I got interested in functional programming through JavaScript. It might have been React w/Redux that initially demonstrated the value of immutability and pure functions to me.
One day I got curious about monads and spent over a month trying to wrap my head around it, which obviously set me on the path to Haskell which is now my favorite thing in the world.
Obviously it's far from pure FP, but I think it's much more at the heart of JS than it is say Python. The Scheme parts of JS are the good parts.
I think some of the resurgence of FP is down to JS. The wealth of libraries available is pretty impressive [0]
[0]: https://github.com/stoeffel/awesome-fp-js