Build Errors

When a Packem build throws or exits with code 1, the message almost always names the exact misconfiguration. This page maps the real errors Packem emits to their fixes: failing on warnings, transformer/bundler mismatches, unknown transformers, and externalization failures.

Build fails because of warnings

Problem

The build prints warnings and then exits with:

Exiting with code (1). You can change this behavior by setting `failOnWarn: false`.

Cause

Packem collects warnings during the build (Build is done with some warnings: followed by the list). When failOnWarn is enabled, any accumulated warning turns into a non-zero exit at the end.

Solution

Either fix the warnings the build reports, or disable the fail-on-warn behavior:

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

export default defineConfig({
  transformer,
  failOnWarn: false
})

transformer set together with bundler: "rolldown"

Problem

The build refuses to start with:

The `transformer` option is not supported when `bundler: "rolldown"`. Rolldown uses its own oxc-based transform — remove `transformer` from your packem config.

Cause

The experimental Rolldown backend ships its own oxc-based transform and always uses it. The esbuild/swc/sucrase/oxc transformer adapters only apply under the default "rollup" bundler. Setting both is a configuration mistake, so Packem refuses to build rather than silently ignoring the option.

Solution

When using Rolldown, omit transformer entirely:

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

export default defineConfig({
  bundler: 'rolldown'
  // no `transformer` here
})

To keep a transformer, stay on the default Rollup backend (drop the bundler option or set bundler: "rollup").

Unknown or missing transformer

Problem

The build throws:

Unknown transformer, check your transformer options or install one of the supported transformers: esbuild, swc, sucrase

Cause

Under the default Rollup backend a transformer is required, and the one you configured could not be resolved — usually because the transformer package is not installed, or the import is wrong.

Solution

Import a transformer and pass it as the transformer option:

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

export default defineConfig({
  transformer
})

Make sure the backing package is installed (for example esbuild, @swc/core, or sucrase). If you run the interactive init and pass an invalid name, you will see Invalid transformer, please choose one of 'swc', 'sucrase' or 'esbuild'.

Externalization / unresolved module errors

Problem

The build fails resolving an import:

Failed to resolve the module "left-pad" imported by "src/index.ts"
Is the module installed? Note:
 ↳ to inline a module into your bundle, install it to "devDependencies".
 ↳ to depend on a module via import/require, install it to "dependencies".

Cause

By default an unresolved import (UNRESOLVED_IMPORT) is a hard error. Packem externalizes everything in dependencies and peerDependencies automatically; anything else must be resolvable on disk to be bundled.

Solution

Place the module where it belongs:

• Install under devDependencies to bundle it into the output.
• Install under dependencies / peerDependencies to keep it external.

To downgrade unresolved imports from error to warning, set unresolvedImportBehavior:

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

export default defineConfig({
  transformer,
  rollup: {
    resolve: {
      unresolvedImportBehavior: 'warn'
    }
  }
})

Duplicate entry names

Problem

The build throws:

Duplicate rollup input name "index" — one maps to "...", another to "...". Each entry must have a unique name.

Cause

Two entries resolved to the same output name but point at different source files. Rollup needs a unique name per input.

Solution

Give the colliding entries distinct names (or distinct output subpaths in your package.json exports) so each maps to one source file.

Output extension map errors

Problem

The build throws one of:

Invalid output extension map: <key> must be "cjs" or "esm"
Invalid output extension map: <key> must not start with a dot. Example: "cjs": "c.js", "esm": "m.js"
Invalid output extension map: <key> must be different from the other key

Cause

outputExtensionMap only accepts the keys cjs and esm, the extension values must not start with a dot, and the two extensions must differ.

Solution

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

export default defineConfig({
  transformer,
  outputExtensionMap: {
    cjs: 'c.js',
    esm: 'm.js'
  }
})

Related

• Common Issues
• TypeScript Problems
• Transformers
• Bundler