What problem does it solve? TypeScript codebases often accumulate unsafe patterns like any types, unvalidated external data, and accidental mutation, leading to runtime crashes and hard-to-trace bugs. This Skill provides a consistent rulebook for writing and reviewing TypeScript with strict type safety, runtime validation at trust boundaries, and immutable data patterns. ## Core Features & Use Cases - Schema-First Validation: Guides defining schemas (e.g., with Zod) at trust boundaries like API responses, user input, and file parsing, deriving types from schemas instead of duplicating declarations. - Strict Typing Rules: Enforces unknown over any, type for data and interface only for behavior contracts, and disciplined use of type assertions with documented checks. - Immutability and Error Handling: Provides patterns for readonly structures, non-mutating array operations, Result types for failures, options objects, and early returns. - Use Case: When reviewing a pull request that fetches data from an external API, use this Skill to ensure the response is validated with a schema, typed via inference, and handled immutably with explicit error contracts. ## Quick Start Review this TypeScript API handler using the typescript skill and fix any type safety or validation issues.
