Type Design

Model the domain explicitly. Do not hide weak modeling behind any, wide assertions, or optional-everything objects.
Prefer unknown over any at trust boundaries, then narrow deliberately.
Use interfaces for extendable object contracts. Use type aliases for unions, intersections, mapped types, and conditional types.
Prefer discriminated unions over boolean flag combinations.
Keep shared types close to the code that owns them unless multiple modules genuinely depend on them.
Make impossible states impossible when the data model allows it.

Functions and APIs

Give exported functions explicit parameter and return contracts.
Use overloads only when they improve the public API. Otherwise prefer a single well-modeled input object.
Prefer result shapes and typed domain errors over vague thrown strings.
Use early returns to keep control flow shallow.

Use this skill for any non-trivial .ts or .tsx change.

Model the domain explicitly. Do not hide weak modeling behind any, wide assertions, or optional-everything objects.
Prefer unknown over any at trust boundaries, then narrow deliberately.
Use interfaces for extendable object contracts. Use type aliases for unions, intersections, mapped types, and conditional types.
Prefer discriminated unions over boolean flag combinations.
Keep shared types close to the code that owns them unless multiple modules genuinely depend on them.
Make impossible states impossible when the data model allows it.

Give exported functions explicit parameter and return contracts.
Use overloads only when they improve the public API. Otherwise prefer a single well-modeled input object.
Prefer result shapes and typed domain errors over vague thrown strings.
Use early returns to keep control flow shallow.