diff --git a/packages/vite/src/node/config.ts b/packages/vite/src/node/config.ts
index ce897d745d2e27..99ec143fa85f0f 100644
--- a/packages/vite/src/node/config.ts
+++ b/packages/vite/src/node/config.ts
@@ -837,10 +837,9 @@ async function loadConfigFromBundledFile(
   fileName: string,
   bundledCode: string
 ): Promise<UserConfig> {
-  const extension = path.extname(fileName)
   const realFileName = fs.realpathSync(fileName)
-  const defaultLoader = _require.extensions[extension]!
-  _require.extensions[extension] = (module: NodeModule, filename: string) => {
+  const defaultLoader = _require.extensions['.js']
+  _require.extensions['.js'] = (module: NodeModule, filename: string) => {
     if (filename === realFileName) {
       ;(module as NodeModuleWithCompile)._compile(bundledCode, filename)
     } else {
@@ -850,9 +849,8 @@ async function loadConfigFromBundledFile(
   // clear cache in case of server restart
   delete _require.cache[_require.resolve(fileName)]
   const raw = _require(fileName)
-  const config = raw.__esModule ? raw.default : raw
-  _require.extensions[extension] = defaultLoader
-  return config
+  _require.extensions['.js'] = defaultLoader
+  return raw.__esModule ? raw.default : raw
 }
 
 export function isDepsOptimizerEnabled(config: ResolvedConfig): boolean {