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typescript-best-practices

Skill2 starsChanged 15 days agoMITView on GitHub

Use when implementing or reviewing TypeScript code, including type contracts and compiler or module configuration.

We split this skill into the rules it gives an agent

3 rules. 0 are common, 2 are distinctive, and 1 is very specific to this project.

  • 0

    Common

    Most of the 22 edited versions we read keep these.

  • 2

    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. Distinctive:

    Use the simplest types that make the required operations safe.

    line 8
  2. Very specific:

    Parse untrusted values: Treat incoming untyped data as unknown.

    line 25
  3. Distinctive:

    Run the repository's checker against the changed files and affected consumers.

    line 54

“Common” is measured: kept by at least 6 in 10 of the 22 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

Use the simplest types that make the required operations safe. Preserve the task's implementation or inspection scope and the repository's public contracts.

Keep types honest

  • Strengthen types where an operation needs it. An empty collection can have a valid result or explicit absence; introduce a non-empty representation only when the contract requires one. Use discriminated unions for dependent fields and compiler-checked exhaustiveness for closed variants. Keep independently optional fields optional. Brand primitives where mixing identities or units causes a concrete error; a brand alone proves no runtime constraint.
  • Use the authoritative contract. Infer from the existing schema or generated source when it owns the same contract. Keep stable public interfaces independent of private implementation details. Preserve legitimate callers and documented states, including ones absent from today's call sites. A generic must express a real relationship; letting a caller choose an unchecked return type proves nothing. Prefer inference over redundant annotations or elaborate type machinery.
  • Parse untrusted values. Treat incoming untyped data as unknown; reuse the project's validator and validate at the boundary that owns the contract. An annotation, generated client type, as, satisfies, or generic argument does not validate runtime data. Use satisfies to check static compatibility, not to repair a mismatch. Do not silence a real mismatch with any, !, double casts, suppression, or weaker compiler settings. Isolate a necessary assertion at a justified boundary and test the invariant it relies on; as const is not a claim that arbitrary input has been validated.
  • Check a predicate's whole promise. A declared value is T must justify both branches: true means T, false excludes T. Property presence or one checked field rarely proves a whole object. Assertion functions need the same scrutiny for the guarantee they claim after returning.
  • Keep guarantees valid until use. readonly and as const do not freeze runtime objects. Mutation through aliases, callbacks, or across an await can invalidate narrowing or a non-empty collection. Preserve ownership, capture a stable value, or recheck at the relevant transition.
  • Represent absence accurately. Arbitrary dictionary keys and array indices can be missing, even with Record<string, T> or a non-empty array. Distinguish omission, explicit undefined, and null when the contract does. strict alone does not enable noUncheckedIndexedAccess or exactOptionalPropertyTypes; do not assume either guarantee or turn a small change into an unrelated configuration migration.
  • Own asynchronous completion. An async function can fit a void callback whose caller ignores its promise. Check whether completion and rejection reach the intended owner. void promise only discards the value. Use error-handling when recovery policy needs design; do not invent a catch-and-log policy here.

Verify the contract

Run the repository's checker against the changed files and affected consumers; transpilation is not type checking. For changed type contracts, check valid and invalid uses with assertions that actually fail on regression. A type alias evaluating to false is not a failing test. Exercise runtime boundary behavior separately. Keep formatting and enforceable style in existing tooling.

For compiler, import/export, or package changes, read compiler-and-modules.md.

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