file-organization

Skill: file-organization

Scope

• Applies to: grouping related implementation files in apps and packages, index.ts public APIs, import paths, folder naming
• Does NOT cover: Next.js lucide-style package import optimization, Fastify route structure, or Drizzle schema design

Assumptions

• TypeScript ESM with .js extensions in import specifiers
• Kebab-case directory names
• App-local group indexes use named re-exports; package subpaths may use export * when the folder is a published module boundary

Principles

• A folder groups 2+ implementation files of one concern
• index.ts is the group's public API when all exports share a runtime
• Drop the folder-name prefix inside the folder (oauth-google.ts → oauth/google.ts)
• Outside the folder, import the group (.../feature/index.js); inside, import siblings directly
• Single-file modules stay files; tests colocate beside implementation
• Mixed runtimes → folder with named entry files, no unifying index

Constraints

MUST

• Use kebab-case folder names
• Use named re-exports in app-local group indexes (avoid export * mega-barrels)
• Resolve basename collisions before creating a folder (auth.ts vs auth/)

MUST NOT

• Add parent mega-barrels (lib/index.ts, components/ui/index.ts)
• Create foo/foo.ts plus a thin re-export index
• Add index.ts in Fastify autoload trees (routes/, plugins/)
• Unify server and client modules in one index when they use different runtimes

SHOULD

• Keep group indexes small and cohesive (one feature domain)
• Use named entry files when re-exporting would create import cycles
• Prefer existing package subpaths over new root barrels

Interactions

• Complements next-v16 (barrel import optimization ≠ feature group indexes)
• Complements typescript-v6 (ESM paths, named exports)
• Repo rules override this skill when they specify exceptions (catalogs mapper, Drizzle schema index)

Patterns

Group index (same runtime)

oauth/
  shared.ts
  google.ts
  index.ts    # named re-exports only

Consumers: import { getOAuthAllowedCallbackUrls } from '../lib/oauth/index.js'

Split runtime (no unifying index)

logger/
  server.ts
  client.ts
  # no index.ts

Consumers import named entry files directly (logger/server, logger/client).

Named entry (cycle avoidance)

catalogs/
  index.ts      # dictionaries only
  mapper.ts     # imports from ./index.js — not re-exported

Anti-patterns

Pattern	Why
Prefix soup at one level	Hard to navigate; use folders
lib/index.ts re-exporting everything	Mega-barrel; slows builds and obscures boundaries
auth.ts beside auth/	Module resolution footgun
Unifying index with cookies + document	Breaks server/client boundaries