Rollup preserveEntrySignatures behavior across major versions

Answer

To keep the old entry’s public API unchanged while exposing a new entry with only its own exports, use preserveEntrySignatures as an object mapping: { oldEntry: 'strict', newEntry: 'exports-only' }. The exact default depends on your Rollup major version.

Confirmed facts (behavior across versions)

• Rollup 1.x: default false (no preservation). Each entry emits only its own exports; chunk exports are not re‑exported.
• Rollup 2.x and 3.x: default 'strict' (preserve entry signatures). Each entry re‑exports all exports from its chunks, matching the entry‑point’s original API.
• Per‑entry object mapping for preserveEntrySignatures is supported starting with Rollup 2.1. Earlier versions accept only a single global value, requiring separate builds or a workaround.
• External dependencies must be declared with the external option (e.g., external: ['lodash', 'react']) and supplied at runtime via a shared global, import map, or UMD/IIFE wrapper. Using the same output format for both bundles prevents duplicate loads and version conflicts.

Steps to verify and apply

1. Check your installed Rollup version: npm list rollup or rollup --version.
2. If version < 2.0:
   • Set a global preserveEntrySignatures: false (default) for both entries.
   • To keep the old entry’s API, you must manually re‑export chunk exports in the old entry’s source or use a separate build with preserveEntrySignatures: true via a plugin.
3. If version ≥ 2.0:
   • Configure preserveEntrySignatures as an object:

     export default {
       input: {
         oldApp: './src/old-entry.js',
         newApp: './src/new-entry.js'
       },
       output: [
         { format: 'esm', dir: 'dist', entryFileNames: '[name].js' }
       ],
       external: ['lodash', 'react'],
       preserveEntrySignatures: {
         oldApp: 'strict',   // keeps original API (re‑exports chunk exports)
         newApp: 'exports-only' // exposes only its own declarations
       }
     };

   • Build: rollup -c.
   • Inspect generated files: dist/oldApp.js should contain re‑exports from chunks; dist/newApp.js should only contain its own exports.
4. Provide external dependencies at runtime (e.g., via a shared window global or an import map) and ensure both bundles use the same format (ESM with import maps, UMD, or IIFE).

Missing diagnostic detail

The recommendation differs between Rollup 1 and Rollup 2+; please confirm your exact Rollup major version so the steps can be finalized.