TypeScript

Version

TypeScript 6.0 (pinned). TypeScript 7 exists, but typescript-eslint does not support it yet (ADR-0009). npm run deps:outdated only proposes TypeScript patch upgrades (.ncurc.cjs).

Split with ESLint (ADR-0010)

What ESLint can check lives in ESLint; tsconfig.json keeps only what belongs to the type system:

Option	Why in TypeScript
strict	any inference, null, this, variance: out of ESLint's reach
noUncheckedIndexedAccess	T[number] becomes T | undefined: a type, not syntax
exactOptionalPropertyTypes	x?: T does not accept an explicit undefined
noImplicitOverride, noPropertyAccessFromIndexSignature	type semantics
noImplicitReturns	replaces consistent-return, which conflicts with unicorn/no-useless-undefined
verbatimModuleSyntax, isolatedModules, noUncheckedSideEffectImports	module emission and resolution

Left to ESLint (off in tsconfig.json): noUnusedLocals, noUnusedParameters, noFallthroughCasesInSwitch.

Files

• tsconfig.json: type checking only (noEmit) of src/, playground/, eslint/, tests/, scripts/, .claude/hooks/, docs/.vitepress/ and root files.
• tsconfig.build.json: emits the declarations dist/types/ from src/ (tests excluded); also used by TypeDoc.

Typed rules

typescript-eslint uses the project service (projectService): strict-boolean-expressions, no-unnecessary-condition, switch-exhaustiveness-check, no-floating-promises… know the types. A file outside tsconfig.json cannot be linted: add it to include.