module: add a condition to if in case of a bug.

lib/internal/modules/esm/get_format.js

@@ -55,25 +55,29 @@ const protocolHandlers = ObjectAssign(ObjectCreate(null), {
   },
   'file:'(parsed, url) {
     const ext = extname(parsed.pathname);
-    let format;
+    let format = null;
 
     if (ext === '.js') {
       format = getPackageType(parsed.href) === 'module' ? 'module' : 'commonjs';
     } else {
       format = extensionFormatMap[ext];
     }
     if (!format) {
-      if (experimentalSpecifierResolution === 'node') {
+      if (
+        experimentalSpecifierResolution === 'node' &&
+        legacyExtensionFormatMap[ext]
+      ) {
         process.emitWarning(
           'The Node.js specifier resolution in ESM is experimental.',
-          'ExperimentalWarning');
+          'ExperimentalWarning'
+        );
         format = legacyExtensionFormatMap[ext];
       } else {
         throw new ERR_UNKNOWN_FILE_EXTENSION(ext, fileURLToPath(url));
       }
     }
 
-    return format || null;
+    return format;
   },
   'node:'() { return 'builtin'; },
 });

test/es-module/test-esm-get-format.js

@@ -0,0 +1,13 @@
+require('../common');
+const assert = require('assert');
+const { defaultGetFormat} = require('internal/modules/esm/get_format');
+
+{
+  const url = new URL('file://example.com/foo/bar.js')
+  assert.strictEqual(defaultGetFormat(url), 'commonjs')
+}
+
+{
+  const url = new URL('file://example.com/foo/bar.whatever')
+  assert.throws(() => defaultGetFormat(url), {name: 'TypeError', message: /Unknown file extension whatever/})
+}