Agent skill
typescript-patterns
TypeScript best practices and patterns. Use when writing TypeScript code, defining types, working with generics, or converting JavaScript to TypeScript.
Install this agent skill to your Project
npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/other/other/typescript-patterns
SKILL.md
TypeScript Patterns
Strict Config (tsconfig.json)
{
"compilerOptions": {
"strict": true,
"noUncheckedIndexedAccess": true,
"noImplicitReturns": true,
"exactOptionalPropertyTypes": true,
"target": "ES2022",
"module": "ESNext",
"moduleResolution": "bundler",
"baseUrl": ".",
"paths": { "@/*": ["src/*"] }
}
}
Type Patterns
Discriminated Union (never use string + optional fields)
// BAD
type ApiResponse = { success: boolean; data?: User; error?: string }
// GOOD
type ApiResponse =
| { success: true; data: User }
| { success: false; error: string }
function handle(res: ApiResponse) {
if (res.success) {
console.log(res.data.email) // TypeScript knows data exists
} else {
console.error(res.error) // TypeScript knows error exists
}
}
Generic Repository
interface Repository<T, ID> {
findById(id: ID): Promise<T | null>
findAll(): Promise<T[]>
create(data: Omit<T, 'id' | 'createdAt'>): Promise<T>
update(id: ID, data: Partial<T>): Promise<T>
delete(id: ID): Promise<void>
}
Branded Types (prevent mixing IDs)
type UserId = number & { readonly _brand: 'UserId' }
type PostId = number & { readonly _brand: 'PostId' }
const userId = 123 as UserId
const postId = 456 as PostId
function getUser(id: UserId): Promise<User> { ... }
getUser(postId) // TypeScript error! Can't pass PostId as UserId
Utility Types
// Pick only what you need
type UserSummary = Pick<User, 'id' | 'name' | 'email'>
// Make all optional for updates
type UserUpdate = Partial<Pick<User, 'name' | 'email'>>
// Require specific fields
type UserCreate = Required<Pick<User, 'email' | 'password'>> & Partial<Pick<User, 'name'>>
// Readonly for immutable data
type Config = Readonly<{ apiUrl: string; timeout: number }>
// Record for maps
const rolePermissions: Record<UserRole, Permission[]> = { ... }
Result Type (instead of throwing everywhere)
type Result<T, E = Error> =
| { ok: true; value: T }
| { ok: false; error: E }
async function safeParseJson<T>(text: string): Promise<Result<T>> {
try {
return { ok: true, value: JSON.parse(text) as T }
} catch (e) {
return { ok: false, error: e as Error }
}
}
Type Guards
function isUser(obj: unknown): obj is User {
return typeof obj === 'object' && obj !== null &&
'id' in obj && 'email' in obj && typeof (obj as User).email === 'string'
}
Rules
- Enable
strict: true— never disable it for individual files - Never use
any— useunknownand narrow with type guards - Prefer
interfacefor object shapes,typefor unions/intersections - Use discriminated unions over optional fields
- Type return values of exported functions explicitly
- Use
as constfor literal arrays/objects that shouldn't be widened - Avoid type assertions (
as X) — use type guards instead - Prefer
readonlyproperties for data that shouldn't change
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