Use when reading or writing TypeScript or JavaScript files (.ts, .tsx, .js, tsconfig.json).

TypeScript Best Practices Follows type first, functional, and error handling patterns from CLAUDE.md. This skill covers language specific idioms only. Pair with React Best Practices When working with React components ( .tsx , .jsx files or @react imports), always load react best practices alongside this skill. This skill covers TypeScript fundamentals; React specific patterns (effects, hooks, refs, component design) are in the dedicated React skill. Make Illegal States Unrepresentable Use the type system to prevent invalid states at compile time. Discriminated unions for mutually exclusive states: Branded types for domain primitives: Const assertions for literal unions: Exhaustive switch with never check: Runtime Validation with Zod Define schemas as single source of truth; infer TypeScript types with z.infer< . Avoid duplicating types and schemas. Use safeParse for user input where failure is expected; use parse at trust boundaries where invalid data is a bug. Compose schemas with .extend() , .pick() , .omit() , .merge() for DRY definitions. Add .transform() for data normalization at parse time (trim strings, parse dates). Optional: type fest For advanced type utilities beyond TypeScript builtins, consider [type fest](https://github.com/sindresorhus/type fest): Opaque<T, Token cleaner branded types than manual & { brand } pattern PartialDeep<T recursive partial for nested objects ReadonlyDeep<T recursive readonly for immutable data SetRequired<T, K / SetOptional<T, K targeted field modifications Simplify<T flatten complex intersection types in IDE tooltips

typescript-best-practices