@rollup/plugin-commonjs 28.0.9: demonstrate esmExternals option for external require calls in @rollup/plugin-commonjs

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: demonstrate esmExternals option for external require calls in @rollup/plugin-commonjs
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:npm/%40rollup/plugin-commonjs@28.0.9
Demonstrate these symbols/APIs:
  - esmExternals

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:3e58126efe02b7bfcc8bc0cc6809534da7c6c57719ebcc68efffff16339fafc5","contract":["External require calls are assumed to be CommonJS by default (esmExternals: false), rendering as default imports rather than namespace imports.","Setting esmExternals: true treats external required dependencies as ES modules, rendering them as namespace imports with getAugmentedNamespace interop wrapper by default.","Setting esmExternals: true combined with requireReturnsDefault: 'namespace' directly references external namespace imports without interop wrapper functions.","Configuring esmExternals with an array or filter function selectively converts specified modules to namespace imports while retaining default imports for other external dependencies."],"goal":"demonstrate esmExternals option for external require calls in @rollup/plugin-commonjs","kind":"HOW","packages":["pkg:npm/%40rollup/plugin-commonjs@28.0.9"],"schemaVersion":1,"symbols":["esmExternals"]},"contractCommand":["node","test/contract.mjs"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"npm","executionContext":"node","language":"javascript","libc":"glibc","libcVersion":"2.39","moduleSystem":"esm","os":"linux","osVersionBucket":"24","packageManager":"npm","packageManagerVersion":"10.9.8","runtime":"node","runtimeVersion":"22.23.2","schemaVersion":1},"license":"MIT-0","packages":["pkg:npm/%40rollup/plugin-commonjs@28.0.9"],"schemaVersion":1,"subject":"pkg:npm/%40rollup/plugin-commonjs@28.0.9","symbols":["esmExternals"],"verifierAdapter":"node-typescript@1"}

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    "node_modules/rollup/node_modules/@types/estree": {
      "version": "1.0.6",
      "resolved": "https://registry.npmjs.org/@types/estree/-/estree-1.0.6.tgz",
      "integrity": "sha512-AYnb1nQyY49te+VRAVgmzfcgjYS91mY5P0TKUDCLEM+gNnA+3T6rWITXRLYCpahpqSQbN5cE+gHpnPyXjHWxcw==",
      "license": "MIT"
    }
  }
}

{
  "name": "sample-rollup-plugin-commonjs-esm-externals",
  "version": "1.0.0",
  "type": "module",
  "license": "MIT-0",
  "scripts": {
    "test": "node test/contract.mjs"
  },
  "dependencies": {
    "@rollup/plugin-commonjs": "28.0.9",
    "rollup": "4.34.8"
  }
}

{
  "schemaVersion": 1,
  "goal": "demonstrate esmExternals option for external require calls in @rollup/plugin-commonjs",
  "kind": "HOW",
  "packages": [
    "pkg:npm/%40rollup/plugin-commonjs@28.0.9"
  ],
  "symbols": [
    "esmExternals"
  ]
}

import * as rollup from 'rollup';
import commonjs from '@rollup/plugin-commonjs';

/**
 * Bundles virtual modules using Rollup and @rollup/plugin-commonjs.
 *
 * @param {string} entryCode - The entry point code (CommonJS).
 * @param {object} [pluginOptions] - Options passed to @rollup/plugin-commonjs.
 * @param {Record<string, string>} [bundledModules] - In-memory modules that should be bundled.
 * @returns {Promise<{ code: string }>} Bundled code output.
 */
export async function bundleCommonJs(entryCode, pluginOptions = {}, bundledModules = {}) {
  const bundle = await rollup.rollup({
    input: 'entry.cjs',
    plugins: [
      {
        name: 'virtual-fixtures',
        resolveId(id) {
          if (id === 'entry.cjs' || (bundledModules && bundledModules[id])) {
            return id;
          }
          // External dependency
          return { id, external: true };
        },
        load(id) {
          if (id === 'entry.cjs') {
            return entryCode;
          }
          if (bundledModules && bundledModules[id]) {
            return bundledModules[id];
          }
          return null;
        }
      },
      commonjs(pluginOptions)
    ]
  });

  const { output } = await bundle.generate({ format: 'es' });
  return { code: output[0].code };
}

import assert from 'node:assert/strict';
import { bundleCommonJs } from '../src/index.js';

async function runContract() {
  const singleExternalFixture = `
const ext = require('external-pkg');
module.exports = { ext };
`;

  // ---------------------------------------------------------------------------
  // Contract Assertion 1:
  // External require calls are assumed to be CommonJS by default (esmExternals: false),
  // rendering as default imports rather than namespace imports.
  // ---------------------------------------------------------------------------
  {
    const defaultResult = await bundleCommonJs(singleExternalFixture);
    assert.match(
      defaultResult.code,
      /import\s+([a-zA-Z0-9_$]+)\s+from\s+['"]external-pkg['"]/,
      'assert external require becomes default import by default'
    );
    assert.doesNotMatch(
      defaultResult.code,
      /import\s+\*\s+as\s+([a-zA-Z0-9_$]+)\s+from\s+['"]external-pkg['"]/,
      'assert no namespace import is created for external dependency by default'
    );

    const explicitFalseResult = await bundleCommonJs(singleExternalFixture, { esmExternals: false });
    assert.match(
      explicitFalseResult.code,
      /import\s+([a-zA-Z0-9_$]+)\s+from\s+['"]external-pkg['"]/,
      'assert explicit esmExternals: false behaves identically to default'
    );
    assert.doesNotMatch(
      explicitFalseResult.code,
      /import\s+\*\s+as\s+/,
      'assert explicit esmExternals: false does not generate namespace imports'
    );
  }

  // ---------------------------------------------------------------------------
  // Contract Assertion 2:
  // Setting esmExternals: true treats external required dependencies as ES modules,
  // rendering them as namespace imports with getAugmentedNamespace interop wrapper by default.
  // ---------------------------------------------------------------------------
  {
    const trueResult = await bundleCommonJs(singleExternalFixture, { esmExternals: true });
    assert.match(
      trueResult.code,
      /import\s+\*\s+as\s+([a-zA-Z0-9_$]+)\s+from\s+['"]external-pkg['"]/,
      'assert esmExternals: true renders external required dependency as namespace import'
    );
    assert.match(
      trueResult.code,
      /getAugmentedNamespace\(/,
      'assert getAugmentedNamespace interop wrapper is used by default when esmExternals is true'
    );
    assert.doesNotMatch(
      trueResult.code,
      /import\s+require\$\$\d+\s+from\s+['"]external-pkg['"]/,
      'assert default import is not used when esmExternals is true'
    );
  }

  // ---------------------------------------------------------------------------
  // Contract Assertion 3:
  // Setting esmExternals: true combined with requireReturnsDefault: 'namespace' directly
  // references external namespace imports without interop wrapper functions.
  // ---------------------------------------------------------------------------
  {
    const nsResult = await bundleCommonJs(singleExternalFixture, {
      esmExternals: true,
      requireReturnsDefault: 'namespace'
    });
    assert.match(
      nsResult.code,
      /import\s+\*\s+as\s+([a-zA-Z0-9_$]+)\s+from\s+['"]external-pkg['"]/,
      'assert namespace import is generated'
    );
    assert.doesNotMatch(
      nsResult.code,
      /getAugmentedNamespace/,
      'assert requireReturnsDefault: "namespace" eliminates interop helper'
    );
  }

  // ---------------------------------------------------------------------------
  // Contract Assertion 4:
  // Configuring esmExternals with an array or filter function selectively converts
  // specified modules to namespace imports while retaining default imports for other external dependencies.
  // ---------------------------------------------------------------------------
  {
    const multiExternalFixture = `
const a = require('ext-esm');
const b = require('ext-cjs');
module.exports = { a, b };
`;

    // Array filter: only 'ext-esm' is treated as ES module namespace import
    const arrayResult = await bundleCommonJs(multiExternalFixture, {
      esmExternals: ['ext-esm']
    });
    assert.match(
      arrayResult.code,
      /import\s+\*\s+as\s+([a-zA-Z0-9_$]+)\s+from\s+['"]ext-esm['"]/,
      'assert specified module in array is imported as namespace'
    );
    assert.match(
      arrayResult.code,
      /import\s+([a-zA-Z0-9_$]+)\s+from\s+['"]ext-cjs['"]/,
      'assert unspecified module in array is imported as default CJS import'
    );
    assert.doesNotMatch(
      arrayResult.code,
      /import\s+\*\s+as\s+([a-zA-Z0-9_$]+)\s+from\s+['"]ext-cjs['"]/,
      'assert unspecified module is not imported as namespace'
    );

    // Function filter: selectively treat 'ext-esm' as ES module
    const fnResult = await bundleCommonJs(multiExternalFixture, {
      esmExternals: (id) => id === 'ext-esm'
    });
    assert.match(
      fnResult.code,
      /import\s+\*\s+as\s+([a-zA-Z0-9_$]+)\s+from\s+['"]ext-esm['"]/,
      'assert matched module by function is imported as namespace'
    );
    assert.match(
      fnResult.code,
      /import\s+([a-zA-Z0-9_$]+)\s+from\s+['"]ext-cjs['"]/,
      'assert unmatched module by function is imported as default CJS import'
    );
  }

  console.log('All contract assertions passed successfully.');
}

runContract().catch((err) => {
  console.error('Contract failed:', err);
  process.exit(1);
});