It's not required to write idiomatic JS, but pure functional code is very much idiomatic. (I.e. pure functional code isn't considered unidiomatic in JS as it might be in some other languages). Many people write the meat of their code in pure functions, and this paradigm is encouraged by React (especially with the invention of hooks).
As for libraries, you can just treat them as stateful external things you have to interact with the same way IO / network calls are stateful.
As for libraries, you can just treat them as stateful external things you have to interact with the same way IO / network calls are stateful.