External accesses will not be able to start the server.

index.js

@@ -92,4 +92,11 @@ if (http.Server && http.WebSocketServer) {
     });
     return true;
   });
+} else {
+
+  document.addEventListener('DOMContentLoaded', function() {
+    document.querySelector('div').textContent =
+      "Sorry, this application can only run as a sandboxed Chrome Application.";
+  });
+
 }