TypeScript best practices

Apply the type-system-discipline principle skill first; this skill grounds it in TypeScript syntax.

| 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 creation. | | unknown over any | External data is unknown. any disables type checking everywhere it touches. | | 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. | | Type guards | Must verify the claim. A lying guard is worse than as because the bug hides behind a name that says it's safe. Name them isX or hasX. | | Exhaustiveness | Inline const _exhaustive: never = x; in default arms so the compiler errors when a new variant is added. | | satisfies over as | Validates the value without widening literal types. | | Boundary validation | Validate where data crosses in; trust types inside. See the boundary-discipline principle skill. | | Schema-derived types | Reach for Pick/Omit/Parameters/ReturnType/Awaited/typeof before declaring a new interface. | | Object args | Pass objects, not positional, so argument order is self-documenting. Skip on hot paths (per-frame render, tokenizers, parsers). |

Examples: references/patterns.md.