Rollup ESM bundle retains unused exports despite tree-shaking

Answer the question first

The Rollup version that controls side‑effect flag handling is the version actually used in the build (check with rollup --version or the devDependencies entry). If you are on Rollup 4.x, side‑effect detection is stricter; on Rollup 3.x it is more permissive.

Likely explanation vs. confirmed facts

Likely explanation: an imported module is being treated as side‑effectful, so Rollup cannot drop its unused export. This happens when the module’s package.json lacks "sideEffects": false or contains top‑level code that Rollup cannot prove pure (e.g., a function call, class definition, or side‑effecting import).

Confirmed facts (observable in the bundle):

1. Search the generated bundle for the unused export’s identifier. If it appears at the top level or inside a function that is never called, the export survived tree‑shaking.
2. Run Rollup with the visualizer plugin (--plugin rollup-plugin-visualizer --visualizer.filename=stats.html) to see which modules are marked as having side effects.
3. If a minifier (e.g., Terser) is listed as an input.plugin, it runs before Rollup’s tree‑shaking pass and can obscure pure‑ness checks, causing dead code to be retained.

Steps needed for this case

• Confirm Rollup version: rollup --version or inspect package.json.
• Check the suspected module’s package.json for "sideEffects": false. If missing, add it (or set treeshake.moduleSideEffects: 'no-external' in config).
• Move any minifier from input.plugins to output.plugins so it runs after bundling.
• Re‑build and verify the export is gone from the bundle.

Ask for one missing diagnostic detail only when it changes the recommendation: we need to know the exact Rollup version (or the name of the imported module) to confirm whether the stricter 4.x side‑effect rules apply.