Edited the post!!!

id comeing trough login request, at the else branch at handleChangeId! it gets the correct id! i try to push at the top!

export function Login({handleChangeId}) {

const login = () => {

//node port egyezés szükséges
Axios.post('http://localhost:3001/login', {

    LoginUsername: usernameLog,
    LoginPassword: passwordLog,
}).then((response) => { 

    if (response.data.message) {
        setLoginCorrect(response.data.message)
    }

    else {
        handleChangeId(response.data[0].id);
        navigate("/App");
    }
});
};
}

than at App.js i try to get the id from the login Route, and push trough profile Route

let id = null;

function changeID(newId) {
  id= newId;
}

ReactDOM.render(
  <BrowserRouter>
    <Routes>
      <Route exact path="/" element={<Login handleChangeId={changeID} />}
      <Route exact path="/profile" element={<Profile id={id} />} />
    </Routes>
  </BrowserRouter>,
  document.getElementById('root')
); />

at Profile.js, i also try to get the id this way, but the value=null every way that i tried! And this is what im looking for - how to read and set the id and get their personal datas, when the users hit profile on the menubar!

export function Profile({ id }) {

    const [costumers, setCostumers] = useState([]);

    useEffect(() => {
        Axios.get(`http://localhost:3001/profile?userId=${id}`)
            .then((response) => {
                if (response) {

                    setCostumers(response.data);
                }
                else {
                    alert("Datas currently unavailable!")
                }
            });
    }, []);
}

CodePudding user response：

Issue

let id = null;

function changeID(newId) {
  id= newId;
}

Doesn't work because id is declared each render cycle with a null value, and simply mutating it doesn't trigger a component rerender with the updated value to be passed to the Profile component.

Solution

Make id part of the component state so updating it triggers a rerender with the updated value closed over in scope.

Example:

const [id, setId] = React.useState(null);

...

const changeID = (newId) => {
  setId(newId);
}

...

<Routes>
  <Route path="/" element={<Login handleChangeId={changeID} />}
  <Route
    path="/profile"
    element={<Profile id={id} />} // <-- updated id state value passed here
  />
</Routes>