About typescript-best-practices
A TypeScript skill for developers that emphasizes types which make invalid states difficult to express. It combines modeling guidance with validation, testing, and telemetry practices.
Key features
- 01Discriminated unions and branded types
- 02Modeling valid values by construction
- 03Validate external data at boundaries
- 04Prefer schemas and derived types
- 05Exhaustive matching and safer narrowing
- 06Real tests and structured telemetry
Use cases
- 01Modeling variants without optional-field bags
- 02Preventing accidental mixing of semantic IDs
- 03Parsing untrusted input into domain types
- 04Reviewing TypeScript code for unsafe casts
Install
Copy the command and run it in a terminal.
$
npx skills add https:/After installing, type/typescript-best-practicesin Claude Code to confirm it loaded.
Bundled filesagents/openai.yamlreferences/patterns.md
Details
From the project READMETypeScript best practices
Call the Skill tool with principle-type-system-discipline 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. |
| 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. Call the Skill tool with principle-boundary-discipline. |
| 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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