From tsup

tsup and Packem solve the same problem — bundling TypeScript libraries — but take different approaches to configuration. tsup is option-driven: you list entries, formats, and flags in tsup.config.ts. Packem is package.json-driven: it reads your exports, main, module, and bin fields to decide what to build and in which formats.

Packem ships a migrate command that rewrites your package.json dependencies and scripts automatically. The bundler config itself still needs a manual pass, which this guide walks you through.

Run the automated migration

From your project root:

packem migrate

This will:

• Replace tsup / tsup-node in your dependencies, devDependencies, and peerDependencies with @visulima/packem.
• Rewrite scripts that invoke tsup to call packem build instead.
• Detect a tsup.config.* file or an inline tsup field in package.json and warn you that the config requires a manual conversion (it is not auto-converted).

Preview every change without writing anything:

packem migrate --dry-run

After running it, install dependencies with your package manager and continue with the config conversion below.

Option mapping

Step-by-step

1. Move your entries into package.json exports

tsup declares entries explicitly:

// tsup.config.ts
import { defineConfig } from 'tsup'

export default defineConfig({
  entry: ['src/index.ts', 'src/cli.ts'],
  format: ['esm', 'cjs'],
  dts: true,
})

Packem reads them from package.json. The exports map below produces both an ESM and a CJS build of src/index.ts, plus declarations, with no entry list:

{
  "type": "module",
  "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"
      }
    }
  },
  "bin": "./dist/cli.mjs",
  "scripts": {
    "build": "packem build",
    "dev": "packem build --watch"
  }
}

Packem matches src/index.ts to the "." export and src/cli.ts to bin. The import/require conditions drive the ESM/CJS split that format controlled in tsup.

2. Write packem.config.ts

Replace tsup.config.ts with packem.config.ts. Pick a transformer — esbuild is the closest match to tsup, which uses esbuild internally:

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

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

Carry over only the flags you actually set in tsup. declaration can be omitted when package.json has a types field — Packem turns it on automatically.

3. Map the remaining options

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

export default defineConfig({
  transformer,
  sourcemap: true,
  minify: true,
  declaration: true,
  runtime: 'node',
  externals: ['some-peer-only-dep'],
})

4. Update scripts

If you skipped packem migrate, change your scripts by hand:

{
  "scripts": {
    "build": "packem build",
    "dev": "packem build --watch"
  }
}

5. Delete the old config

Remove tsup.config.ts (and any inline tsup field in package.json) once the build passes.

Gotchas

• No entry and no format options. This is the biggest mental shift. Entries come from package.json exports/bin; output format comes from the export condition and file extension. If a build is missing, check that the matching exports entry and the corresponding src/ file both exist.
• Declarations follow your exports. With dual ESM/CJS exports that point at .d.mts and .d.cts, Packem emits matching extension-specific declaration files. A single .d.ts is enough for an ESM-only package.
• dependencies are external by default. tsup users often rely on noExternal or external lists. In Packem, anything in dependencies/peerDependencies is already external. Use --no-external only if you want everything inlined.
• onSuccess as a function needs the config file. The --onSuccess flag takes a shell string. To run JavaScript (e.g. start a dev server) use the onSuccess function form in packem.config.ts.
• Config conversion is manual. packem migrate never rewrites tsup.config.ts — it only warns. Translate the options yourself using the table above.

Next steps

• Configuration — Full option reference.
• Transformers — esbuild, swc, OXC, and sucrase.
• Package.json Setup — How exports drive entries and formats.