Using the External Editor
Once your widget is in the external editor, you're free to organize the code as you wish. Experienced JS developers recognize having separate files makes coding easier as many files hold smaller amounts of code.
WidgetStudio automatically creates the three files: index.html, data.json, and config.json. You're free to add other files as you wish. As long as you link them back to the HTML file appropriately, WidgetStudio will be able to work with this.
In the picture above of a widget, two additional files were created: "flexdatalist.js" and "index.css".
And they are linked properly: