esm: bypass CJS loader in default load under `--default-type=module`

This allows user to opt-out from using the monkey-patchable CJS loader,
even to load CJS modules.

PR-URL: https://github.com/nodejs/node/pull/50004
Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com>

doc/api/module.md

@@ -622,7 +622,8 @@ Omitting vs providing a `source` for `'commonjs'` has very different effects:
   registered hooks. This behavior for nullish `source` is temporary — in the
   future, nullish `source` will not be supported.
 
-The Node.js internal `load` implementation, which is the value of `next` for the
+When `node` is run with `--experimental-default-type=commonjs`, the Node.js
+internal `load` implementation, which is the value of `next` for the
 last hook in the `load` chain, returns `null` for `source` when `format` is
 `'commonjs'` for backward compatibility. Here is an example hook that would
 opt-in to using the non-default behavior:

lib/internal/modules/esm/load.js

@@ -18,6 +18,8 @@ const policy = getOptionValue('--experimental-policy') ?
   null;
 const experimentalNetworkImports =
   getOptionValue('--experimental-network-imports');
+const defaultType =
+  getOptionValue('--experimental-default-type');
 
 const { Buffer: { from: BufferFrom } } = require('buffer');
 
@@ -140,7 +142,7 @@ async function defaultLoad(url, context = kEmptyObject) {
     // Now that we have the source for the module, run `defaultGetFormat` again in case we detect ESM syntax.
     format ??= await defaultGetFormat(urlInstance, contextToPass);
 
-    if (format === 'commonjs' && contextToPass !== context) {
+    if (format === 'commonjs' && contextToPass !== context && defaultType !== 'module') {
       // For backward compatibility reasons, we need to discard the source in
       // order for the CJS loader to re-fetch it.
       source = null;

test/es-module/test-esm-type-flag-errors.mjs

@@ -1,31 +1,101 @@
 import { spawnPromisified } from '../common/index.mjs';
 import * as fixtures from '../common/fixtures.mjs';
 import { describe, it } from 'node:test';
-import { match, strictEqual } from 'node:assert';
+import { deepStrictEqual, match, strictEqual } from 'node:assert';
 
-describe('--experimental-default-type=module should not affect the interpretation of files with unknown extensions',
-         { concurrency: true }, () => {
-           it('should error on an entry point with an unknown extension', async () => {
-             const { code, signal, stdout, stderr } = await spawnPromisified(process.execPath, [
-               '--experimental-default-type=module',
-               fixtures.path('es-modules/package-type-module/extension.unknown'),
-             ]);
+describe('--experimental-default-type=module', { concurrency: true }, () => {
+  describe('should not affect the interpretation of files with unknown extensions', { concurrency: true }, () => {
+    it('should error on an entry point with an unknown extension', async () => {
+      const { code, signal, stdout, stderr } = await spawnPromisified(process.execPath, [
+        '--experimental-default-type=module',
+        fixtures.path('es-modules/package-type-module/extension.unknown'),
+      ]);
 
-             match(stderr, /ERR_UNKNOWN_FILE_EXTENSION/);
-             strictEqual(stdout, '');
-             strictEqual(code, 1);
-             strictEqual(signal, null);
-           });
+      match(stderr, /ERR_UNKNOWN_FILE_EXTENSION/);
+      strictEqual(stdout, '');
+      strictEqual(code, 1);
+      strictEqual(signal, null);
+    });
 
-           it('should error on an import with an unknown extension', async () => {
-             const { code, signal, stdout, stderr } = await spawnPromisified(process.execPath, [
-               '--experimental-default-type=module',
-               fixtures.path('es-modules/package-type-module/imports-unknownext.mjs'),
-             ]);
+    it('should error on an import with an unknown extension', async () => {
+      const { code, signal, stdout, stderr } = await spawnPromisified(process.execPath, [
+        '--experimental-default-type=module',
+        fixtures.path('es-modules/package-type-module/imports-unknownext.mjs'),
+      ]);
 
-             match(stderr, /ERR_UNKNOWN_FILE_EXTENSION/);
-             strictEqual(stdout, '');
-             strictEqual(code, 1);
-             strictEqual(signal, null);
-           });
-         });
+      match(stderr, /ERR_UNKNOWN_FILE_EXTENSION/);
+      strictEqual(stdout, '');
+      strictEqual(code, 1);
+      strictEqual(signal, null);
+    });
+  });
+
+  it('should affect CJS .js files (imported, required, entry points)', async () => {
+    const result = await spawnPromisified(process.execPath, [
+      '--experimental-default-type=module',
+      fixtures.path('es-modules/package-type-commonjs/echo-require-cache.js'),
+    ]);
+
+    deepStrictEqual(result, {
+      code: 0,
+      stderr: '',
+      stdout: 'undefined\n',
+      signal: null,
+    });
+  });
+
+  it('should affect .cjs files that are imported', async () => {
+    const result = await spawnPromisified(process.execPath, [
+      '--experimental-default-type=module',
+      '-e',
+      `import ${JSON.stringify(fixtures.fileURL('es-module-require-cache/echo.cjs'))}`,
+    ]);
+
+    deepStrictEqual(result, {
+      code: 0,
+      stderr: '',
+      stdout: 'undefined\n',
+      signal: null,
+    });
+  });
+
+  it('should affect entry point .cjs files (with no hooks)', async () => {
+    const { stderr, stdout, code } = await spawnPromisified(process.execPath, [
+      '--experimental-default-type=module',
+      fixtures.path('es-module-require-cache/echo.cjs'),
+    ]);
+
+    strictEqual(stderr, '');
+    match(stdout, /^undefined\n$/);
+    strictEqual(code, 0);
+  });
+
+  it('should affect entry point .cjs files (when any hooks is registered)', async () => {
+    const result = await spawnPromisified(process.execPath, [
+      '--experimental-default-type=module',
+      '--import',
+      'data:text/javascript,import{register}from"node:module";register("data:text/javascript,");',
+      fixtures.path('es-module-require-cache/echo.cjs'),
+    ]);
+
+    deepStrictEqual(result, {
+      code: 0,
+      stderr: '',
+      stdout: 'undefined\n',
+      signal: null,
+    });
+  });
+
+  it('should not affect CJS from input-type', async () => {
+    const { stderr, stdout, code } = await spawnPromisified(process.execPath, [
+      '--experimental-default-type=module',
+      '--input-type=commonjs',
+      '-p',
+      'require.cache',
+    ]);
+
+    strictEqual(stderr, '');
+    match(stdout, /^\[Object: null prototype\] \{\}\n$/);
+    strictEqual(code, 0);
+  });
+});

test/fixtures/es-module-require-cache/echo.cjs

@@ -0,0 +1 @@
+console.log(require.cache);

test/fixtures/es-modules/package-type-commonjs/echo-require-cache.js

@@ -0,0 +1 @@
+console.log(require.cache);