module: support ES modules without file extension within module scope

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 @@
 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 @@
 > 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,

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/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);

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);
 }
 
 /**

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');
+}));

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');
+  }));
+}