just-works: Skill for Claude Code

TypeScript Coding

Project conventions override these defaults. Read 2-3 nearby files, tsconfig.json (strict, target, module, moduleResolution), and package.json (runtime, dependency versions) before writing.

Never rules

• Never use any. Use unknown and narrow with type guards, or generics with constraints.
• Never use @ts-ignore. Use @ts-expect-error with a reason; it fails the build once the error disappears.
• Never use as on external data (API responses, JSON.parse, storage, env, form data). It performs no runtime check. Validate at the boundary with Zod safeParse or an equivalent.
• Never use ! without proof. Prefer ??, ?., or an explicit if.
• Never leave a Promise floating. await it, chain .catch(), or void it with an error handler.
• Never mutate parameters or shared objects. Copy (structuredClone, spread) before changing; destructure to omit properties instead of using delete.
• Never use export * in barrel files. Names collide silently and go-to-definition lands on the barrel. Use explicit named re-exports, one barrel level at most, at package boundaries.
• Never import types without type. Use import type or inline type qualifiers, and verbatimModuleSyntax where the project can enable it.
• Prefer as const objects with a derived union over enum. Enums emit runtime code, numeric enums accept arbitrary numbers, and enums fail under erasableSyntaxOnly and Node's type stripping. Keep enum where the project already uses it.
• Never leave a switch over a union without an exhaustiveness check. End with default: { const _exhaustive: never = value; }.
• Treat items[0] as possibly undefined. Without noUncheckedIndexedAccess, TypeScript types it as T. Narrow before use, and suggest the flag in new projects; do not change an existing project's tsconfig.json unprompted.

Error handling

Use throw for programmer mistakes and internal code. Use a Result discriminated union for expected domain failures and public API boundaries, so the caller must handle them. When wrapping an error, pass { cause: error } (new Error(msg, { cause: error }), or a custom error class with a machine-readable code) to keep the original stack. catch (error) is unknown: narrow with instanceof before reading properties.