| name | ts-best-practices |
| description | TypeScript type safety guidelines for writing maximally type-safe code. Apply these patterns when writing or reviewing any TypeScript: discriminated unions, type narrowing, type guards, exhaustiveness checks, avoiding `as` casts, preferring `unknown` over `any`, and making impossible states unrepresentable. Use this skill whenever writing TypeScript code, reviewing TypeScript for type safety issues, or when the user mentions type safety, type narrowing, discriminated unions, or asks to make types stricter/more explicit.
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Type Safety
This project's TypeScript policy. Apply when writing or reviewing TypeScript.
| Rule | Summary |
|---|
No as casts | Every as is a potential runtime crash. Validate at boundaries, then cast only if earned. Prefer Zod/Valibot over manual validation. |
unknown over any | any disables type checking for everything it touches. External data is always unknown. |
| Discriminated unions | Model variants with a shared literal discriminant. No optional-field bags. |
| Narrowing hierarchy | Prefer: discriminated union switch > in operator > typeof/instanceof > type guard > as |
| Type guards | Must actually verify the claim. Name them isX or hasX. Prefer discriminant narrowing when possible. |
| Exhaustiveness checks | Always add default: never arm to switches over discriminated unions. Use an absurd() helper to reduce boilerplate. |
satisfies over as | When verifying a value matches a type without widening, use satisfies to preserve literal types. |
| Impossible states | If a bug requires asking "can this combination happen?" the type is too loose. Tighten it. |
Read references/patterns.md for code examples of each rule.
