Build fails with Vite 8 (Rolldown): cannot resolve preact-render-to-string

[kazuma0129]

• Check if updating to the latest preact-iso version resolves the issue

Describe the bug

When building a client-side SPA that uses only preact-iso's routing features (Router, LocationProvider, Route), the build fails with Vite 8 (Rolldown) because preact-render-to-string is not installed.

To Reproduce

Reproduction repository: https://github.com/kazuma0129/preact-iso-vite8-repro
StackBlitz: https://stackblitz.com/github/kazuma0129/preact-iso-vite8-repro

Steps to reproduce:

1. Clone the repository
2. Run npm install
3. Run npm run build
4. See error:

Error: [vite]: Rolldown failed to resolve import "preact-render-to-string" from "node_modules/preact-iso/src/prerender.js".
This is most likely unintended because it can break your application at runtime.

Expected behavior

Build should succeed since the app only uses client-side routing features and does not use the prerender function.

Root Cause Analysis

• preact-iso/src/index.js exports a prerender function via dynamic import: import('./prerender.js')
• That prerender.js statically imports preact-render-to-string
• Vite 8 uses Rolldown which strictly resolves all imports including dynamic imports
• Since preact-render-to-string is not installed (not needed for client-side only apps), the build fails

Workaround

Add to vite.config.js:

export default defineConfig({
  build: {
    rolldownOptions: {
      external: ['preact-render-to-string'],
    },
  },
});

Suggested Fix

1. Make preact-render-to-string an optional peerDependency
2. Or remove the prerender export from the main entry point (preact-iso) and keep it only accessible via the subpath preact-iso/prerender

Environment

• preact-iso: 2.11.1
• vite: 8.0.0
• Node.js: 22.x