My first touch with functional programming was with Javascript, discovered it through some blog posts and I thought it was an interesting code paradigm!
So I read some books about it, particularly "Mostly adequate guide to functional programming". Since then I try to make as much FP as I can, getting used with terminology and wrap my head around those concepts!
I do some Elm and Clojure for practicing with real functional & immutable constraints.
I am still in the process of understanding fully functional programming, but I must say that I am a way better developer now with FP than then with POO & Imperative!