module: support ES modules without file extension within module scope

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

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';
   }
 

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

@@ -0,0 +1,23 @@
+// 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, [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 +1,80 @@
+// 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, [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');
+  }));
+}