JavaScript is a bad example. We rely on so many libraries which are not functional. React one of the biggest is not functional at all.

Also JavaScript biggest advantage is that you can mix up. And it’s biggest disadvantage is that non experience developers will mix up too.

Also you need to cloning multi level objects.

To work 100% FP in JavaScript you need the libraries and you need to have a flat data structure every where.

I don’t know how many JSON APIs you see who work like that.

JavaScript has also a rich set of language features you can use to save your time and still have quality code.