Summary
Enforces a tight set of TypeScript patterns that treat the type system as a proof assistant rather than documentation. Every rule here exists to close a gap where TypeScript would otherwise let you lie to yourself: discriminated unions over optional fields, branded primitives to prevent ID mixups, unknown for external data, and exhaustiveness checks that break the build when you add a variant. The narrowing hierarchy and type guard rules are especially useful if you're tired of as casts hiding bugs. Also pushes object parameters for readability and real tests over mocks, which is opinionated but defensible. If you want TypeScript to actually catch your mistakes instead of rubber stamping them, this is a solid foundation.
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npx -y skills add cursor/plugins --skill typescript-best-practices --agent claude-codeInstalls into .claude/skills of the current project.
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SKILL.md
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. |
| 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. |
| 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 | Parse where data crosses in, into a named domain type. Record<string, unknown> (however spelled) stops at that parse. 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). |
| Real tests | Don't mock what you can run. Prefer the framework's real test primitives with leak/disposable checks, and verify UI in a running build. Mock only what you can't run locally. |
| Structured telemetry | Prefer structured logger diagnostics with enough context to debug from an id. No console.log in shipped code. |
Examples: references/patterns.md.
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Keeps your Mac awake while Claude Code, Codex or Cursor works. Lets it sleep when they are done.
Try free for 7 days →Block distracting apps from your iPhone permanently without a 3rd party app. Free and open source.
Block now (100% free) →Notes, actions and memory. Without a meeting bot. First month 100% off.
Download for free →belt cli automatically finds the best tools and skills for your agent. image, video, music, tts...
one prompt install →Free an open-source skills that make AI coding agents easier to understand, verify, and control.
Download for free →Connect your Claude agent to live crypto prices and trading routes via 1inch
Get the MCP →First SeenJun 23, 2026
