I don't think the point here is to write your entire app in C#. TypeScript is already close enough. I think the point is that you have some existing libraries

I don't think the point here is to write your entire app in C#. TypeScript is already close enough. I think the point is that you have some existing libraries that would be useful in a web application, you could go ahead and compile those down to TS and use them within your TS code.