Bundler

Packem can build with one of two backends. By default it uses Rollup — the full-featured, fully-supported path. You can opt into the experimental Rolldown backend (a Rust-based, oxc-powered bundler) with the bundler option.

export default defineConfig({
  bundler: 'rollup',   // default
  // or
  bundler: 'rolldown', // experimental
})

CLI flag

You can also select the backend per build without editing the config file:

packem build --bundler rolldown

The flag accepts rollup or rolldown; any other value is rejected.

Rolldown backend (experimental)

When you select Rolldown, keep these in mind:

• No transformer option. Rolldown ships its own oxc-based transform and always uses it, so omit the transformer option when bundler is 'rolldown'. The esbuild / swc / sucrase / oxc transformer adapters only apply under the default 'rollup' backend.

  // ✅ Rolldown — no transformer
  export default defineConfig({
    bundler: 'rolldown',
  })
  
  // ❌ Rolldown + transformer is not supported
  export default defineConfig({
    bundler: 'rolldown',
    transformer: esbuild, // remove this
  })

• Declaration files route through Rollup. Even with bundler: 'rolldown', .d.ts generation (and DTS watching) still runs through Rollup. Rollup is pulled in automatically when Rolldown is combined with declaration: true, so you don't need to configure anything — but Rollup remains a build-time dependency in that case.

Choosing a backend

For the detailed support matrix and the criteria for graduating Rolldown out of experimental status, see Rolldown backend status.

Related Options

• Transformers - Choose the code transformer (Rollup backend only)
• Declaration Files - TypeScript .d.ts generation