JavaScript can double your codebase so make sure you need it first

There isn't a lot of detail on exactly how javascript is used in this codebase so it is hard to comment. Having worked in .NET web shops there is a tendency to be very back-end (and .NET) oriented and not really having a lot of deep expertise in the web, particularly on the front-end. I don't want to assume this (the author mentioned UX and some other things which tends to lead me to believe they do have this kind of expertise). There is also a tendency to avoid technologies that can make working with the web easier. I'm talking about things like Ember or Angular, build tools like Grunt or some kind of asset pipeline for work so javascript can be more organized and structured and you can write less of it.

In the end web development on the .NET platform (can be) is harder and front-end development doubly so. I'll admit this is my experience so YMMV.

In the author's defence though I agree Javascript, when compared with many other languages, particularly Java or C# is often unwieldily, there is little help for how code is structured or organized and architecture of any sort is more challenging. That said it can be done, just not the way you might be used to in a more OOP C# or Java.

I'm working on a fairly substantial SPA now, and while I think these costs are worth it for my use case, they certainly are real costs and should be carefully considered.