typescript-best-practices

• SKILL.mdcore · 2,734 bytes
• references/patterns.mddoc · 9,287 bytes

# TypeScript best practices

Apply the **type-system-discipline** principle skill first.

| Rule | Summary |
|------|---------|
| Discriminated unions | Model variants with a `kind` literal discriminant so impossible states can't be represented. No optional-field bags. |
| Branded types | Brand primitives with `& { readonly __brand: "X" }` so they can't be mixed up. Validate once at the boundary. A brand comes from a verifying type guard (`input is AgentId`), never from `as`. |
| Constructive modeling | Build the shape so the illegal value can't be constructed. `[T, ...T[]]` for non-empty, `[T, T][]` for even length, `start` plus `duration` for a range. Not a runtime guard, not a wish for refinement types. |
| Simplest total type | Keep `T[]` while every operation on it stays total. Strengthen to `NonEmpty<T>` only where the loose type forces `!`, a cast, or a "should never happen" throw. |
| `unknown` over `any` | External data is `unknown`. |
| Schemas before guards | Before hand-writing a property-by-property type guard, use the repository's runtime schema library and infer the type from the schema, such as `z.infer`. |
| No `as` casts | Every `as` is a runtime crash waiting. Cast only after validation. |
| Narrowing hierarchy | Discriminant switch > `in` operator > `typeof`/`instanceof` > user-defined type guard > `as`. |