noctcore-architecture/barrel-purity

A barrel (index.ts / index.tsx) must contain only re-exports — never local declarations, side effects, or default-exported values.

Recommended preset: error · Autofix: no · Suggestions: no · Type information: not needed

Why

A barrel exists to present a folder’s public surface. The moment it also declares something — a const, a function, a type, a default-exported value — or does something — a side-effect import, a top-level call — the barrel becomes a real module with behavior. Every consumer that imports the folder now silently pulls that behavior in, and the folder no longer has a clean, movable boundary.

What it flags

Only index.ts / index.tsx (and the other index extensions) are inspected. Each top-level statement that is not a pure re-export is reported.

Correct · src/components/Card/index.ts

export { Card } from './Card';          // named re-export
export * from './Card.types';           // star re-export
export * as card from './Card';         // namespace re-export
export { default as CardView } from './Card'; // default re-export
import { a } from './a';                // import that feeds a re-export
export { a };                           //   ...its matching specifier
export type { Props } from './Card';    // type re-export
import Card from './Card';
export default Card;                    // re-export of a binding by name

Incorrect · src/components/Card/index.ts · 7 reports

export const helper = 1;                // local declaration
export function build() {}              // local declaration
export type T = string;                 // local declaration
export default () => 1;                 // a value, not a re-export
import './styles.css';                  // side-effect import
console.log('hi');                      // side-effect statement
const cache = new Map();                // non-export code

An import that carries bindings is allowed because it feeds a re-export; a specifier-less import './x' is a side effect and is flagged.

What it does not flag

• Any file that is not an index barrel: a helper.ts full of logic is none of this rule’s business.
• Pure re-exports in every form shown in the good example, including an import whose bindings feed a specifier-only export { a } and export default Card of a binding by name.
• Barrels whose path matches an allow glob.

Options

Option	Type	Default	Meaning
allow	string[]	[]	Globs (supporting **, *, ?) of barrel paths to exempt entirely.

'noctcore-architecture/barrel-purity': ['error', {
  allow: ['**/legacy/**'],
}]

When not to use it

If you deliberately keep small helpers or feature flags inside a package’s index.ts, this rule will fight you. Prefer moving them into a sibling module and re-exporting — but if you cannot, disable the rule for those paths via allow.