In this book, we cover basic app development and also explain how advanced development takes place. Most web and mobile apps today are single-page apps (SPAs). Because of this, Salesforce has also introduced Lightning Experience, which is a SPA or an OPA. We develop modern and interactive desktop apps and mobile apps with Lightning Framework. So we need to upgrade from Salesforce Classic, which is the old UI, to Lightning Experience, which is the modern UI. For the brevity of the book we are not diving deep into advanced concepts such as client-side controllers, server-side controllers, event debugging, lack of service, and more.