The TypeScript Narrows

The TypeScript Narrows is a Claude Skill and an ESLint plugin that complement each other. Together they help AI agents narrow the "five different ways to do X" to a single, opinionated, best-practice way to do X. Some examples:

• No any, no !, no type assertions — narrow instead
• No enum — use as const objects with discriminated unions
• No default exports, no barrel files
• No .forEach(), no .reduce() — use for...of
• No floating promises, no misused promises
• return await inside try/catch (always)
• ?? over ||, ?. over manual null checks
• Exhaustive switch — every union variant handled
• Result types for expected failures, throw only for bugs
• Readonly<T> params, readonly properties, no mutable exports
• Branded types for domain IDs and value objects
• Early returns to fail fast and avoid deep nesting
• strict: true plus noUncheckedIndexedAccess
• ...and many more

The Claude skill and the ESLint plugin extend @typescript-eslint and cover over 50 opinions on type safety, error handling, async patters, naming conventions, and more.

"The TypeScript Narrows" is a play on "The Deathly Hallows".

Repo Structure

• Opinions -- The source of truth for both the Claude Skill and the ESLint plugin (docs/opinions/INDEX.md)
• Claude Plugin -- All opinions structured for AI-assisted TypeScript development (plugin/the-typescript-narrows/skills/typescript-narrows/SKILL.md)
• ESLint Plugin -- The subset of the opinions that can be enforced with ESLint rules, including typescript-eslint's strict rules and others (eslint-plugin/)

Getting Started

For AI-assisted development: Install the Claude plugin from the marketplace:

claude plugin add-marketplace github:sethlivingston/claude-marketplace
claude plugin install the-typescript-narrows

The skill triggers automatically when writing or reviewing TypeScript. See the plugin README for details.

For automated enforcement: Install @sethlivingston/eslint-plugin-typescript-narrows to lint against these opinions in CI and your editor:

npm install --save-dev @sethlivingston/eslint-plugin-typescript-narrows

See eslint-plugin README for setup instructions.

Both artifacts work independently, but they're designed to complement each other -- the skill covers opinions that can't be expressed as lint rules.

Coverage

See docs/TRACEABILITY.md for the full traceability matrix mapping all opinions to their enforcement mechanisms.

License

MIT