loader, docs, test: named exports from commonjs modules

doc/api/esm.md

@@ -83,12 +83,20 @@ All CommonJS, JSON, and C++ modules can be used with `import`.
 Modules loaded this way will only be loaded once, even if their query
 or fragment string differs between `import` statements.
 
-When loaded via `import` these modules will provide a single `default` export
-representing the value of `module.exports` at the time they finished evaluating.
+CommonJS modules, when imported, will be handled in one of two ways. By default
+they will provide a single `default` export representing the value of
+`module.exports` at the time they finish evaluating. However, they may also
+provide `@@esModule` or `__esModule` to use named exports, representing each
+enumerable key of `module.exports` at the time they finish evaluating.
+`@@esModule` is available as `require('module').esModule` and prefered over
+`__esModule` In both cases, this should be thought of  like a "snapshot" of
+the exports at the time of importing; asynchronously modifying `module.exports`
+will not affect the values of the exports. Builtin libraries are provided with
+named exports as if they were using `@@esModule`.
 
 ```js
-import fs from 'fs';
-fs.readFile('./foo.txt', (err, body) => {
+import { readFile } from 'fs';
+readFile('./foo.txt', (err, body) => {
   if (err) {
     console.error(err);
   } else {
@@ -97,6 +105,16 @@ fs.readFile('./foo.txt', (err, body) => {
 });
 ```
 
+```js
+// main.mjs
+import { part } from './other.js';
+
+// other.js
+import { esModule } from 'module';
+exports.part = () => {};
+exports[esModule] = true;
+```
+
 ## Loader hooks
 
 <!-- type=misc -->

lib/internal/loader/ModuleRequest.js

@@ -19,6 +19,8 @@ const search = require('internal/loader/search');
 const asyncReadFile = require('util').promisify(require('fs').readFile);
 const debug = require('util').debuglog('esm');
 
+const esModuleSymbol = exports.esModuleSymbol = Symbol('esModule');
+
 const realpathCache = new Map();
 
 const loaders = new Map();
@@ -36,21 +38,31 @@ loaders.set('esm', async (url) => {
 
 // Strategy for loading a node-style CommonJS module
 loaders.set('cjs', async (url) => {
-  return createDynamicModule(['default'], url, (reflect) => {
-    debug(`Loading CJSModule ${url}`);
-    const CJSModule = require('module');
-    const pathname = internalURLModule.getPathFromURL(new URL(url));
-    CJSModule._load(pathname);
+  debug(`Loading CJSModule ${url}`);
+  const CJSModule = require('module');
+  const pathname = internalURLModule.getPathFromURL(new URL(url));
+  const exports = CJSModule._load(pathname);
+  const es = Boolean(exports[esModuleSymbol] || exports.__esModule);
+  const keys = es ? Object.keys(exports) : ['default'];
+  return createDynamicModule(keys, url, (reflect) => {
+    if (es) {
+      for (const key of keys)
+        reflect.exports[key].set(exports[key]);
+    } else {
+      reflect.exports.default.set(exports);
+    }
   });
 });
 
 // Strategy for loading a node builtin CommonJS module that isn't
 // through normal resolution
 loaders.set('builtin', async (url) => {
-  return createDynamicModule(['default'], url, (reflect) => {
-    debug(`Loading BuiltinModule ${url}`);
-    const exports = NativeModule.require(url.substr(5));
+  debug(`Loading BuiltinModule ${url}`);
+  const exports = NativeModule.require(url.substr(5));
+  const keys = Object.keys(exports);
+  return createDynamicModule(['default', ...keys], url, (reflect) => {
     reflect.exports.default.set(exports);
+    for (const key of keys) reflect.exports[key].set(exports[key]);
   });
 });
 

lib/module.js

@@ -42,6 +42,7 @@ const errors = require('internal/errors');
 const Loader = require('internal/loader/Loader');
 const ModuleJob = require('internal/loader/ModuleJob');
 const { createDynamicModule } = require('internal/loader/ModuleWrap');
+const { esModuleSymbol } = require('internal/loader/ModuleRequest');
 let ESMLoader;
 
 function stat(filename) {
@@ -79,6 +80,7 @@ Module._pathCache = Object.create(null);
 Module._extensions = Object.create(null);
 var modulePaths = [];
 Module.globalPaths = [];
+Module.esModule = esModuleSymbol;
 
 Module.wrap = function(script) {
   return Module.wrapper[0] + script + Module.wrapper[1];

test/es-module/test-esm-namespace.mjs

@@ -1,7 +1,14 @@
 // Flags: --experimental-modules
 /* eslint-disable required-modules */
 
-import * as fs from 'fs';
 import assert from 'assert';
+import fs, { readFile } from 'fs';
+import main, { named } from
+  '../fixtures/es-module-loaders/cjs-to-es-namespace.js';
 
-assert.deepStrictEqual(Object.keys(fs), ['default']);
+assert(fs);
+assert(fs.readFile);
+assert.strictEqual(fs.readFile, readFile);
+
+assert.strictEqual(main, 1);
+assert.strictEqual(named, true);

test/es-module/test-reserved-keywords.mjs

@@ -0,0 +1,8 @@
+// Flags: --experimental-modules
+/* eslint-disable required-modules */
+
+import assert from 'assert';
+import { enum as e } from
+  '../fixtures/es-module-loaders/reserved-keywords.js';
+
+assert(e);

test/fixtures/es-module-loaders/cjs-to-es-namespace.js

@@ -0,0 +1,6 @@
+const { esModule } = require('module');
+
+exports.named = true;
+exports.default = 1;
+
+exports[esModule] = true;

test/fixtures/es-module-loaders/reserved-keywords.js

@@ -0,0 +1,8 @@
+const { esModule } = require('module');
+
+module.exports = {
+  enum: 'enum',
+  class: 'class',
+  delete: 'delete',
+  [esModule]: true,
+};