UserProvider.js

const persistUserToStore = user => localStorage.setItem(
    "user",
    JSON.stringify(user)
);

class UserProvider extends Component {

    state = {
        user: {....}
    };

    persistUser = () => {
        persistUserToStore(this.state.user);
    }

    componentDidMount() {
        window.addEventListener("beforeunload", this.persistUser);
    }

    componentWillUnmount() {
        this.persistUser();
        window.removeEventListener("beforeunload", this.persistUser);
    }
}