put factor creation in function

src/app.js

@@ -1,3 +1,4 @@
+function createApp() {
   const express = require('express');
   const cookieParser = require('cookie-parser');
   const path = require('path');
@@ -5,6 +6,8 @@ const path = require('path');
   const app = express();
   const port = 3000;
 
+  app.set('port', port);
+
   app.use(cookieParser());
 
   app.use(express.json());
@@ -26,6 +29,10 @@ app.use(express.static(path.join(__dirname, 'public')));
 
   app.disable('etag'); //avoids 304 responses
 
-app.listen(port, () => {
-  console.log(`Server started on port ${port}`);
+  return app;
+}
+
+const app = createApp();
+app.listen(app.get('port'), () => {
+  console.log(`Server started on port ${app.get('port')}`);
 });