Issues with Google Closure Compiler and modern JavaScript

Hi Ralf, I'm honestly not the best to answer your question. All I know is that XWiki 18.0.0 will use the latest version of the Closure compiler, but there hasn't been any update on https://github.com/google/closure-compiler/issues/2899 in quite a while.

WDYT Marius Dumitru Florea and Manuel Leduc ?

Unfortunately, the Closure compiler still doesn't support them: https://github.com/google/closure-compiler/issues/4286 - they are aware of the issue but unfortunately it won't come anytime soon according to this comment:

I'm sorry to say that the work to add support for private fields continues to be pushed back by other, more urgent work. I had expected to finally get it started in the first half of 2026, but now it looks like that will not happen.

So we'll either need to wait or move to another JavaScript minifier.