How would you structure JavaScript module boundaries so code remains reusable and unused code can be removed safely?

Designs explicit ESM boundaries with focused exports, controlled side effects, and dependency direction that supports safe bundling.

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TL;DR

I organize modules around cohesive responsibilities and expose a small public API with static ESM imports and exports. I keep initialization side effects separate from reusable definitions, because bundlers can remove unused exports only when they can analyze the graph safely. Consumers should depend on the public boundary rather than reaching into internal files.

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Judgment about public module contracts, side effects, circular dependencies, and the limits of tree shaking.

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Core answer
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Overview
Tree shaking is a build optimization, not an architectural guarantee. Clear static module boundaries make both reuse and analysis easier.
I organize modules around cohesive responsibilities and expose a small public API with static ESM imports and exports. I keep initialization side effects separate from reusable definitions, because bundlers can remove unused exports only when they can analyze the graph safely. Consumers should depend on the public boundary rather than reaching into internal files.
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Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
A feature package might export its component, contract, and factory from one intentional entry point while keeping adapters internal. I avoid a giant barrel that accidentally imports every feature or creates circular dependencies. I verify the production bundle because dynamic imports, CommonJS interop, package side-effect declarations, and re-export patterns can change what the tool can eliminate.

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Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Assuming unused source text is always removed automatically.
Putting registration or global mutation inside widely imported modules.
Using broad barrel files without checking their dependency graph.
Publishing internal file paths as an unofficial public API.

How would you structure JavaScript module boundaries so code remains reusable and unused code can be removed safely?

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