loader, docs, test: named exports from commonjs modules

doc/api/esm.md

@@ -83,12 +83,19 @@ 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 `@@esModuleInterop` or `__esModule` to use named exports, representing
+each enumerable key of `module.exports` at the time they finish evaluating.
+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 `@@esModuleInterop`.
 
 ```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 +104,15 @@ fs.readFile('./foo.txt', (err, body) => {
 });
 ```
 
+```js
+// main.mjs
+import { part } from './other.js';
+
+// other.js
+exports.part = () => {};
+exports[Symbol.for('esModuleInterop')] = 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 esModuleInterop = Symbol.for('esModuleInterop');
+
 const realpathCache = new Map();
 
 const loaders = new Map();
@@ -36,21 +38,32 @@ 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[esModuleInterop] || exports.__esModule);
+  const keys = es ? Object.keys(exports) : ['default'];
+  return createDynamicModule(keys, url, (reflect) => {
+    if (es) {
+      for (let i = 0; i < keys.length; i++)
+        reflect.exports[keys[i]].set(exports[keys[i]]);
+    } 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 (let i = 0; i < keys.length; i++)
+      reflect.exports[keys[i]].set(exports[keys[i]]);
   });
 });
 

test/es-module/test-esm-cjs-esmodule.mjs

@@ -0,0 +1,9 @@
+// Flags: --experimental-modules
+/* eslint-disable required-modules */
+
+import assert from 'assert';
+import eightyfour, { named as fourtytwo } from
+  '../fixtures/es-module-loaders/babel-to-esm.js';
+
+assert.strictEqual(eightyfour, 84);
+assert.strictEqual(fourtytwo, 42);

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/babel-to-esm.js

@@ -0,0 +1,15 @@
+"use strict";
+
+/*
+created by babel with es2015 preset
+```
+export const named = 42;
+export default 84;
+```
+*/
+
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+var named = exports.named = 42;
+exports.default = 84;

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

@@ -0,0 +1,4 @@
+exports.named = true;
+exports.default = 1;
+
+exports[Symbol.for('esModuleInterop')] = true;

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

@@ -0,0 +1,6 @@
+module.exports = {
+  enum: 'enum',
+  class: 'class',
+  delete: 'delete',
+  [Symbol.for('esModuleInterop')]: true,
+};