esm: handle extension-less "bin" files

lib/internal/modules/esm/get_format.js

@@ -74,7 +74,7 @@ function getLegacyExtensionFormat(ext) {
 
 function getFileProtocolModuleFormat(url, ignoreErrors) {
   const ext = extname(url.pathname);
-  if (ext === '.js') {
+  if (ext === '.js' || !ext) {
     return getPackageType(url) === 'module' ? 'module' : 'commonjs';
   }
 

lib/internal/modules/run_main.js

@@ -28,20 +28,26 @@ function resolveMainPath(main) {
 }
 
 function shouldUseESMLoader(mainPath) {
+  // General indicators of user intention
+
   const userLoader = getOptionValue('--experimental-loader');
-  if (userLoader)
-    return true;
+  // This flag is a user's only way to opt into ESMLoader (which can handle both
+  // CJS & ESM), so assume that's what is happening here.
+  if (userLoader) return true;
+
   const esModuleSpecifierResolution =
     getOptionValue('--experimental-specifier-resolution');
-  if (esModuleSpecifierResolution === 'node')
-    return true;
-  // Determine the module format of the main
-  if (mainPath && StringPrototypeEndsWith(mainPath, '.mjs'))
-    return true;
-  if (!mainPath || StringPrototypeEndsWith(mainPath, '.cjs'))
-    return false;
+  // This flag is only applicable to ESM, so assume it signals opting in.
+  if (esModuleSpecifierResolution === 'node') return true;
+
+  // Specific indicators of user intention
+
+  // Check trump-cards
+  if (mainPath && StringPrototypeEndsWith(mainPath, '.mjs')) return true;
+  if (!mainPath || StringPrototypeEndsWith(mainPath, '.cjs')) return false;
+
   const pkg = readPackageScope(mainPath);
-  return pkg && pkg.data.type === 'module';
+  return pkg?.data?.type === 'module';
 }
 
 function runMainESM(mainPath) {

test/es-module/test-esm-resolve-type.js

@@ -38,13 +38,28 @@ try {
   [
     [ '/es-modules/package-type-module/index.js', 'module' ],
     [ '/es-modules/package-type-commonjs/index.js', 'commonjs' ],
+    // "commonjs" is the default type, so it is assumed when there is no "type"
+    // or specific file extension (`.cjs` or `.mjs`)
+    // It happens to be "correct" here:
+    [ '/es-modules/packages-with-bin/no-type-but-commonjs/bin/foo', 'commonjs'],
+    // It happens to be "wrong" here:
+    [ '/es-modules/packages-with-bin/no-type-but-module/bin/foo', 'commonjs' ],
+    [ '/es-modules/packages-with-bin/type-commonjs/bin/foo', 'commonjs' ],
+    [ '/es-modules/packages-with-bin/type-module/bin/foo', 'module' ],
     [ '/es-modules/package-without-type/index.js', 'commonjs' ],
     [ '/es-modules/package-without-pjson/index.js', 'commonjs' ],
-  ].forEach((testVariant) => {
-    const [ testScript, expectedType ] = testVariant;
+  ].forEach(([ testScript, expectedType ]) => {
     const resolvedPath = path.resolve(fixtures.path(testScript));
     const resolveResult = resolve(url.pathToFileURL(resolvedPath));
-    assert.strictEqual(resolveResult.format, expectedType);
+    assert.strictEqual(
+      resolveResult.format,
+      expectedType,
+      new assert.AssertionError({
+        actual: resolveResult?.format,
+        expected: expectedType,
+        message: `Expectation failed for "${testScript}"`
+      })
+    );
   });
 
   /**

test/es-module/test-esm-unknown-or-no-extension.js

@@ -5,13 +5,11 @@ const fixtures = require('../common/fixtures');
 const { spawn } = require('child_process');
 const assert = require('assert');
 
-// 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`.
+// In a "type": "module" package scope, files with unknown extensions should
+// throw; both when used as a main entry point and also when referenced via
+// `import`.
 
 [
-  '/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',
 ].forEach((fixturePath) => {

test/fixtures/es-modules/packages-with-bin/no-type-but-commonjs/bin/foo

@@ -0,0 +1 @@
+require('assert');

test/fixtures/es-modules/packages-with-bin/no-type-but-commonjs/package.json

@@ -0,0 +1,4 @@
+{
+  "bin": "./bin/foo",
+  "name": "bin-without-type"
+}

test/fixtures/es-modules/packages-with-bin/no-type-but-module/bin/foo

@@ -0,0 +1 @@
+import 'assert';

test/fixtures/es-modules/packages-with-bin/no-type-but-module/package.json

@@ -0,0 +1,4 @@
+{
+  "bin": "./bin/foo",
+  "name": "bin-without-type"
+}

test/fixtures/es-modules/packages-with-bin/type-commonjs/bin/foo

@@ -0,0 +1 @@
+require('assert');

test/fixtures/es-modules/packages-with-bin/type-commonjs/package.json

@@ -0,0 +1,5 @@
+{
+  "bin": "./bin/foo",
+  "name": "bin-without-type",
+  "type": "commonjs"
+}

test/fixtures/es-modules/packages-with-bin/type-module/bin/foo

@@ -0,0 +1 @@
+import 'assert';

test/fixtures/es-modules/packages-with-bin/type-module/package.json

@@ -0,0 +1,5 @@
+{
+  "bin": "./bin/foo",
+  "name": "bin-without-type",
+  "type": "module"
+}