"The desktop app’s are fully native, and will remain so. They utilize React Native for a few components, but otherwise are written in C & C++."
Microsoft Office rewrite in React.js nears completion
The whole of Microsoft Office 365 software suite is being rewritten in React.js.
Home - Office UI Fabric (developer.microsoft.com)
"Built with React
Fabric’s robust, up-to-date components are built with React"