Private Chat React Native; what is happening is useEffect

PrivateChat-RN-useEffect

useEffect(() => {
  readUser();
  const unsubscribe = chatsRef.onSnapshot((querySnapshot) => {
      const messagesFirestore = querySnapshot
          .docChanges()
          .filter(({ type }) => type === 'added')
          .map(({ doc }) => {
              const message = doc.data();
              return {
                  ...message,
                  createdAt: message.createdAt.toDate(),
              };
          })
          .sort((a, b) => b.createdAt.getTime() - a.createdAt.getTime());
      appendMessages(messagesFirestore);
  });
  return () => unsubscribe();
}, []);

const appendMessages = useCallback(
  (messages) => {
      setMessages((previousMessages) =>
          GiftedChat.append(previousMessages, messages)
      );
  },
  [messages]
);

async function readUser() {
  const user = await AsyncStorage.getItem('user');
  if (user) {
      setUser(JSON.parse(user));
  }
}