My experience suggests the opposite of what this article claims. Claude Code is ridiculously good with vanilla JavaScript, provided that your code is well written. I tried it with a TypeScript code base and it wasn't anywhere near as good.

With JS, Claude has very high success rate. Only issue I had with it was that one time it forgot to update one part of the code which was in a different file but as soon as I told it, it updated it perfectly.

With TypeScript my experience was that it struggles to find things. Writing tests was a major pain because it kept trying to grep the build output because it had to mock out one of the functions in the test and it just couldn't figure it out.

Also typed code it produces is more complex to solve the same problem with more different files and it struggles to get the right context. Also TS is more verbose (this is objectively true and measurable); requires more tokens so it literally costs more.