SharePoint Online as an app platform

Extending and customizing SharePoint has also evolved. With on-premises SharePoint versions, customizations were commonly made by modifying master pages and page layouts with SharePoint Designer or another HTML editor, running server-side C# code hooked into different events or loaded as a control during page load, or even creating customized APIs for accessing data. Development environments were heavy, consumed a lot of RAM, and required a local installation of SharePoint Server, which also required other services such as a Microsoft SQL database engine. The client-side implementation was quite commonly made by loading JavaScript using Script Editor web parts or injecting code directly to master pages or page layouts.

Since SharePoint Online is a SaaS offering, and actual servers are managed by Microsoft, there aren’t possibilities to run custom server-side code as in SharePoint Server environments hosted in customers’ data centers...