Keep internals private

How to do it

Mark helpers private or keep them out of the module's public entry file. Give each package one public entry point. Callers never import from another module's internal folders.

Example

// Bad: callers reach into the module's internals
import { formatLine } from '../invoices/internal/format.js';

// Good: one public entry point, internals stay hidden
import { renderInvoice } from '../invoices/index.js';

How to check

A tool fails on unused exports and on imports of another module's internals. Examples are knip for unused exports, and dependency-cruiser, Deptrac or import-linter for internal imports.