module: support __cjsUnwrapDefault interop flag in require(esm)

Author: guybedford

doc/api/modules.md

@@ -1416,10 +1416,13 @@ function loadESMFromCJS(mod, filename) {
     // createRequiredModuleFacade() to `wrap` which is a ModuleWrap wrapping
     // over the original module.
 
-    // We don't do this to modules that don't have default exports to avoid
-    // the unnecessary overhead. If __esModule is already defined, we will
-    // also skip the extension to allow users to override it.
-    if (!ObjectHasOwn(namespace, 'default') || ObjectHasOwn(namespace, '__esModule')) {
+    // We don't do this to modules that are marked as CJS ESM or that
+    // don't have default exports to avoid the unnecessary overhead.
+    // If __esModule is already defined, we will also skip the extension
+    // to allow users to override it.
+    if (namespace.__cjsUnwrapDefault) {
+      mod.exports = namespace.default;
+    } else if (!ObjectHasOwn(namespace, 'default') || ObjectHasOwn(namespace, '__esModule')) {
       mod.exports = namespace;
     } else {
       mod.exports = createRequiredModuleFacade(wrap);

lib/internal/modules/cjs/loader.js

@@ -0,0 +1,51 @@
+// Flags: --experimental-require-module
+import { mustCall } from '../common/index.mjs';
+import assert from 'assert';
+import { directRequireFixture, importFixture } from '../fixtures/pkgexports.mjs';
+
+const tests = {
+  'string': 'cjs',
+  'object': { a: 'cjs a', b: 'cjs b' },
+  'fauxesmdefault': { default: 'faux esm default' },
+  'fauxesmmixed': { default: 'faux esm default', a: 'faux esm a', b: 'faux esm b' },
+  'fauxesmnamed': { a: 'faux esm a', b: 'faux esm b' }
+};
+
+// This test demonstrates interop between CJS and CJS represented as ESM
+// under the new `export const __cjsModule = true` pattern, for the above cases.
+for (const [test, exactShape] of Object.entries(tests)) {
+  // Each case represents a CJS dependency, which has the expected shape in CJS:
+  assert.deepStrictEqual(directRequireFixture(`interop-cjsdep-${test}`), exactShape);
+
+  // Each dependency is reexported through CJS as if it is a library being consumed,
+  // which in CJS is fully shape-preserving:
+  assert.deepStrictEqual(directRequireFixture(`interop-cjs/${test}`), exactShape);
+
+  // Now we have ESM conversions of these dependencies, using __cjsModule = true,
+  // staring with the conversion of those dependencies into ESM under require(esm):
+  assert.deepStrictEqual(directRequireFixture(`interop-cjsdep-${test}-esm`), exactShape);
+
+  // When importing these ESM conversions, from require(esm), we should preserve the shape:
+  assert.deepStrictEqual(directRequireFixture(`interop-cjs/${test}-esm`), exactShape);
+
+  // Now if the importer itself is converted into ESM, it should still be able to load the original
+  // CJS and reexport it, preserving the shape:
+  assert.deepStrictEqual(directRequireFixture(`interop-cjs-esm/${test}`), exactShape);
+
+  // And then if we have the converted CJS to ESM importing from converted CJS to ESM,
+  // that should also work:
+  assert.deepStrictEqual(directRequireFixture(`interop-cjs-esm/${test}-esm`), exactShape);
+
+  // Finally, the CJS ESM representation under `import()` should match all these cases equivalently,
+  // where the CJS module is exported as the default export:
+  const esmCjsImport = await importFixture(`interop-cjsdep-${test}`);
+  assert.deepStrictEqual(esmCjsImport.default, exactShape);
+
+  assert.deepStrictEqual((await importFixture(`interop-cjsdep-${test}`)).default, exactShape);
+  assert.deepStrictEqual((await importFixture(`interop-cjs/${test}`)).default, exactShape);
+  assert.deepStrictEqual((await importFixture(`interop-cjsdep-${test}-esm`)).default, exactShape);
+  assert.deepStrictEqual((await importFixture(`interop-cjs/${test}-esm`)).default, exactShape);
+  assert.deepStrictEqual((await importFixture(`interop-cjs-esm/${test}`)).default, exactShape);
+  assert.deepStrictEqual((await importFixture(`interop-cjs-esm/${test}-esm`)).default, exactShape);
+
+}