ESM and CommonJS

ESM uses static `import`/`export` with live bindings — importers observe updates from the exporter. CommonJS uses runtime `require` and `module.exports`.

`"type": "module"` in package.json, `exports` maps, and tool interop rules determine default import shapes. Circular dependencies can expose partially initialized modules.

ESM enables static analysis and tree shaking; CJS is more dynamic.

On interviews, explain the concept with a concrete example and name the runtime behavior interviewers probe.

Common pitfalls include mixing similar APIs and forgetting edge cases during live coding.

The trade-off is often clarity versus performance or safety versus convenience.

Checklist:

• Know static versus runtime loading.
• Check package type and exports field.
• Treat default import interop as tool-specific.