArray<UserRole> = ['admin', 'editor', 'moderator'];

// ❌ Avoid constant with incorrect values const DASHBOARD_ACCESS_ROLES = ['admin', 'contributor', 'analyst'] as const;

// ✅ Use readonly constant of narrowed type const DASHBOARD_ACCESS_ROLES = ['admin', 'editor', 'moderator'] as const satisfies ReadonlyArray<UserRole>; ```

Object constants:

```ts type OrderStatus = { pending: 'pending' | 'idle'; fulfilled: boolean; error: string; };

// ❌ Avoid mutable constant of wide type const IDLE_ORDER: OrderStatus = { pending: 'idle', fulfilled: true, error: 'Shipping Error', };

// ❌ Avoid constant with incorrect values const IDLE_ORDER = { pending: 'done', fulfilled: 'partially', error: 116, } as const;

// ✅ Use readonly constant of narrowed type const IDLE_ORDER = { pending: 'idle', fulfilled: true, error: 'Shipping Error', } as const satisfies OrderStatus; ```

### Template Literal Types

Embrace template literal types as they allow you to create precise and type-safe string constructs by interpolating values. They are a powerful alternative to using the wide string type, providing better type safety.

Template literal types constrain values known to TypeScript at compile time. They do not validate strings received at runtime.

Adopting template literal types brings several advantages:

- Prevent errors caused by typos or invalid strings. - Provide better type safety and autocompletion support. - Improve code maintainability and readability.

Template literal types are useful in various practical scenarios, such as:

- String Patterns - Use template literal types to enforce specific string patterns during type checking.

```ts // ❌ Avoid const appVersion = '2.6'; // ✅ Use type Version = `v${number}.${number}.${number}`; const appVersion: Version = 'v2.6.1'; ```

- API Endpoints - Use template literal types to restrict values to valid API routes.

```ts // ❌ Avoid const userEndpoint = '/api/usersss'; // Type 'string' - Typo 'usersss': the route doesn't exist, leading to a runtime error. // ✅ Use type ApiRoute = 'users' | 'posts' | 'comments'; type ApiEndpoint = `/api/${ApiRoute}`; // Type ApiEndpoint = "/api/users" | "/api/posts" | "/api/comments" const userEndpoint: ApiEndpoint = '/api/users'; ```

- Internationalization Keys - Avoid relying on raw strings for translation keys, which can lead to typos and missing translations. Use template literal types to constrain their structure.

```ts // ❌ Avoid const homeTitle = 'translation.homesss.title'; // Type 'string' - Typo 'homesss': the translation doesn't exist, leading to a runtime error. // ✅ Use type LocaleKeyPages = 'home' | 'about' | 'contact'; type TranslationKey = `translation.${LocaleKeyPages}.${string}`; // Type TranslationKey = `translation.home.${string}` | `translation.about.${string}` | `translation.contact.${string}` const homeTitle: TranslationKey = 'translation.home.title'; ```

- CSS Utilities - Avoid raw strings for color values, which can lead to invalid or non-existent colors. Use template literal types to enforce known color names and require custom values to start with `#`.

```ts // ❌ Avoid const color = 'blue-450'; // Type 'string' - Color 'blue-450' doesn't exist, leading to a runtime error. // ✅ Use type BaseColor = 'blue' | 'red' | 'yellow' | 'gray'; type Variant = 50 | 100 | 200 | 300 | 400; type Color = `${BaseColor}-${Variant}` | `#${string}`; // Type Color = "blue-50" | "blue-100" | "blue-200" ... | "red-50" | "red-100" ... | #${string} const iconColor: Color = 'blue-400'; const customColor: Color = '#AD3128'; ```

- Database queries - Avoid using raw strings for table or column names, which can lead to typos. Use template literal types to define valid table and column combinations.

<!-- prettier-ignore-start --> ```ts // ❌ Avoid const query = 'SELECT name FROM usersss WHERE age > 30'; // Type 'string' - Typo 'usersss': table doesn't exist, leading to a runtime error. // ✅ Use type Table = 'users' | 'posts' | 'comments'; type Column<TTableName extends Table> = TTableName extends 'users' ? 'id' | 'name' | 'age' : TTableName extends 'posts' ? 'id' | 'title' | 'content' : TTableName extends 'comments' ? 'id' | 'postId' | 'text' : never;

type Query<TTableName extends Table> = `SELECT ${Column<TTableName>} FROM ${TTableName} WHERE ${string}`; const userQuery: Query<'users'> = 'SELECT name FROM users WHERE age > 30'; // Accepted by Query<'users'> const invalidQuery: Query<'users'> = 'SELECT title FROM users WHERE age > 30'; // Error: 'title' is not a column in 'users' table. ``` <!-- prettier-ignore-end -->

### Type any & unknown

The `any` type must not be used because it bypasses type checking and allows unsafe operations and assignments. This can mask serious programming errors.

When dealing with ambiguous data, use `unknown`, which is the type-safe counterpart of `any`. Anything can be assigned to `unknown`, but it must be narrowed before accessing its properties or assigning it to a more specific type.

```ts // ❌ Avoid any const foo: any = 'five'; const bar: number = foo; // no type error

// ✅ Use unknown const foo: unknown = 5; const bar: number = foo; // type error - Type 'unknown' is not assignable to type 'number'

// Narrow the type before dereferencing it using: // Type guard const isNumber = (num: unknown): num is number => { return typeof num === 'number'; }; if (!isNumber(foo)) { throw Error(`API provided a fault value for field 'foo':${foo}. Should be a number!`); } const bar: number = foo; ```

### Type & Non-nullability Assertions

Type assertions `user as User` and non-nullability assertions `user!.name` are unsafe. Both only silence the TypeScript compiler and increase the risk of crashing the application at runtime. They can only be used as an exception, such as a third-party library type mismatch, with a strong rationale for why they are introduced into the codebase.

```ts type User = { id: string; username: string; avatar: string | null }; // ❌ Avoid type assertions const user = { name: 'Nika' } as User;

// ❌ Avoid non-nullability assertions const getUsername = (user: User | null) => user!.username; // Runtime error when user is null ```

### Type Errors

When a TypeScript error cannot be mitigated, use `@ts-expect-error` as a last resort to suppress it.

This directive notifies the compiler when the suppressed error no longer exists, ensuring errors are revisited once they’re obsolete, unlike `@ts-ignore`, which can silently linger even after the error is resolved.

- Always use `@ts-expect-error` with a clear description explaining why it is necessary. - Avoid `@ts-ignore`, as it does not track suppressed errors.

<Rule href="https://typescript-eslint.io/rules/ban-ts-comment/#allow-with-description">{`'@typescript-eslint/ban-ts-comment': [ 'error', { 'ts-expect-error': 'allow-with-description' }, ]`}