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ngctongcto/oh-my-pstack/skills/typescript-best-practices/SKILL.md

typescript-best-practices

Skill1 starsChanged 38 days agoMITView on GitHub

TypeScript best practices. Use when reading or editing any .ts or .tsx file, modeling types, narrowing unknown data, or reviewing TypeScript for unsound casts, untyped boundaries, and non-exhaustive unions.

77 other projects use a version of this, 55 of them edited.

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Projects with it also use poteto-help, make-bot-ui and 6 more.

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The file

We split this skill into the rules it gives an agent

1 rule. 0 are common, 0 are distinctive, and 1 is very specific to this project.

  • 0

    Common

    Most of the 26 edited versions we read keep these.

  • 0

    Distinctive

    This author's own choices. Worth a look.

  • 1

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Very specific:

    Check the result with xd://lsp diagnostics on the files you edited, and use xd://lsp definitions and references, not text search, to find …

    line 30

“Common” is measured: kept by at least 6 in 10 of the 26 edited versions of this file we could read. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

Keep what fits and drop the rest. Your version, ready for your agents.
Files
SKILL.md

TypeScript best practices

Apply the type-system-discipline principle skill first.

RuleSummary
Discriminated unionsModel variants with a kind literal discriminant so impossible states can't be represented. No optional-field bags.
Branded typesBrand primitives with & { readonly __brand: "X" } so they can't be mixed up. Validate once at the boundary.
Constructive modelingBuild 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 typeKeep 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 anyExternal data is unknown.
Schemas before guardsBefore 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 castsEvery as is a runtime crash waiting. Cast only after validation.
Narrowing hierarchyDiscriminant switch > in operator > typeof/instanceof > user-defined type guard > as.
Type guardsMust 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.
ExhaustivenessInline const _exhaustive: never = x; in default arms so the compiler errors when a new variant is added.
satisfies over asValidates the value without widening literal types.
Boundary validationParse 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 typesReach for Pick/Omit/Parameters/ReturnType/Awaited/typeof before declaring a new interface.
Object argsPass objects, not positional, so argument order is self-documenting. Skip on hot paths (per-frame render, tokenizers, parsers).
Real testsDon'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 telemetryPrefer structured logger diagnostics with enough context to debug from an id. No console.log in shipped code.

Check the result with xd://lsp diagnostics on the files you edited, and use xd://lsp definitions and references, not text search, to find every use of a type before you change its shape.

Examples: references/patterns.md.

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