# Typescript Language Patterns

> Modern TypeScript standards for type safety and maintainability. Use when working with types, interfaces, generics, enums, unions, or tsconfig settings. Use when this capability is needed.

- Skill: `tomevault-io/typescript-language-patterns-3` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add tomevault-io/typescript-language-patterns-3`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/typescript-language-patterns-3/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: tomevault-io (https://skillmd.com/u/tomevault-io)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/tomevault-io/typescript-language-patterns-3

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# TypeScript Language Patterns

## **Priority: P0 (CRITICAL)**

## Implementation Guidelines

- **Type Annotations**: Explicit params/returns. Infer locals.
- **Interfaces vs Types**: `interface` for APIs. `type` for unions.
- **Strict Mode**: `strict: true`. Null Safety: `?.` and `??`.
- **Enums**: Literal unions or `as const`. **No runtime `enum`**.
- **Generics**: Reusable, type-safe code.
- **Type Guards**: `typeof`, `instanceof`, predicates.
- **Utility Types**: `Partial`, `Pick`, `Omit`, `Record`.
- **Immutability**: `readonly` arrays/objects. Const Assertions: `as const`, `satisfies`.
- **Template Literals**: `on${Capitalize<string>}`.
- **Discriminated Unions**: Literal `kind` property.
- **Advanced**: Mapped, Conditional, Indexed types.
- **Access**: Default `public`. Use `private`/`protected` or `#private`.
- **Branded Types**: `string & { __brand: 'Id' }`.

## Anti-Patterns

- **No `any`**: NEVER use `any`. Use `unknown` or specific interfaces.
- **No `Function`**: Use signature `() => void`.
- **No `enum`**: Runtime cost.
- **No `!`**: Use narrowing.
- **NO LINT DISABLE**: PROHIBITED. Fix issues properly.

## Testing

- **Mocking**: Use `jest.Mocked<T>` or `as unknown as T`.
- **Checklist**: Check method existence, match error constants, satisfy required properties.
- **References**: See [references/TESTING.md](references/TESTING.md) for common issues/solutions.

## Code

```typescript
// Branded Type
type UserId = string & { __brand: 'Id' };

// Satisfies (Validate + Infer)
const cfg = { port: 3000 } satisfies Record<string, number>;

// Discriminated Union
type Result<T> = { kind: 'ok'; data: T } | { kind: 'err'; error: Error };
```

## Reference & Examples

For advanced type patterns and utility types:
See [references/REFERENCE.md](references/REFERENCE.md).

## Related Topics

best-practices | security | tooling

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> Source: [ntluong95/agent-skills-dlnm](https://github.com/ntluong95/agent-skills-dlnm) — distributed by [TomeVault](https://tomevault.io).
<!-- tomevault:4.0:skill_md:2026-06-15 -->