SharePoint Framework and the missing script and/or content editor web part. Here are three sources to learn about how you could roll your own:
Next to client-side web parts, extensions are new types of customizations, SharePoint developers can build to extend SharePoint. In the developer preview you can build
- field customizers, which control the rendering of fields in list view, similar to JSLink
- list view command sets, which you can use to add options to list menu, similar to Ribbon and ECB extensions using custom actions
- application customizers, with which you can embed custom scripts to run on all pages, similar to script injection using custom actions
Additionally, the preview includes placeholders which you could compare to server-side delegate controls and an API to build dialog windows.
The Visual Studio Extension for SharePoint Framework wraps the command-line UI of the Microsoft Yeoman Generator (yo @microsoft/sharepoint) into a familiar Windows Forms experience, executes the generator project scaffolding behind the scenes and creates a Visual Studio project that includes all the necessary files for a complete web part project