Duplicate packages

Overview

When several of your dependencies depend on different versions of the same package — or the same version resolved from different locations (for example, distinct peer-dependency variants under pnpm) — that package ends up bundled more than once. This inflates bundle size and can cause subtle bugs when two copies of a library don't share state.

Packem ships a built-in detector that walks the final module graph after each build and reports any package that was bundled multiple times, along with the files that imported each copy.

It is enabled by default.

Example output

WARNING  packages ms is bundled multiple times!

  ms:
    - 2.0.0 imported by packages/pkg1/index.js
    - 2.1.3 imported by packages/pkg2/index.js

A runnable demo lives in examples/duplicated-packages.

Configuration

Configure (or disable) it through the rollup.detectDuplicated option:

import { defineConfig } from "@visulima/packem/config";

export default defineConfig({
  rollup: {
    detectDuplicated: {
      // Stop reporting specific packages/versions. Use "*" to ignore all versions.
      ignore: {
        axios: ["0.27.2"],
        lodash: ["*"],
      },

      // Only report duplicates imported directly by your own source,
      // ignoring those pulled in transitively by another dependency.
      // @default true
      deep: true,

      // Fail the build (non-zero exit) when duplicates are found.
      // @default false
      throwErrorWhenDuplicated: false,

      // Build a custom report from the collected data instead of the default one.
      // customErrorMessage: (packagesInfo) => `...`,
    },
  },
});

Disable it

import { defineConfig } from "@visulima/packem/config";

export default defineConfig({
  rollup: {
    detectDuplicated: false,
  },
});

Options

Notes

• The detector inspects the bundled module graph, so dependencies that are externalized (not bundled) are never flagged.
• Detection runs at buildEnd, which means it works correctly with Packem's build cache and watch mode — there is no extra module resolution cost.
• Set detectDuplicated: false if you don't want the check (for example, in a project that intentionally bundles multiple versions).

Tests

See packages/packem/__tests__/intigration/detect-duplicated.test.ts.