module: add __esModule to require()'d ESM

Tooling in the ecosystem have been using the __esModule property to
recognize transpiled ESM in consuming code. For example, a 'log'
package written in ESM:

export function log(val) { console.log(val); }

Can be transpiled as:

exports.__esModule = true;
exports.default = function log(val) { console.log(val); }

The consuming code may be written like this in ESM:

import log from 'log'

Which gets transpiled to:

const _mod = require('log');
const log = _mod.__esModule ? _mod.default : _mod;

So to allow transpiled consuming code to recognize require()'d real ESM
as ESM and pick up the default exports, we add a __esModule property by
building a source text module facade for any module that has a default
export and add .__esModule = true to the exports. We don't do this to
modules that don't have default exports to avoid the unnecessary
overhead. This maintains the enumerability of the re-exported names
and the live binding of the exports.

The source of the facade is defined as a constant per-isolate property
required_module_facade_source_string, which looks like this

export * from 'original';
export { default } from 'original';
export const __esModule = true;

And the 'original' module request is always resolved by
createRequiredModuleFacade() to wrap which is a ModuleWrap wrapping
over the original module.

PR-URL: nodejs#52166
Refs: nodejs#52134
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Guy Bedford <guybedford@gmail.com>
Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com>

doc/api/modules.md

@@ -188,33 +188,51 @@ loaded by `require()` meets the following requirements:
      `"type": "commonjs"`, and `--experimental-detect-module` is enabled.
 
 `require()` will load the requested module as an ES Module, and return
-the module name space object. In this case it is similar to dynamic
+the module namespace object. In this case it is similar to dynamic
 `import()` but is run synchronously and returns the name space object
 directly.
 
+With the following ES Modules:
+
 ```mjs
-// point.mjs
+// distance.mjs
 export function distance(a, b) { return (b.x - a.x) ** 2 + (b.y - a.y) ** 2; }
+```
+
+```mjs
+// point.mjs
 class Point {
   constructor(x, y) { this.x = x; this.y = y; }
 }
 export default Point;
 ```
 
+A CommonJS module can load them with `require()` under `--experimental-detect-module`:
+
 ```cjs
-const required = require('./point.mjs');
+const distance = require('./distance.mjs');
+console.log(distance);
 // [Module: null prototype] {
-//   default: [class Point],
 //   distance: [Function: distance]
 // }
-console.log(required);
 
-(async () => {
-  const imported = await import('./point.mjs');
-  console.log(imported === required);  // true
-})();
+const point = require('./point.mjs');
+console.log(point);
+// [Module: null prototype] {
+//   default: [class Point],
+//   __esModule: true,
+// }
 ```
 
+For interoperability with existing tools that convert ES Modules into CommonJS,
+which could then load real ES Modules through `require()`, the returned namespace
+would contain a `__esModule: true` property if it has a `default` export so that
+consuming code generated by tools can recognize the default exports in real
+ES Modules. If the namespace already defines `__esModule`, this would not be added.
+This property is experimental and can change in the future. It should only be used
+by tools converting ES modules into CommonJS modules, following existing ecosystem
+conventions. Code authored directly in CommonJS should avoid depending on it.
+
 If the module being `require()`'d contains top-level `await`, or the module
 graph it `import`s contains top-level `await`,
 [`ERR_REQUIRE_ASYNC_MODULE`][] will be thrown. In this case, users should

lib/internal/modules/cjs/loader.js

@@ -41,6 +41,7 @@ const {
   ObjectFreeze,
   ObjectGetOwnPropertyDescriptor,
   ObjectGetPrototypeOf,
+  ObjectHasOwn,
   ObjectKeys,
   ObjectPrototype,
   ObjectPrototypeHasOwnProperty,
@@ -71,7 +72,7 @@ const {
   },
 } = internalBinding('util');
 
-const { kEvaluated } = internalBinding('module_wrap');
+const { kEvaluated, createRequiredModuleFacade } = internalBinding('module_wrap');
 
 // Internal properties for Module instances.
 /**
@@ -1333,9 +1334,55 @@ function loadESMFromCJS(mod, filename) {
     // ESM won't be accessible via process.mainModule.
     setOwnProperty(process, 'mainModule', undefined);
   } else {
-    // TODO(joyeecheung): we may want to invent optional special handling for default exports here.
-    // For now, it's good enough to be identical to what `import()` returns.
-    mod.exports = cascadedLoader.importSyncForRequire(mod, filename, source, isMain, mod[kModuleParent]);
+    const {
+      wrap,
+      namespace,
+    } = cascadedLoader.importSyncForRequire(mod, filename, source, isMain, mod[kModuleParent]);
+    // Tooling in the ecosystem have been using the __esModule property to recognize
+    // transpiled ESM in consuming code. For example, a 'log' package written in ESM:
+    //
+    // export default function log(val) { console.log(val); }
+    //
+    // Can be transpiled as:
+    //
+    // exports.__esModule = true;
+    // exports.default = function log(val) { console.log(val); }
+    //
+    // The consuming code may be written like this in ESM:
+    //
+    // import log from 'log'
+    //
+    // Which gets transpiled to:
+    //
+    // const _mod = require('log');
+    // const log = _mod.__esModule ? _mod.default : _mod;
+    //
+    // So to allow transpiled consuming code to recognize require()'d real ESM
+    // as ESM and pick up the default exports, we add a __esModule property by
+    // building a source text module facade for any module that has a default
+    // export and add .__esModule = true to the exports. This maintains the
+    // enumerability of the re-exported names and the live binding of the exports,
+    // without incurring a non-trivial per-access overhead on the exports.
+    //
+    // The source of the facade is defined as a constant per-isolate property
+    // required_module_default_facade_source_string, which looks like this
+    //
+    // export * from 'original';
+    // export { default } from 'original';
+    // export const __esModule = true;
+    //
+    // And the 'original' module request is always resolved by
+    // 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')) {
+      mod.exports = namespace;
+    } else {
+      mod.exports = createRequiredModuleFacade(wrap);
+    }
   }
 }
 

lib/internal/modules/esm/loader.js

@@ -280,7 +280,7 @@ class ModuleLoader {
    * @param {string} source Source code. TODO(joyeecheung): pass the raw buffer.
    * @param {string} isMain Whether this module is a main module.
    * @param {CJSModule|undefined} parent Parent module, if any.
-   * @returns {{ModuleWrap}}
+   * @returns {{wrap: ModuleWrap, namespace: ModuleNamespaceObject}}
    */
   importSyncForRequire(mod, filename, source, isMain, parent) {
     const url = pathToFileURL(filename).href;
@@ -305,7 +305,7 @@ class ModuleLoader {
         }
         throw new ERR_REQUIRE_CYCLE_MODULE(message);
       }
-      return job.module.getNamespaceSync();
+      return { wrap: job.module, namespace: job.module.getNamespaceSync() };
     }
     // TODO(joyeecheung): refactor this so that we pre-parse in C++ and hit the
     // cache here, or use a carrier object to carry the compiled module script
@@ -317,7 +317,7 @@ class ModuleLoader {
     job = new ModuleJobSync(this, url, kEmptyObject, wrap, isMain, inspectBrk);
     this.loadCache.set(url, kImplicitTypeAttribute, job);
     mod[kRequiredModuleSymbol] = job.module;
-    return job.runSync().namespace;
+    return { wrap: job.module, namespace: job.runSync().namespace };
   }
 
   /**

src/env.h

@@ -1057,6 +1057,8 @@ class Environment : public MemoryRetainer {
   std::vector<std::string> supported_hash_algorithms;
 #endif  // HAVE_OPENSSL
 
+  v8::Global<v8::Module> temporary_required_module_facade_original;
+
  private:
   inline void ThrowError(v8::Local<v8::Value> (*fun)(v8::Local<v8::String>,
                                                      v8::Local<v8::Value>),

src/env_properties.h

@@ -256,6 +256,7 @@
   V(openssl_error_stack, "opensslErrorStack")                                  \
   V(options_string, "options")                                                 \
   V(order_string, "order")                                                     \
+  V(original_string, "original")                                               \
   V(output_string, "output")                                                   \
   V(overlapped_string, "overlapped")                                           \
   V(parse_error_string, "Parse Error")                                         \
@@ -289,6 +290,11 @@
   V(regexp_string, "regexp")                                                   \
   V(rename_string, "rename")                                                   \
   V(replacement_string, "replacement")                                         \
+  V(required_module_facade_url_string,                                         \
+    "node:internal/require_module_default_facade")                             \
+  V(required_module_facade_source_string,                                      \
+    "export * from 'original'; export { default } from 'original'; export "    \
+    "const __esModule = true;")                                                \
   V(require_string, "require")                                                 \
   V(resource_string, "resource")                                               \
   V(retry_string, "retry")                                                     \

src/module_wrap.cc

@@ -1019,6 +1019,69 @@ void ModuleWrap::CreateCachedData(const FunctionCallbackInfo<Value>& args) {
   }
 }
 
+// This v8::Module::ResolveModuleCallback simply links `import 'original'`
+// to the env->temporary_required_module_facade_original() which is stashed
+// right before this callback is called and will be restored as soon as
+// v8::Module::Instantiate() returns.
+MaybeLocal<Module> LinkRequireFacadeWithOriginal(
+    Local<Context> context,
+    Local<String> specifier,
+    Local<FixedArray> import_attributes,
+    Local<Module> referrer) {
+  Environment* env = Environment::GetCurrent(context);
+  Isolate* isolate = context->GetIsolate();
+  CHECK(specifier->Equals(context, env->original_string()).ToChecked());
+  CHECK(!env->temporary_required_module_facade_original.IsEmpty());
+  return env->temporary_required_module_facade_original.Get(isolate);
+}
+
+// Wraps an existing source text module with a facade that adds
+// .__esModule = true to the exports.
+// See env->required_module_facade_source_string() for the source.
+void ModuleWrap::CreateRequiredModuleFacade(
+    const FunctionCallbackInfo<Value>& args) {
+  Isolate* isolate = args.GetIsolate();
+  Local<Context> context = isolate->GetCurrentContext();
+  Environment* env = Environment::GetCurrent(context);
+  CHECK(args[0]->IsObject());  // original module
+  Local<Object> wrap = args[0].As<Object>();
+  ModuleWrap* original;
+  ASSIGN_OR_RETURN_UNWRAP(&original, wrap);
+
+  // Use the same facade source and URL to hit the compilation cache.
+  ScriptOrigin origin(env->required_module_facade_url_string(),
+                      0,               // line offset
+                      0,               // column offset
+                      true,            // is cross origin
+                      -1,              // script id
+                      Local<Value>(),  // source map URL
+                      false,           // is opaque (?)
+                      false,           // is WASM
+                      true);           // is ES Module
+  ScriptCompiler::Source source(env->required_module_facade_source_string(),
+                                origin);
+
+  // The module facade instantiation simply links `import 'original'` in the
+  // facade with the original module and should never fail.
+  Local<Module> facade =
+      ScriptCompiler::CompileModule(isolate, &source).ToLocalChecked();
+  // Stash the original module in temporary_required_module_facade_original
+  // for the LinkRequireFacadeWithOriginal() callback to pick it up.
+  CHECK(env->temporary_required_module_facade_original.IsEmpty());
+  env->temporary_required_module_facade_original.Reset(
+      isolate, original->module_.Get(isolate));
+  CHECK(facade->InstantiateModule(context, LinkRequireFacadeWithOriginal)
+            .IsJust());
+  env->temporary_required_module_facade_original.Reset();
+
+  // The evaluation of the facade is synchronous.
+  Local<Value> evaluated = facade->Evaluate(context).ToLocalChecked();
+  CHECK(evaluated->IsPromise());
+  CHECK_EQ(evaluated.As<Promise>()->State(), Promise::PromiseState::kFulfilled);
+
+  args.GetReturnValue().Set(facade->GetModuleNamespace());
+}
+
 void ModuleWrap::CreatePerIsolateProperties(IsolateData* isolate_data,
                                             Local<ObjectTemplate> target) {
   Isolate* isolate = isolate_data->isolate();
@@ -1051,6 +1114,10 @@ void ModuleWrap::CreatePerIsolateProperties(IsolateData* isolate_data,
             target,
             "setInitializeImportMetaObjectCallback",
             SetInitializeImportMetaObjectCallback);
+  SetMethod(isolate,
+            target,
+            "createRequiredModuleFacade",
+            CreateRequiredModuleFacade);
 }
 
 void ModuleWrap::CreatePerContextProperties(Local<Object> target,
@@ -1091,6 +1158,8 @@ void ModuleWrap::RegisterExternalReferences(
   registry->Register(GetStatus);
   registry->Register(GetError);
 
+  registry->Register(CreateRequiredModuleFacade);
+
   registry->Register(SetImportModuleDynamicallyCallback);
   registry->Register(SetInitializeImportMetaObjectCallback);
 }

src/module_wrap.h

@@ -88,6 +88,9 @@ class ModuleWrap : public BaseObject {
       std::optional<v8::ScriptCompiler::CachedData*> user_cached_data,
       bool* cache_rejected);
 
+  static void CreateRequiredModuleFacade(
+      const v8::FunctionCallbackInfo<v8::Value>& args);
+
  private:
   ModuleWrap(Realm* realm,
              v8::Local<v8::Object> object,

test/common/index.js

@@ -964,9 +964,14 @@ function getPrintedStackTrace(stderr) {
  * @param {object} mod result returned by require()
  * @param {object} expectation shape of expected namespace.
  */
-function expectRequiredModule(mod, expectation) {
+function expectRequiredModule(mod, expectation, checkESModule = true) {
+  const clone = { ...mod };
+  if (Object.hasOwn(mod, 'default') && checkESModule) {
+    assert.strictEqual(mod.__esModule, true);
+    delete clone.__esModule;
+  }
   assert(isModuleNamespaceObject(mod));
-  assert.deepStrictEqual({ ...mod }, { ...expectation });
+  assert.deepStrictEqual(clone, { ...expectation });
 }
 
 const common = {

test/es-module/test-require-module-default-extension.js

@@ -1,13 +1,11 @@
 // Flags: --experimental-require-module
 'use strict';
 
-require('../common');
+const { expectRequiredModule } = require('../common');
 const assert = require('assert');
-const { isModuleNamespaceObject } = require('util/types');
 
 const mod = require('../fixtures/es-modules/package-default-extension/index.mjs');
-assert.deepStrictEqual({ ...mod }, { entry: 'mjs' });
-assert(isModuleNamespaceObject(mod));
+expectRequiredModule(mod, { entry: 'mjs' });
 
 assert.throws(() => {
   const mod = require('../fixtures/es-modules/package-default-extension');

test/es-module/test-require-module-defined-esmodule.js

@@ -0,0 +1,23 @@
+// Flags: --experimental-require-module
+'use strict';
+const common = require('../common');
+
+// If an ESM already defines __esModule to be something else,
+// require(esm) should allow the user override.
+{
+  const mod = require('../fixtures/es-modules/export-es-module.mjs');
+  common.expectRequiredModule(
+    mod,
+    { default: { hello: 'world' }, __esModule: 'test' },
+    false,
+  );
+}
+
+{
+  const mod = require('../fixtures/es-modules/export-es-module-2.mjs');
+  common.expectRequiredModule(
+    mod,
+    { default: { hello: 'world' }, __esModule: false },
+    false,
+  );
+}

test/es-module/test-require-module-dynamic-import-1.js

@@ -21,8 +21,7 @@ const { pathToFileURL } = require('url');
     const url = pathToFileURL(path.resolve(__dirname, id));
     const imported = await import(url);
     const required = require(id);
-    assert.strictEqual(imported, required,
-                       `import()'ed and require()'ed result of ${id} was not reference equal`);
+    common.expectRequiredModule(required, imported);
   }
 
   const id = '../fixtures/es-modules/data-import.mjs';

test/es-module/test-require-module-dynamic-import-2.js

@@ -21,8 +21,7 @@ const path = require('path');
     const url = pathToFileURL(path.resolve(__dirname, id));
     const required = require(id);
     const imported = await import(url);
-    assert.strictEqual(imported, required,
-                       `import()'ed and require()'ed result of ${id} was not reference equal`);
+    common.expectRequiredModule(required, imported);
   }
 
   const id = '../fixtures/es-modules/data-import.mjs';

test/es-module/test-require-module-dynamic-import-3.js

@@ -5,10 +5,9 @@
 // be loaded by dynamic import().
 
 const common = require('../common');
-const assert = require('assert');
 
 (async () => {
   const required = require('../fixtures/es-modules/require-and-import/load.cjs');
   const imported = await import('../fixtures/es-modules/require-and-import/load.mjs');
-  assert.deepStrictEqual({ ...required }, { ...imported });
+  common.expectRequiredModule(required, imported);
 })().then(common.mustCall());

test/es-module/test-require-module-dynamic-import-4.js

@@ -5,10 +5,9 @@
 // be loaded by require().
 
 const common = require('../common');
-const assert = require('assert');
 
 (async () => {
   const imported = await import('../fixtures/es-modules/require-and-import/load.mjs');
   const required = require('../fixtures/es-modules/require-and-import/load.cjs');
-  assert.deepStrictEqual({ ...required }, { ...imported });
+  common.expectRequiredModule(required, imported);
 })().then(common.mustCall());