diff --git a/core/loader/src/LoaderUtil.ts b/core/loader/src/LoaderUtil.ts
index 6f281a45..8cfcf0a6 100644
--- a/core/loader/src/LoaderUtil.ts
+++ b/core/loader/src/LoaderUtil.ts
@@ -9,8 +9,6 @@ const Module = module.constructor.length > 1
   /* istanbul ignore next */
   : BuiltinModule;
 
-const EXTENSION = Object.keys((Module as any)._extensions).find(t => t === '.ts') ? '.ts' : '.js';
-
 interface LoaderUtilConfig {
   extraFilePattern?: string[];
 }
@@ -21,7 +19,9 @@ export class LoaderUtil {
     this.config = config;
   }
 
-  static extension = EXTENSION;
+  static get extension() {
+    return Object.keys((Module as any)._extensions).find(t => t === '.ts') ? '.ts' : '.js';
+  }
 
   static filePattern(): string[] {
     const extensions = Object.keys((Module as any)._extensions);