diff --git a/lib/core.js b/lib/core.js
index 074c5e6a..b7ed0dc2 100644
--- a/lib/core.js
+++ b/lib/core.js
@@ -137,6 +137,9 @@ SystemJSLoader.prototype.config = function(cfg) {
     getBaseURLObj.call(this);
   }
 
+  if (cfg.defaultJSExtensions)
+    this.defaultJSExtensions = cfg.defaultJSExtensions;
+
   if (cfg.paths) {
     for (var p in cfg.paths)
       this.paths[p] = cfg.paths[p];