esm: empty ext from pkg type/main doesnt affect format

This ensures files with unknown extensions like foo.bar are not
loaded as CJS/ESM when imported as a main entry point and makes
sure that those files would maintain the same format even if loaded
after the main entrypoint.

PR-URL: nodejs#31021
Reviewed-By: Guy Bedford <guybedford@gmail.com>

doc/api/esm.md

@@ -1154,15 +1154,13 @@ updates.
 In the following algorithms, all subroutine errors are propagated as errors
 of these top-level routines unless stated otherwise.
 
-_isMain_ is **true** when resolving the Node.js application entry point.
-
 _defaultEnv_ is the conditional environment name priority array,
 `["node", "import"]`.
 
 <details>
 <summary>Resolver algorithm specification</summary>
 
-**ESM_RESOLVE**(_specifier_, _parentURL_, _isMain_)
+**ESM_RESOLVE**(_specifier_, _parentURL_)
 
 > 1. Let _resolvedURL_ be **undefined**.
 > 1. If _specifier_ is a valid URL, then
@@ -1183,7 +1181,7 @@ _defaultEnv_ is the conditional environment name priority array,
 > 1. If the file at _resolvedURL_ does not exist, then
 >    1. Throw a _Module Not Found_ error.
 > 1. Set _resolvedURL_ to the real path of _resolvedURL_.
-> 1. Let _format_ be the result of **ESM_FORMAT**(_resolvedURL_, _isMain_).
+> 1. Let _format_ be the result of **ESM_FORMAT**(_resolvedURL_).
 > 1. Load _resolvedURL_ as module format, _format_.
 
 **PACKAGE_RESOLVE**(_packageSpecifier_, _parentURL_)
@@ -1334,20 +1332,20 @@ _defaultEnv_ is the conditional environment name priority array,
 >       1. Return _resolved_.
 > 1. Throw a _Module Not Found_ error.
 
-**ESM_FORMAT**(_url_, _isMain_)
+**ESM_FORMAT**(_url_)
 
-> 1. Assert: _url_ corresponds to an existing file.
+> 1. Assert: _url_ corresponds to an existing file pathname.
 > 1. Let _pjson_ be the result of **READ_PACKAGE_SCOPE**(_url_).
 > 1. If _url_ ends in _".mjs"_, then
 >    1. Return _"module"_.
 > 1. If _url_ ends in _".cjs"_, then
 >    1. Return _"commonjs"_.
 > 1. If _pjson?.type_ exists and is _"module"_, then
->    1. If _isMain_ is **true** or _url_ ends in _".js"_, then
+>    1. If _url_ ends in _".js"_ or lacks a file extension, then
 >       1. Return _"module"_.
 >    1. Throw an _Unsupported File Extension_ error.
 > 1. Otherwise,
->    1. If _isMain_ is **true**, then
+>    1. If _url_ lacks a file extension, then
 >       1. Return _"commonjs"_.
 >    1. Throw an _Unsupported File Extension_ error.
 

lib/internal/modules/esm/default_resolve.js

@@ -106,9 +106,12 @@ function resolve(specifier, parentURL) {
   }
 
   const ext = extname(url.pathname);
-  let format = extensionFormatMap[ext];
-  if (ext === '.js' || (!format && isMain))
+  let format;
+  if (ext === '.js' || ext === '') {
     format = getPackageType(url.href) === TYPE_MODULE ? 'module' : 'commonjs';
+  } else {
+    format = extensionFormatMap[ext];
+  }
   if (!format) {
     if (experimentalSpeciferResolution === 'node') {
       process.emitWarning(

test/es-module/test-esm-unknown-main.js

@@ -0,0 +1,81 @@
+'use strict';
+
+const common = require('../common');
+const fixtures = require('../common/fixtures');
+const { spawn } = require('child_process');
+const assert = require('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');
+  }));
+}

test/fixtures/es-modules/package-type-module/extension.unknown

@@ -0,0 +1 @@
+throw new Error('NO, NEVER');

test/fixtures/es-modules/package-type-module/imports-noext.mjs

@@ -0,0 +1 @@
+import './noext-esm';

test/fixtures/es-modules/package-type-module/imports-unknownext.mjs

@@ -0,0 +1 @@
+import './extension.unknown';