module: support ES modules without file extension within module scope

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
@@ -2199,6 +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`

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

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.
 .

lib/internal/modules/esm/get_format.js

@@ -16,6 +16,8 @@ const {
 
 const experimentalNetworkImports =
   getOptionValue('--experimental-network-imports');
+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;
@@ -74,7 +76,7 @@ function extname(url) {
  */
 function getFileProtocolModuleFormat(url, context, ignoreErrors) {
   const ext = extname(url);
-  if (ext === '.js') {
+  if (ext === '.js' || (experimentalExtensionlessModules && ext === '')) {
     return getPackageType(url) === 'module' ? 'module' : 'commonjs';
   }
 

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,

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;

test/parallel/test-esm-no-extension.mjs

@@ -0,0 +1,26 @@
+// Flags: --experimental-extensionless-modules
+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, [
+  '--experimental-extensionless-modules',
+  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');
+}));

test/parallel/test-esm-unknown-main.mjs

@@ -0,0 +1,92 @@
+// Flags: --experimental-extensionless-modules
+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, [
+    '--experimental-extensionless-modules',
+    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, [
+    '--experimental-extensionless-modules',
+    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, [
+    '--experimental-extensionless-modules',
+    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, [
+    '--experimental-extensionless-modules',
+    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');
+  }));
+}