vfs: address remaining review feedback from avivkeller

Inline VFS_FD_BASE constant since it was only used once. Replace
custom getFormatFromExtension with extensionFormatMap from
internal/modules/esm/formats.

Author: mcollina

lib/internal/vfs/fd.js

@@ -10,8 +10,7 @@ const kFd = Symbol('kFd');
 const kEntry = Symbol('kEntry');
 
 // FD range: 10000+ to avoid conflicts with real fds
-const VFS_FD_BASE = 10_000;
-let nextFd = VFS_FD_BASE;
+let nextFd = 10_000;
 
 // Global registry of open virtual file descriptors
 const openFDs = new SafeMap();

lib/internal/vfs/module_hooks.js

@@ -5,11 +5,11 @@ const {
   ArrayPrototypePush,
   ArrayPrototypeSplice,
   FunctionPrototypeCall,
-  StringPrototypeEndsWith,
   StringPrototypeStartsWith,
 } = primordials;
 
-const { dirname, isAbsolute, normalize, resolve } = require('path');
+const { dirname, extname, isAbsolute, normalize, resolve } = require('path');
+const { extensionFormatMap } = require('internal/modules/esm/formats');
 const { isURL, pathToFileURL, fileURLToPath, toPathIfFileURL, URL } = require('internal/url');
 const { kEmptyObject } = require('internal/util');
 const { validateObject } = require('internal/validators');
@@ -321,22 +321,18 @@ function findVFSForWatch(filename) {
 
 /**
  * Determine module format from file extension.
- * @param {string} url The file URL
- * @returns {string} The format ('module', 'commonjs', or 'json')
+ * Uses the shared extensionFormatMap, falling back to commonjs for .js
+ * and unknown extensions since VFS does not check package.json "type".
+ * @param {string} filePath The file path
+ * @returns {string} The format ('module', 'commonjs', 'json', etc.)
  */
-function getFormatFromExtension(url) {
-  if (StringPrototypeEndsWith(url, '.mjs')) {
-    return 'module';
-  }
-  if (StringPrototypeEndsWith(url, '.cjs')) {
+function getFormatFromExtension(filePath) {
+  const ext = extname(filePath);
+  if (ext === '.js') {
+    // TODO: Check package.json "type" field for proper detection
     return 'commonjs';
   }
-  if (StringPrototypeEndsWith(url, '.json')) {
-    return 'json';
-  }
-  // Default to commonjs for .js files
-  // TODO: Check package.json "type" field for proper detection
-  return 'commonjs';
+  return extensionFormatMap[ext] ?? 'commonjs';
 }
 
 /**