[wruza]

they don't grasp FP. But don't we use higher-order functions, closures, combinators all the time in most mainstream languages?

We don’t really use them in mainstream languages. They exist as idioms for shallow application, but one doesn’t go full-on combinators in javascript, for at least performance reasons. What we do in mainstream is as FP as walking stairs compared to rope climbing.

That said I think deep FP makes no sense and only entertains bored intellectuals by being a subspecies of code golf.