typescript

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Write type-safe TypeScript. Use when writing or reviewing TypeScript code. Covers type safety best practices, avoiding dangerous features (any/as), and proper narrowing techniques.

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[Code Review]

When & Why to Use This Skill

This Claude skill acts as a rigorous TypeScript architect, enforcing strict type-safety standards to eliminate runtime errors and technical debt. It focuses on replacing unsafe patterns like 'any' and 'as' with robust alternatives such as 'unknown', 'satisfies', and discriminated unions. By providing a systematic checklist for type narrowing and validation, it ensures that TypeScript codebases remain maintainable, predictable, and fully leveraged for maximum developer productivity.

Use Cases

  • Automated Code Audits: Use the skill during PR reviews to identify and replace dangerous type assertions (as) or 'any' types with safer narrowing techniques.
  • Refactoring Legacy Code: Systematically upgrade loosely typed TypeScript files to strict mode by following the skill's step-by-step checklist for fixing type errors.
  • Safe Data Validation: Implement runtime-to-compile-time type safety by using the skill's guidance on integrating validation libraries like valibot for unknown API responses.
  • Complex Logic Design: Utilize discriminated unions and 'as const' patterns suggested by the skill to create exhaustive and type-safe state machines or UI components.
nametypescript
descriptionWrite type-safe TypeScript. Use when writing or reviewing TypeScript code. Covers type safety best practices, avoiding dangerous features (any/as), and proper narrowing techniques.

TypeScript

Checklist

  1. No any - Use unknown and validate with valibot
  2. No as - Fix types at source
  3. No ! - Explicit checks instead
  4. NO value is T - Use a validator library like valibot instead
  5. Discriminated unions - Add type field to narrow

Good Practices

  1. as const - For literal types only
  2. value satisfies T - Type-checks value without changing its type (safe, unlike is T)
  3. Builtin narrowing - typeof, Array.isArray, in, instanceof are always safe
  4. T extends SomeType if T is required to have some trait
  5. foo?.bar - Use ?. for safe property access
  6. Tagged unions - type User = { role: "admin"; permissions: string[] } | { role: "guest"; expiresAt: Date } then narrow with if (user.role === "admin")
  7. When refactoring - Fix ONE type error at a time (type inference changes)
  8. Const string unions - type Status = "pending" | "active" | "done" instead of enums

When Stuck - NEVER Give Up

IMPORTANT: Every time you encounter a type error, repeat this checklist in your response before attempting a fix.

When you're stuck and considering any or as, STOP:

  1. Read the library's documentation - Most type issues come from misunderstanding the library API
  2. Search GitHub issues (library repo) - Someone likely faced the same problem
  3. Read error message more carefully
  4. Use valibot to validate (if the type is unknown at runtime)
  5. Ask the user

any and as are admitting defeat. Always use choose another method.