Transformers

The transformer option selects the engine that converts your TypeScript and modern JavaScript source into the output Packem bundles. It is the option-reference companion to the Transformers guide, which compares the engines in detail.

import { defineConfig } from '@visulima/packem/config'
import transformer from '@visulima/packem/transformer/esbuild'

export default defineConfig({
  transformer,
})

Setting the transformer

transformer is a function (TransformerFn), not a string. You import the adapter you want from @visulima/packem and pass it straight through to defineConfig.

import esbuild from '@visulima/packem/transformer/esbuild'
import swc from '@visulima/packem/transformer/swc'
import oxc from '@visulima/packem/transformer/oxc'
import sucrase from '@visulima/packem/transformer/sucrase'

There are four transformer import paths:

When each transformer applies

The transformer option only takes effect under the default 'rollup' bundler. The esbuild / swc / oxc / sucrase adapters are Rollup plugins, so they are only invoked on the Rollup path.

When you select bundler: 'rolldown', Rolldown ships its own oxc-based transform and always uses it. In that case the transformer option must be omitted — passing one is a configuration mistake and Packem will refuse to build.

// ✅ Rollup (default) — transformer required
export default defineConfig({
  bundler: 'rollup',
  transformer: esbuild,
})

// ✅ Rolldown — no transformer
export default defineConfig({
  bundler: 'rolldown',
})

Switching transformers conditionally

Because transformer is just a value, you can swap engines based on the environment:

import esbuild from '@visulima/packem/transformer/esbuild'
import swc from '@visulima/packem/transformer/swc'

const isDev = process.env.NODE_ENV === 'development'

export default defineConfig({
  transformer: isDev ? esbuild : swc,
})

Related Options

• Bundler - Rollup (transformer-driven) vs Rolldown (built-in oxc)
• Target - Compilation targets passed to the transformer
• Minification - Output minification