From 887ee9be7ac74eb1e6481231b88ab972f34be454 Mon Sep 17 00:00:00 2001
From: LiviaMedeiros <livia@cirno.name>
Date: Thu, 7 Sep 2023 20:19:45 +0800
Subject: [PATCH 1/8] module: support ES modules without file extension within
 `module` scope

---
 lib/internal/errors.js                 |  8 +-------
 lib/internal/modules/esm/get_format.js | 20 ++++----------------
 2 files changed, 5 insertions(+), 23 deletions(-)

diff --git a/lib/internal/errors.js b/lib/internal/errors.js
index bed02da76a3bab..6ac77686569cbe 100644
--- a/lib/internal/errors.js
+++ b/lib/internal/errors.js
@@ -1690,13 +1690,7 @@ E('ERR_UNHANDLED_ERROR',
 E('ERR_UNKNOWN_BUILTIN_MODULE', 'No such built-in module: %s', Error);
 E('ERR_UNKNOWN_CREDENTIAL', '%s identifier does not exist: %s', Error);
 E('ERR_UNKNOWN_ENCODING', 'Unknown encoding: %s', TypeError);
-E('ERR_UNKNOWN_FILE_EXTENSION', (ext, path, suggestion) => {
-  let msg = `Unknown file extension "${ext}" for ${path}`;
-  if (suggestion) {
-    msg += `. ${suggestion}`;
-  }
-  return msg;
-}, TypeError);
+E('ERR_UNKNOWN_FILE_EXTENSION', 'Unknown file extension %s for %s', TypeError);
 E('ERR_UNKNOWN_MODULE_FORMAT', 'Unknown module format: %s for URL %s',
   RangeError);
 E('ERR_UNKNOWN_SIGNAL', 'Unknown signal: %s', TypeError);
diff --git a/lib/internal/modules/esm/get_format.js b/lib/internal/modules/esm/get_format.js
index 4ac9c011d153f4..4ee7725db44f3f 100644
--- a/lib/internal/modules/esm/get_format.js
+++ b/lib/internal/modules/esm/get_format.js
@@ -7,7 +7,6 @@ const {
   StringPrototypeCharCodeAt,
   StringPrototypeSlice,
 } = primordials;
-const { basename, relative } = require('path');
 const { getOptionValue } = require('internal/options');
 const {
   extensionFormatMap,
@@ -16,7 +15,7 @@ const {
 
 const experimentalNetworkImports =
   getOptionValue('--experimental-network-imports');
-const { getPackageType, getPackageScopeConfig } = require('internal/modules/esm/resolve');
+const { getPackageType } = require('internal/modules/esm/resolve');
 const { fileURLToPath } = require('internal/url');
 const { ERR_UNKNOWN_FILE_EXTENSION } = require('internal/errors').codes;
 
@@ -74,7 +73,7 @@ function extname(url) {
  */
 function getFileProtocolModuleFormat(url, context, ignoreErrors) {
   const ext = extname(url);
-  if (ext === '.js') {
+  if (ext === '.js' || ext === '') {
     return getPackageType(url) === 'module' ? 'module' : 'commonjs';
   }
 
@@ -83,20 +82,9 @@ function getFileProtocolModuleFormat(url, context, ignoreErrors) {
 
   // Explicit undefined return indicates load hook should rerun format check
   if (ignoreErrors) { return undefined; }
+
   const filepath = fileURLToPath(url);
-  let suggestion = '';
-  if (getPackageType(url) === 'module' && ext === '') {
-    const config = getPackageScopeConfig(url);
-    const fileBasename = basename(filepath);
-    const relativePath = StringPrototypeSlice(relative(config.pjsonPath, filepath), 1);
-    suggestion = 'Loading extensionless files is not supported inside of ' +
-      '"type":"module" package.json contexts. The package.json file ' +
-      `${config.pjsonPath} caused this "type":"module" context. Try ` +
-      `changing ${filepath} to have a file extension. Note the "bin" ` +
-      'field of package.json can point to a file with an extension, for example ' +
-      `{"type":"module","bin":{"${fileBasename}":"${relativePath}.js"}}`;
-  }
-  throw new ERR_UNKNOWN_FILE_EXTENSION(ext, filepath, suggestion);
+  throw new ERR_UNKNOWN_FILE_EXTENSION(ext, filepath);
 }
 
 /**

From fcb8be93f08e423d247f40dfd255412511bc5b6e Mon Sep 17 00:00:00 2001
From: LiviaMedeiros <livia@cirno.name>
Date: Thu, 7 Sep 2023 20:23:10 +0800
Subject: [PATCH 2/8] module: test ES modules without file extension within
 `module` scope

---
 .../test-esm-unknown-or-no-extension.js       | 36 ---------
 test/parallel/test-esm-no-extension.mjs       | 22 ++++++
 test/parallel/test-esm-unknown-main.mjs       | 79 +++++++++++++++++++
 3 files changed, 101 insertions(+), 36 deletions(-)
 delete mode 100644 test/es-module/test-esm-unknown-or-no-extension.js
 create mode 100644 test/parallel/test-esm-no-extension.mjs
 create mode 100644 test/parallel/test-esm-unknown-main.mjs

diff --git a/test/es-module/test-esm-unknown-or-no-extension.js b/test/es-module/test-esm-unknown-or-no-extension.js
deleted file mode 100644
index 3f0660e5aa9225..00000000000000
--- a/test/es-module/test-esm-unknown-or-no-extension.js
+++ /dev/null
@@ -1,36 +0,0 @@
-'use strict';
-
-const { spawnPromisified } = require('../common');
-const fixtures = require('../common/fixtures.js');
-const assert = require('node:assert');
-const { execPath } = require('node:process');
-const { describe, it } = require('node:test');
-
-
-// In a "type": "module" package scope, files with unknown extensions or no
-// extensions should throw; both when used as a main entry point and also when
-// referenced via `import`.
-describe('ESM: extensionless and unknown specifiers', { concurrency: true }, () => {
-  for (
-    const fixturePath of [
-      '/es-modules/package-type-module/noext-esm',
-      '/es-modules/package-type-module/imports-noext.mjs',
-      '/es-modules/package-type-module/extension.unknown',
-      '/es-modules/package-type-module/imports-unknownext.mjs',
-    ]
-  ) {
-    it('should throw', async () => {
-      const entry = fixtures.path(fixturePath);
-      const { code, signal, stderr, stdout } = await spawnPromisified(execPath, [entry]);
-
-      assert.strictEqual(code, 1);
-      assert.strictEqual(signal, null);
-      assert.strictEqual(stdout, '');
-      assert.ok(stderr.includes('ERR_UNKNOWN_FILE_EXTENSION'));
-      if (fixturePath.includes('noext')) {
-        // Check for explanation to users
-        assert.ok(stderr.includes('extensionless'));
-      }
-    });
-  }
-});
diff --git a/test/parallel/test-esm-no-extension.mjs b/test/parallel/test-esm-no-extension.mjs
new file mode 100644
index 00000000000000..047e124b3fd4e8
--- /dev/null
+++ b/test/parallel/test-esm-no-extension.mjs
@@ -0,0 +1,22 @@
+import * as common from '../common/index.mjs';
+import * as fixtures from '../common/fixtures.mjs';
+import { spawn } from 'node:child_process';
+import assert from 'node:assert';
+
+const entry = fixtures.path('/es-modules/package-type-module/noext-esm');
+
+// Run a module that does not have extension.
+// This is to ensure that "type": "module" applies to extensionless files.
+
+const child = spawn(process.execPath, [entry]);
+
+let stdout = '';
+child.stdout.setEncoding('utf8');
+child.stdout.on('data', (data) => {
+  stdout += data;
+});
+child.on('close', common.mustCall((code, signal) => {
+  assert.strictEqual(code, 0);
+  assert.strictEqual(signal, null);
+  assert.strictEqual(stdout, 'executed\n');
+}));
diff --git a/test/parallel/test-esm-unknown-main.mjs b/test/parallel/test-esm-unknown-main.mjs
new file mode 100644
index 00000000000000..d460393e19dbc0
--- /dev/null
+++ b/test/parallel/test-esm-unknown-main.mjs
@@ -0,0 +1,79 @@
+import * as common from '../common/index.mjs';
+import * as fixtures from '../common/fixtures.mjs';
+import { spawn } from 'node:child_process';
+import assert from 'node:assert';
+
+{
+  const entry = fixtures.path(
+    '/es-modules/package-type-module/extension.unknown'
+  );
+  const child = spawn(process.execPath, [entry]);
+  let stdout = '';
+  let stderr = '';
+  child.stderr.setEncoding('utf8');
+  child.stdout.setEncoding('utf8');
+  child.stdout.on('data', (data) => {
+    stdout += data;
+  });
+  child.stderr.on('data', (data) => {
+    stderr += data;
+  });
+  child.on('close', common.mustCall((code, signal) => {
+    assert.strictEqual(code, 1);
+    assert.strictEqual(signal, null);
+    assert.strictEqual(stdout, '');
+    assert.ok(stderr.indexOf('ERR_UNKNOWN_FILE_EXTENSION') !== -1);
+  }));
+}
+{
+  const entry = fixtures.path(
+    '/es-modules/package-type-module/imports-unknownext.mjs'
+  );
+  const child = spawn(process.execPath, [entry]);
+  let stdout = '';
+  let stderr = '';
+  child.stderr.setEncoding('utf8');
+  child.stdout.setEncoding('utf8');
+  child.stdout.on('data', (data) => {
+    stdout += data;
+  });
+  child.stderr.on('data', (data) => {
+    stderr += data;
+  });
+  child.on('close', common.mustCall((code, signal) => {
+    assert.strictEqual(code, 1);
+    assert.strictEqual(signal, null);
+    assert.strictEqual(stdout, '');
+    assert.ok(stderr.indexOf('ERR_UNKNOWN_FILE_EXTENSION') !== -1);
+  }));
+}
+{
+  const entry = fixtures.path('/es-modules/package-type-module/noext-esm');
+  const child = spawn(process.execPath, [entry]);
+  let stdout = '';
+  child.stdout.setEncoding('utf8');
+  child.stdout.on('data', (data) => {
+    stdout += data;
+  });
+  child.on('close', common.mustCall((code, signal) => {
+    assert.strictEqual(code, 0);
+    assert.strictEqual(signal, null);
+    assert.strictEqual(stdout, 'executed\n');
+  }));
+}
+{
+  const entry = fixtures.path(
+    '/es-modules/package-type-module/imports-noext.mjs'
+  );
+  const child = spawn(process.execPath, [entry]);
+  let stdout = '';
+  child.stdout.setEncoding('utf8');
+  child.stdout.on('data', (data) => {
+    stdout += data;
+  });
+  child.on('close', common.mustCall((code, signal) => {
+    assert.strictEqual(code, 0);
+    assert.strictEqual(signal, null);
+    assert.strictEqual(stdout, 'executed\n');
+  }));
+}

From bfaedc470c42f26e9a0644ea11e91625cd565f38 Mon Sep 17 00:00:00 2001
From: LiviaMedeiros <livia@cirno.name>
Date: Thu, 7 Sep 2023 20:49:31 +0800
Subject: [PATCH 3/8] module: document ES modules without file extension within
 `module` scope

---
 doc/api/esm.md | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/doc/api/esm.md b/doc/api/esm.md
index 79793aef9cfe24..8b6f9c1e33fb46 100644
--- a/doc/api/esm.md
+++ b/doc/api/esm.md
@@ -7,6 +7,9 @@
 <!-- YAML
 added: v8.5.0
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/49531
+    description: ES modules within `module` package can be extensionless.
   - version: v20.0.0
     pr-url: https://github.com/nodejs/node/pull/44710
     description: Module customization hooks are executed off the main thread.
@@ -156,15 +159,6 @@ via the paths defined in [`"exports"`][].
 For details on these package resolution rules that apply to bare specifiers in
 the Node.js module resolution, see the [packages documentation](packages.md).
 
-### Mandatory file extensions
-
-A file extension must be provided when using the `import` keyword to resolve
-relative or absolute specifiers. Directory indexes (e.g. `'./startup/index.js'`)
-must also be fully specified.
-
-This behavior matches how `import` behaves in browser environments, assuming a
-typically configured server.
-
 ### URLs
 
 ES modules are resolved and cached as URLs. This means that special characters
@@ -1008,7 +1002,7 @@ _isImports_, _conditions_)
 > 5. Let _packageURL_ be the result of **LOOKUP\_PACKAGE\_SCOPE**(_url_).
 > 6. Let _pjson_ be the result of **READ\_PACKAGE\_JSON**(_packageURL_).
 > 7. If _pjson?.type_ exists and is _"module"_, then
->    1. If _url_ ends in _".js"_, then
+>    1. If _url_ ends in _".js"_ or lacks file extension, then
 >       1. Return _"module"_.
 >    2. Return **undefined**.
 > 8. Otherwise,

From 500ea04cc5c0a596696defd5fb77228a0492581a Mon Sep 17 00:00:00 2001
From: LiviaMedeiros <livia@cirno.name>
Date: Thu, 7 Sep 2023 20:56:15 +0800
Subject: [PATCH 4/8] squash: remove obsolete link from `packages.md`

---
 doc/api/packages.md | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/doc/api/packages.md b/doc/api/packages.md
index 09f32e6c160892..a900d1cc0abe33 100644
--- a/doc/api/packages.md
+++ b/doc/api/packages.md
@@ -393,8 +393,7 @@ and other JavaScript runtimes, using the extensionless style can result in
 bloated import map definitions. Explicit file extensions can avoid this issue by
 enabling the import map to utilize a [packages folder mapping][] to map multiple
 subpaths where possible instead of a separate map entry per package subpath
-export. This also mirrors the requirement of using [the full specifier path][]
-in relative and absolute import specifiers.
+export.
 
 ### Exports sugar
 
@@ -1352,4 +1351,3 @@ This field defines [subpath imports][] for the current package.
 [subpath imports]: #subpath-imports
 [supported package managers]: corepack.md#supported-package-managers
 [the dual CommonJS/ES module packages section]: #dual-commonjses-module-packages
-[the full specifier path]: esm.md#mandatory-file-extensions

From a82e860c7daa8ee5f0d10823a41b73591c6eb40a Mon Sep 17 00:00:00 2001
From: LiviaMedeiros <livia@cirno.name>
Date: Tue, 12 Sep 2023 00:06:26 +0800
Subject: [PATCH 5/8] rework: hide under `--experimental-extensionless-modules`

---
 doc/api/esm.md                                | 14 +++++---
 lib/internal/errors.js                        |  8 ++++-
 lib/internal/modules/esm/get_format.js        | 22 +++++++++---
 .../test-esm-unknown-or-no-extension.js       | 36 +++++++++++++++++++
 test/parallel/test-esm-no-extension.mjs       |  1 +
 test/parallel/test-esm-unknown-main.mjs       |  1 +
 6 files changed, 73 insertions(+), 9 deletions(-)
 create mode 100644 test/es-module/test-esm-unknown-or-no-extension.js

diff --git a/doc/api/esm.md b/doc/api/esm.md
index 8b6f9c1e33fb46..79793aef9cfe24 100644
--- a/doc/api/esm.md
+++ b/doc/api/esm.md
@@ -7,9 +7,6 @@
 <!-- YAML
 added: v8.5.0
 changes:
-  - version: REPLACEME
-    pr-url: https://github.com/nodejs/node/pull/49531
-    description: ES modules within `module` package can be extensionless.
   - version: v20.0.0
     pr-url: https://github.com/nodejs/node/pull/44710
     description: Module customization hooks are executed off the main thread.
@@ -159,6 +156,15 @@ via the paths defined in [`"exports"`][].
 For details on these package resolution rules that apply to bare specifiers in
 the Node.js module resolution, see the [packages documentation](packages.md).
 
+### Mandatory file extensions
+
+A file extension must be provided when using the `import` keyword to resolve
+relative or absolute specifiers. Directory indexes (e.g. `'./startup/index.js'`)
+must also be fully specified.
+
+This behavior matches how `import` behaves in browser environments, assuming a
+typically configured server.
+
 ### URLs
 
 ES modules are resolved and cached as URLs. This means that special characters
@@ -1002,7 +1008,7 @@ _isImports_, _conditions_)
 > 5. Let _packageURL_ be the result of **LOOKUP\_PACKAGE\_SCOPE**(_url_).
 > 6. Let _pjson_ be the result of **READ\_PACKAGE\_JSON**(_packageURL_).
 > 7. If _pjson?.type_ exists and is _"module"_, then
->    1. If _url_ ends in _".js"_ or lacks file extension, then
+>    1. If _url_ ends in _".js"_, then
 >       1. Return _"module"_.
 >    2. Return **undefined**.
 > 8. Otherwise,
diff --git a/lib/internal/errors.js b/lib/internal/errors.js
index 6ac77686569cbe..bed02da76a3bab 100644
--- a/lib/internal/errors.js
+++ b/lib/internal/errors.js
@@ -1690,7 +1690,13 @@ E('ERR_UNHANDLED_ERROR',
 E('ERR_UNKNOWN_BUILTIN_MODULE', 'No such built-in module: %s', Error);
 E('ERR_UNKNOWN_CREDENTIAL', '%s identifier does not exist: %s', Error);
 E('ERR_UNKNOWN_ENCODING', 'Unknown encoding: %s', TypeError);
-E('ERR_UNKNOWN_FILE_EXTENSION', 'Unknown file extension %s for %s', TypeError);
+E('ERR_UNKNOWN_FILE_EXTENSION', (ext, path, suggestion) => {
+  let msg = `Unknown file extension "${ext}" for ${path}`;
+  if (suggestion) {
+    msg += `. ${suggestion}`;
+  }
+  return msg;
+}, TypeError);
 E('ERR_UNKNOWN_MODULE_FORMAT', 'Unknown module format: %s for URL %s',
   RangeError);
 E('ERR_UNKNOWN_SIGNAL', 'Unknown signal: %s', TypeError);
diff --git a/lib/internal/modules/esm/get_format.js b/lib/internal/modules/esm/get_format.js
index 4ee7725db44f3f..ef170c14d2efa8 100644
--- a/lib/internal/modules/esm/get_format.js
+++ b/lib/internal/modules/esm/get_format.js
@@ -7,6 +7,7 @@ const {
   StringPrototypeCharCodeAt,
   StringPrototypeSlice,
 } = primordials;
+const { basename, relative } = require('path');
 const { getOptionValue } = require('internal/options');
 const {
   extensionFormatMap,
@@ -15,7 +16,9 @@ const {
 
 const experimentalNetworkImports =
   getOptionValue('--experimental-network-imports');
-const { getPackageType } = require('internal/modules/esm/resolve');
+const experimentalExtensionlessModules =
+  getOptionValue('--experimental-extensionless-modules');
+const { getPackageType, getPackageScopeConfig } = require('internal/modules/esm/resolve');
 const { fileURLToPath } = require('internal/url');
 const { ERR_UNKNOWN_FILE_EXTENSION } = require('internal/errors').codes;
 
@@ -73,7 +76,7 @@ function extname(url) {
  */
 function getFileProtocolModuleFormat(url, context, ignoreErrors) {
   const ext = extname(url);
-  if (ext === '.js' || ext === '') {
+  if (ext === '.js' || (experimentalExtensionlessModules && ext === '')) {
     return getPackageType(url) === 'module' ? 'module' : 'commonjs';
   }
 
@@ -82,9 +85,20 @@ function getFileProtocolModuleFormat(url, context, ignoreErrors) {
 
   // Explicit undefined return indicates load hook should rerun format check
   if (ignoreErrors) { return undefined; }
-
   const filepath = fileURLToPath(url);
-  throw new ERR_UNKNOWN_FILE_EXTENSION(ext, filepath);
+  let suggestion = '';
+  if (getPackageType(url) === 'module' && ext === '') {
+    const config = getPackageScopeConfig(url);
+    const fileBasename = basename(filepath);
+    const relativePath = StringPrototypeSlice(relative(config.pjsonPath, filepath), 1);
+    suggestion = 'Loading extensionless files is not supported inside of ' +
+      '"type":"module" package.json contexts. The package.json file ' +
+      `${config.pjsonPath} caused this "type":"module" context. Try ` +
+      `changing ${filepath} to have a file extension. Note the "bin" ` +
+      'field of package.json can point to a file with an extension, for example ' +
+      `{"type":"module","bin":{"${fileBasename}":"${relativePath}.js"}}`;
+  }
+  throw new ERR_UNKNOWN_FILE_EXTENSION(ext, filepath, suggestion);
 }
 
 /**
diff --git a/test/es-module/test-esm-unknown-or-no-extension.js b/test/es-module/test-esm-unknown-or-no-extension.js
new file mode 100644
index 00000000000000..3f0660e5aa9225
--- /dev/null
+++ b/test/es-module/test-esm-unknown-or-no-extension.js
@@ -0,0 +1,36 @@
+'use strict';
+
+const { spawnPromisified } = require('../common');
+const fixtures = require('../common/fixtures.js');
+const assert = require('node:assert');
+const { execPath } = require('node:process');
+const { describe, it } = require('node:test');
+
+
+// In a "type": "module" package scope, files with unknown extensions or no
+// extensions should throw; both when used as a main entry point and also when
+// referenced via `import`.
+describe('ESM: extensionless and unknown specifiers', { concurrency: true }, () => {
+  for (
+    const fixturePath of [
+      '/es-modules/package-type-module/noext-esm',
+      '/es-modules/package-type-module/imports-noext.mjs',
+      '/es-modules/package-type-module/extension.unknown',
+      '/es-modules/package-type-module/imports-unknownext.mjs',
+    ]
+  ) {
+    it('should throw', async () => {
+      const entry = fixtures.path(fixturePath);
+      const { code, signal, stderr, stdout } = await spawnPromisified(execPath, [entry]);
+
+      assert.strictEqual(code, 1);
+      assert.strictEqual(signal, null);
+      assert.strictEqual(stdout, '');
+      assert.ok(stderr.includes('ERR_UNKNOWN_FILE_EXTENSION'));
+      if (fixturePath.includes('noext')) {
+        // Check for explanation to users
+        assert.ok(stderr.includes('extensionless'));
+      }
+    });
+  }
+});
diff --git a/test/parallel/test-esm-no-extension.mjs b/test/parallel/test-esm-no-extension.mjs
index 047e124b3fd4e8..66c3afaacad67f 100644
--- a/test/parallel/test-esm-no-extension.mjs
+++ b/test/parallel/test-esm-no-extension.mjs
@@ -1,3 +1,4 @@
+// Flags: --experimental-extensionless-modules
 import * as common from '../common/index.mjs';
 import * as fixtures from '../common/fixtures.mjs';
 import { spawn } from 'node:child_process';
diff --git a/test/parallel/test-esm-unknown-main.mjs b/test/parallel/test-esm-unknown-main.mjs
index d460393e19dbc0..6c9efe5148d5f8 100644
--- a/test/parallel/test-esm-unknown-main.mjs
+++ b/test/parallel/test-esm-unknown-main.mjs
@@ -1,3 +1,4 @@
+// Flags: --experimental-extensionless-modules
 import * as common from '../common/index.mjs';
 import * as fixtures from '../common/fixtures.mjs';
 import { spawn } from 'node:child_process';

From 92747ab41ebd79a6fb8630527ec9b8aa4fc55c93 Mon Sep 17 00:00:00 2001
From: LiviaMedeiros <livia@cirno.name>
Date: Tue, 12 Sep 2023 01:24:24 +0800
Subject: [PATCH 6/8] squash: add `--experimental-extensionless-modules` flag

---
 doc/api/cli.md      | 8 ++++++++
 doc/node.1          | 3 +++
 src/node_options.cc | 4 ++++
 src/node_options.h  | 1 +
 4 files changed, 16 insertions(+)

diff --git a/doc/api/cli.md b/doc/api/cli.md
index beee94f0a21239..4cfeef06c61695 100644
--- a/doc/api/cli.md
+++ b/doc/api/cli.md
@@ -593,6 +593,14 @@ added: v11.8.0
 
 Use the specified file as a security policy.
 
+### `--experimental-extensionless-modules`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+Enable extensionless modules support.
+
 ### `--no-experimental-fetch`
 
 <!-- YAML
diff --git a/doc/node.1 b/doc/node.1
index d7095e90fdf4d6..9a578fd33b7d3f 100644
--- a/doc/node.1
+++ b/doc/node.1
@@ -178,6 +178,9 @@ Use this flag to enable ShadowRealm support.
 .It Fl -experimental-test-coverage
 Enable code coverage in the test runner.
 .
+.It Fl -experimental-extensionless-modules
+Enable extensionless modules support.
+.
 .It Fl -no-experimental-fetch
 Disable experimental support for the Fetch API.
 .
diff --git a/src/node_options.cc b/src/node_options.cc
index 6ea85e3399be69..8701e77b6b74ee 100644
--- a/src/node_options.cc
+++ b/src/node_options.cc
@@ -365,6 +365,10 @@ EnvironmentOptionsParser::EnvironmentOptionsParser() {
             &EnvironmentOptions::enable_source_maps,
             kAllowedInEnvvar);
   AddOption("--experimental-abortcontroller", "", NoOp{}, kAllowedInEnvvar);
+  AddOption("--experimental-extensionless-modules",
+            "load extensionless files in module scope as ES modules",
+            &EnvironmentOptions::experimental_extensionless_modules,
+            kAllowedInEnvvar);
   AddOption("--experimental-fetch",
             "experimental Fetch API",
             &EnvironmentOptions::experimental_fetch,
diff --git a/src/node_options.h b/src/node_options.h
index bc18a45e681a3c..1c2c09cb214104 100644
--- a/src/node_options.h
+++ b/src/node_options.h
@@ -110,6 +110,7 @@ class EnvironmentOptions : public Options {
   std::vector<std::string> conditions;
   std::string dns_result_order;
   bool enable_source_maps = false;
+  bool experimental_extensionless_modules = false;
   bool experimental_fetch = true;
   bool experimental_global_customevent = true;
   bool experimental_global_web_crypto = true;

From d054cd071564560e719ffe6573997539abfdd49c Mon Sep 17 00:00:00 2001
From: LiviaMedeiros <livia@cirno.name>
Date: Tue, 12 Sep 2023 03:38:31 +0800
Subject: [PATCH 7/8] squash: document flag

---
 doc/api/cli.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/doc/api/cli.md b/doc/api/cli.md
index 4cfeef06c61695..378bf13e42e93e 100644
--- a/doc/api/cli.md
+++ b/doc/api/cli.md
@@ -2207,6 +2207,7 @@ Node.js options that are allowed are:
 * `--enable-network-family-autoselection`
 * `--enable-source-maps`
 * `--experimental-abortcontroller`
+* `--experimental-extensionless-modules`
 * `--experimental-import-meta-resolve`
 * `--experimental-json-modules`
 * `--experimental-loader`

From a177750afe16fd3533ca9e708e2b396374cc9c88 Mon Sep 17 00:00:00 2001
From: LiviaMedeiros <livia@cirno.name>
Date: Tue, 12 Sep 2023 03:38:43 +0800
Subject: [PATCH 8/8] squash: add flag to tests

---
 test/parallel/test-esm-no-extension.mjs |  5 ++++-
 test/parallel/test-esm-unknown-main.mjs | 20 ++++++++++++++++----
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/test/parallel/test-esm-no-extension.mjs b/test/parallel/test-esm-no-extension.mjs
index 66c3afaacad67f..025306ab41ea93 100644
--- a/test/parallel/test-esm-no-extension.mjs
+++ b/test/parallel/test-esm-no-extension.mjs
@@ -9,7 +9,10 @@ const entry = fixtures.path('/es-modules/package-type-module/noext-esm');
 // Run a module that does not have extension.
 // This is to ensure that "type": "module" applies to extensionless files.
 
-const child = spawn(process.execPath, [entry]);
+const child = spawn(process.execPath, [
+  '--experimental-extensionless-modules',
+  entry,
+]);
 
 let stdout = '';
 child.stdout.setEncoding('utf8');
diff --git a/test/parallel/test-esm-unknown-main.mjs b/test/parallel/test-esm-unknown-main.mjs
index 6c9efe5148d5f8..5adc9ec582831f 100644
--- a/test/parallel/test-esm-unknown-main.mjs
+++ b/test/parallel/test-esm-unknown-main.mjs
@@ -8,7 +8,10 @@ import assert from 'node:assert';
   const entry = fixtures.path(
     '/es-modules/package-type-module/extension.unknown'
   );
-  const child = spawn(process.execPath, [entry]);
+  const child = spawn(process.execPath, [
+    '--experimental-extensionless-modules',
+    entry,
+  ]);
   let stdout = '';
   let stderr = '';
   child.stderr.setEncoding('utf8');
@@ -30,7 +33,10 @@ import assert from 'node:assert';
   const entry = fixtures.path(
     '/es-modules/package-type-module/imports-unknownext.mjs'
   );
-  const child = spawn(process.execPath, [entry]);
+  const child = spawn(process.execPath, [
+    '--experimental-extensionless-modules',
+    entry,
+  ]);
   let stdout = '';
   let stderr = '';
   child.stderr.setEncoding('utf8');
@@ -50,7 +56,10 @@ import assert from 'node:assert';
 }
 {
   const entry = fixtures.path('/es-modules/package-type-module/noext-esm');
-  const child = spawn(process.execPath, [entry]);
+  const child = spawn(process.execPath, [
+    '--experimental-extensionless-modules',
+    entry,
+  ]);
   let stdout = '';
   child.stdout.setEncoding('utf8');
   child.stdout.on('data', (data) => {
@@ -66,7 +75,10 @@ import assert from 'node:assert';
   const entry = fixtures.path(
     '/es-modules/package-type-module/imports-noext.mjs'
   );
-  const child = spawn(process.execPath, [entry]);
+  const child = spawn(process.execPath, [
+    '--experimental-extensionless-modules',
+    entry,
+  ]);
   let stdout = '';
   child.stdout.setEncoding('utf8');
   child.stdout.on('data', (data) => {