In React terms, "functional" almost always implies the opposite of purely functional. It just means that the component is declared by a function instead of a class. First time you call it, the function can assign state that it may reference in future calls, not unlike the methods of a class. How they actually go about this is however almost entirely weird.
Classes in JS are just functions anyway, and functions can have state. Saving the state of a function is a different thing, but of course JS allows that, too. Why is it weird?
Class instances are objects and it's the objects that have state. Functions can only abuse themselves as objects to store state.
function johnson() { johnson.state = {}; }
This is however shared state and the state will be reset whenever the function is called a second time. So what you normally do with a function is to pass the state via arguments and now the same function can work with different states. React could totally do this via props or as a second argument.
function Johnson(props, state) {}
Instead they changed the laws of functions so that a function can act different the first time it is called and also be called the first time multiple times if and when the function calls a functions that looks like JavaScript but follow different rules [1].
