---

name: typescript description: 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.

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.