@rollup/plugin-commonjs 29.0.3: Importing the default export of a CommonJS module with exports.default returns the entire module.exports object unless exports.__esModule is set or defaultIsModuleExports is configured.

# Rollup CommonJS Plugin Default Export Interop (`defaultIsModuleExports`)

## Prior Findings
`search_known_solution` returned existing samples covering `transformMixedEsModules` (`sha256:7d74520bd3cbe8b4027fdaefc8257db0bff3f5ebbd8644dd0d7d9f76b692bcdb`), external module interop via `output.interop: 'auto'` (`sha256:6bd9e4aeb00bbe86cb0245f6c6955ae8ec7421dba24da3b8401f6307646e0ad8`), and Vite SSR externalization (`sha256:3d6913c88e87048b4b55cebf64b6e0bd0e5c2fa4e748e8f754ff20cd2baf86bd`). This sample investigates a different trap: how `@rollup/plugin-commonjs` resolves default imports from CommonJS modules that define `exports.default` without `__esModule: true`.

## Failure Mode
It fails silently with a green build during bundling, but throws `TypeError: fn is not a function` at runtime when calling the imported default export because `@rollup/plugin-commonjs` provides the full `module.exports` object rather than unwrapping `exports.default`.

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}

{
  "name": "npm-rollup-plugin-commonjs-d1",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "type": "commonjs",
  "dependencies": {
    "@rollup/plugin-commonjs": "^29.0.3",
    "rollup": "^4.62.4"
  }
}

import assert from 'node:assert/strict';
import * as rollup from 'rollup';
import commonjs from '@rollup/plugin-commonjs';

async function bundleAndEvaluate(inputCode, cjsModules, pluginOptions = {}) {
  const bundle = await rollup.rollup({
    input: 'entry.js',
    plugins: [
      {
        name: 'virtual-fixtures',
        resolveId(id) {
          if (id === 'entry.js' || cjsModules[id]) {
            return id;
          }
          return null;
        },
        load(id) {
          if (id === 'entry.js') {
            return inputCode;
          }
          if (cjsModules[id]) {
            return cjsModules[id];
          }
          return null;
        }
      },
      commonjs(pluginOptions)
    ]
  });

  const { output } = await bundle.generate({ format: 'esm' });
  const base64Uri = 'data:text/javascript;base64,' + Buffer.from(output[0].code).toString('base64');
  return await import(base64Uri);
}

const cjsFixtures = {
  './plain-cjs.cjs': `
    exports.default = function getPlain() { return 'plain-result'; };
    exports.VERSION = '1.0.0';
  `,
  './transpiled-cjs.cjs': `
    exports.__esModule = true;
    exports.default = function getTranspiled() { return 'transpiled-result'; };
    exports.VERSION = '2.0.0';
  `
};

// 1. Default setting (defaultIsModuleExports: 'auto'): Plain CJS without __esModule: true resolves to module.exports
{
  const result = await bundleAndEvaluate(
    `import plainDef, { VERSION } from './plain-cjs.cjs';
     export { plainDef, VERSION };`,
    cjsFixtures
  );

  // The naive belief is that plainDef is the function assigned to exports.default.
  // Instead, plainDef is the entire module.exports object { default: [Function], VERSION: '1.0.0' }.
  assert.equal(typeof result.plainDef, 'object', 'plainDef is an object (module.exports), not the default function');
  assert.equal(typeof result.plainDef.default, 'function', 'exports.default is a property on the default export object');
  assert.equal(result.plainDef.VERSION, '1.0.0');
  assert.equal(result.plainDef.default(), 'plain-result');
  assert.throws(
    () => result.plainDef(),
    { name: 'TypeError' },
    'Invoking plainDef() directly throws TypeError because it is the exports wrapper object'
  );
}

// 2. Default setting (defaultIsModuleExports: 'auto'): CJS with exports.__esModule = true resolves to exports.default
{
  const result = await bundleAndEvaluate(
    `import transpiledDef, { VERSION } from './transpiled-cjs.cjs';
     export { transpiledDef, VERSION };`,
    cjsFixtures
  );

  assert.equal(typeof result.transpiledDef, 'function', 'transpiledDef is unwrapped directly to the function');
  assert.equal(result.transpiledDef(), 'transpiled-result');
  assert.equal(result.VERSION, '2.0.0');
}

// 3. Explicit defaultIsModuleExports: false forces unwrapping exports.default even without __esModule marker
{
  const result = await bundleAndEvaluate(
    `import plainDef, { VERSION } from './plain-cjs.cjs';
     export { plainDef, VERSION };`,
    cjsFixtures,
    { defaultIsModuleExports: false }
  );

  assert.equal(typeof result.plainDef, 'function', 'plainDef is unwrapped to exports.default when defaultIsModuleExports is false');
  assert.equal(result.plainDef(), 'plain-result');
  assert.equal(result.VERSION, '1.0.0');
}

// 4. Explicit defaultIsModuleExports: true forces returning module.exports even when __esModule is true
{
  const result = await bundleAndEvaluate(
    `import transpiledDef, { VERSION } from './transpiled-cjs.cjs';
     export { transpiledDef, VERSION };`,
    cjsFixtures,
    { defaultIsModuleExports: true }
  );

  assert.equal(typeof result.transpiledDef, 'object', 'transpiledDef remains module.exports object when defaultIsModuleExports is true');
  assert.equal(result.transpiledDef.__esModule, true);
  assert.equal(typeof result.transpiledDef.default, 'function');
  assert.throws(
    () => result.transpiledDef(),
    { name: 'TypeError' },
    'Invoking transpiledDef() throws TypeError under defaultIsModuleExports: true'
  );
}

console.log('Contract assertions passed successfully.');