esm: treat modules with no extension as .js

lib/internal/modules/esm/get_format.js

@@ -50,7 +50,7 @@ function defaultGetFormat(url, context, defaultGetFormatUnused) {
   } else if (parsed.protocol === 'file:') {
     const ext = extname(parsed.pathname);
     let format;
-    if (ext === '.js') {
+    if (!ext || ext === '.js') {
       format = getPackageType(parsed.href) === 'module' ? 'module' : 'commonjs';
     } else {
       format = extensionFormatMap[ext];

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

@@ -0,0 +1,34 @@
+'use strict';
+
+const common = require('../common');
+const fixtures = require('../common/fixtures');
+const { spawn } = require('child_process');
+const assert = require('assert');
+
+// In a "type": "module" package scope, files with no extensions should
+// not 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',
+].forEach((fixturePath) => {
+  const entry = fixtures.path(fixturePath);
+  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, 0);
+    assert.strictEqual(signal, null);
+    assert.strictEqual(stdout, 'executed\n');
+    assert.ok(stderr.indexOf('ERR_UNKNOWN_FILE_EXTENSION') === -1);
+  }));
+});

test/es-module/test-esm-unknown-or-no-extension.js → test/es-module/test-esm-unknown-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
+// 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) => {