[Updating Arrays in State] examples in the beta documentation are using impure function to explain concepts which doesn't seems right

Hi, learning react and going through the new react doc (beta).
I finished reading about pure functions, but then in a later section (update arrays in state), it showcase multiple examples that seem contradictory to the concept of pure functions in React...
OR I simply did not really understand that concept and If that is the case, I am sorry in advance.

let nextId = 0;

export default function List() {
  const [name, setName] = useState('');
  const [artists, setArtists] = useState([]);

  return (
    <>
      <h1>Inspiring sculptors:</h1>
      <input
        value={name}
        onChange={e => setName(e.target.value)}
      />
      <button onClick={() => {
        setName('');
        artists.push({
          id: nextId++,
          name: name,
        });
      }}>Add</button>

  //rest of the code...

isnt using a variable from outside (in this case: let nextId = 0) and just modifying it against the concept of pure function in react?
If this is indeed an impure function, I just think it would be better to promote using pure functions in the visual examples just to not confuse new learners or at least mention that it was made on purpose.

Beginne mit dem Abschnitt „Adding to an array“ in der verlinkten Dokumentation „Update Arrays in State“ und vergleiche die Beispiele darin mit den Hinweisen zu Pure Functions. Ermittle, ob die Beispiele zu nextId und artists.push eine Klarstellung oder einen Ersatz benötigen, und betrachte die Arbeit als abgeschlossen, wenn die Dokumentation diese Konzepte nicht mehr verwechselt.