From unbuild

unbuild and Packem share a lineage: both are Rollup-based library bundlers that lean on package.json and a build.config style file, and both can infer entries automatically. Migrating is mostly a one-to-one translation of unbuild's BuildConfig into Packem's defineConfig, with Packem doing even more inference from your exports map.

Packem ships a migrate command that handles the package.json rewrites for you.

Run the automated migration

From your project root:

packem migrate

This will:

• Replace unbuild in your dependencies, devDependencies, and peerDependencies with @visulima/packem.
• Rewrite scripts that invoke unbuild to call packem build instead.
• Detect a build.config.* file and warn you that it needs a manual conversion (it is not auto-converted).

Preview without writing anything:

packem migrate --dry-run

Option mapping

unbuild's build.config.ts exports a BuildConfig. Most fields have a direct Packem counterpart:

Step-by-step

1. Translate build.config.ts

A common unbuild config:

// build.config.ts
import { defineBuildConfig } from 'unbuild'

export default defineBuildConfig({
  entries: ['src/index'],
  declaration: true,
  clean: true,
  rollup: {
    emitCJS: true,
  },
})

The emitCJS flag and the explicit entry both move into package.json — a require condition produces the CJS output, and the "." export plus src/index.ts replace the entries list:

{
  "files": ["dist"],
  "main": "./dist/index.cjs",
  "module": "./dist/index.mjs",
  "types": "./dist/index.d.ts",
  "exports": {
    ".": {
      "import": { "types": "./dist/index.d.mts", "default": "./dist/index.mjs" },
      "require": { "types": "./dist/index.d.cts", "default": "./dist/index.cjs" }
    }
  },
  "scripts": {
    "build": "packem build"
  }
}

Then packem.config.ts only needs a transformer; declaration is auto-detected from the types field, and clean is already the default:

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

export default defineConfig({
  transformer,
})

2. Keep explicit entries when you need them

If you relied on unbuild's entries array for entries outside your public exports, use Packem's entries option. It accepts a string or a richer object with per-entry declaration, runtime, cjs/esm, and outDir:

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

export default defineConfig({
  transformer,
  entries: [
    'src/index.ts',
    { input: 'src/cli.ts', executable: true },
  ],
})

3. Map the mkdist builder to unbundle

unbuild's mkdist builder transpiles a directory file-by-file without bundling. Packem's equivalent is unbundle mode, which preserves the source file structure instead of bundling into single files:

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

export default defineConfig({
  transformer,
  unbundle: true,
})

Or per build: packem build --unbundle.

4. Carry over hooks, aliases, and externals

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

export default defineConfig({
  transformer,
  alias: {
    '~': './src',
  },
  externals: ['some-undeclared-peer'],
  hooks: {
    // Same lifecycle-hook idea as unbuild's `hooks`
  },
})

5. Delete build.config.ts

Once packem build produces the same dist/, remove build.config.ts.

Gotchas

• A transformer is required. unbuild picks its compiler for you; Packem makes it an explicit choice under the default Rollup backend. Add transformer to your config or the build will refuse to run.
• emitCJS → export conditions. There is no emitCJS flag. Dual output comes from having both import and require conditions (or .mjs/.cjs extensions) in package.json.
• mkdist becomes unbundle. If you used the mkdist builder for unbundled, directory-preserving output, switch to unbundle: true rather than expecting a bundled single file.
• Inlining dependencies is opt-in. unbuild's inlineDependencies has no single equivalent — declared deps are external by default; use --no-external to inline everything, or curate externals.
• Config conversion is manual. packem migrate rewrites package.json only and warns about build.config.*. Translate the config yourself using the table above.

Next steps

• Configuration — Full option reference.
• Unbundle — Preserve source file structure (the mkdist equivalent).
• Entry — Configure explicit entry points.