I'm not sure that it uses React Native, but modern add-ins for Office are JS, so any injected modern add-in will be using JS rather than COM/C/C++. This gave add-ins portability between all supported Office platforms, increased security, and made administration much easier.
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React Native, absolutely - as with Office 365, using WinUI 3 as the underlying UI layer.
Running a full javascript framework in a javascript runtime.
When did office start using React Native? Office has been around well before the creation of javascript, let alone react.
I'm not sure that it uses React Native, but modern add-ins for Office are JS, so any injected modern add-in will be using JS rather than COM/C/C++. This gave add-ins portability between all supported Office platforms, increased security, and made administration much easier.