TypeScript Core Development

Reference hub for TypeScript work. Verify versions, compiler options, and APIs against npm (npm view <pkg> version) and the installed toolchain — never from trained knowledge.

Reference routing

Load references on demand — not all at once.

| Reference | Use when | |---|---| | strict-configuration.md | tsconfig strictness anatomy, compiler defaults, ESLint flat config | | common-errors.md | The genuinely confusing compiler errors (TS2742, TS1484, TS1479, ...) | | error-handling.md | Result types (True Myth), Effect error channel, boundary strategy | | patterns.md | ts-pattern, builder/newtype idioms, satisfies / const type params / NoInfer / using | | testing.md | vitest config, mocking, testing-library setup, MSW, coverage | | principles.md | Type-safety posture: unknown at boundaries, parse-don't-validate, exhaustiveness | | project-structure.md | exports/imports maps, #-subpath imports, project references, run-TS-directly | | dependencies.md | Version policy, updates, knip/publint/arethetypeswrong, audit, provenance | | packages-always-use.md | The curated default stack (zod, Effect, True Myth, LogTape, ...) | | packages-utilities.md | type-fest (Tagged), es-toolkit, ts-pattern, TypeID | | packages-cli.md | CLI framework decision table, @inquirer/prompts, Ink 7, ora, listr2 |

For code-review checklists use the devs:code-review skill; React UI work belongs to devs:react-core / the react-dev agent.

Decision guides

Package manager: detect from the lockfile in existing projects and conform (pnpm-lock.yaml/bun.lock/yarn.lock/package-lock.json); prefer pnpm otherwise. Wrap network-touching commands in Socket Firewall when available: sfw pnpm add …, sfw npm install.

Test runner: vitest by default; node:test when zero-dependency matters. Details: testing.md.

Error strategy: exceptions at the edges, typed Results (True Myth) or Effect in the core — the decision table is in error-handling.md.

Package selection: the curated stack in packages-always-use.md (with do-not-use lists). Prefer the platform (fetch, node:test, Temporal) when it covers the need.

Invoking tsc: through the project's scripts or ./node_modules/.bin/tsc — bare npx tsc resolves to a squatter package, not TypeScript.

tsconfig templates (${CLAUDE_SKILL_DIR}/assets/tsconfig-templates/)

TS-7-verified. strict-base.json is the shared baseline (strict + the valuable non-implied flags + verbatimModuleSyntax); variants extend it — copy the base plus one variant:

• strict-node.json — Node service: nodenext, types: ["node"], erasableSyntaxOnly (sources run directly under Node's type stripping).
• library.json — published library: nodenext resolution, declarations + isolatedDeclarations.
• strict-react.json — React app: bundler resolution, jsx: react-jsx, noEmit.
• monorepo-base.json — workspace base: composite + declarations; packages add include/rootDir/outDir and project references.

All nodenext variants require "type": "module" in package.json (TS1287 otherwise — the comment in each file says so).

This skill deliberately ships no scaffolding scripts: framework CLIs (npm create vite, pnpm create ...) own scaffolding in this ecosystem; these templates and the greenfield spec in the typescript-dev agent cover the TypeScript layer.