You can write pure code in almost any language, though some make it easier than others with features like lexical closures and algebraic data types. But is that the idiomatic form? Is it evaluated efficiently? And can you count on libraries to be written the same way?
Javascript without any support for mutation or other side effects wouldn't really be recognizable as Javascript any more.
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.
