Shims

When you publish ESM and CommonJS output from the same source, two interop problems show up: CommonJS globals (__dirname, __filename, require) do not exist in ESM, and default-export shapes differ between the two module systems. Packem addresses both with two real options.

cjsInterop

cjsInterop enables CommonJS interop handling for ESM modules. It is a top-level BuildOptions field and defaults to false.

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

export default defineConfig({
  cjsInterop: true,
})

When enabled, Packem applies its CommonJS-interop Rollup plugin and adjusts warning handling (for example, it suppresses the MIXED_EXPORTS warning that would otherwise fire). The plugin's own behaviour can be tuned through rollup.cjsInterop, which defaults to { addDefaultProperty: false }:

export default defineConfig({
  cjsInterop: true,
  rollup: {
    cjsInterop: {
      addDefaultProperty: true,
    },
  },
})

ESM __dirname / __filename / require shim

The ESM shim rewrites CommonJS-only globals so that code using __dirname, __filename, or a global require(...) keeps working when emitted as ESM. It is configured via rollup.shim (an EsmShimCjsSyntaxOptions | false).

export default defineConfig({
  cjsInterop: true,
  rollup: {
    shim: {
      // EsmShimCjsSyntaxOptions (e.g. include/exclude filters)
    },
  },
})

What the shim injects

The shim scans each chunk for __filename, __dirname, and global require(...) usage and, only when found, prepends the matching imports after the last static import. For example:

• __filename / __dirname → import __cjs_url__ from "node:url" (and node:path for __dirname)
• global require(...) → import __cjs_mod__ from "node:module"

Each injected line is marked with a // -- packem CommonJS … shim -- comment, so Packem will not double-shim already-shimmed output. To disable the shim entirely, set rollup.shim: false.

Related Options

• Bundler - Shims run on the Rollup backend
• Package Exports - How ESM/CJS entries map to exports
• Target - Node target affects which interop you need