localStorage在ReactJS和react-router-v4代码上重新调整`undefined`

Question

我正在构建一个简单的路由器来构建我的身份验证机制。该机制将使用存储在localStorage中的JWT来了解我的身份验证。这是我的代码：

import React, { Component } from "react";
import PropTypes from "prop-types"; 
import {
    BrowserRouter as Router,
    Switch,
    Route,
    Redirect
} 

const isAuthenticated = () => {

    let token = localStorage.getItem("jwtToken");

    console.log('Local Storage JWT TOKEN:');
    console.log(token); // <<<<=== undefined

    if (token) {
        return true;
    }

    return false;
};


const AuthenticatedRoute = ({ component: Component, ...rest }) =>

    <Route
        {...rest}
        render={props =>
            isAuthenticated()
                ? <Component {...props} />
                : <Redirect
                        to={{
                            pathname: "/landing",
                            state: { from: props.location }
                        }}
                    />}
    />;

class App extends Component {
    render() {

        return (
            <Router>
                <Switch>
                    <Route exact path="/landing" component={Landing} />
                    <Route exact path="/logout" component={Logout} />
                    <Route exact path="/pagenotfound" component={PageNotFound} />
                    <AuthenticatedRoute exact path="/" component={AppNav} />
                    <AuthenticatedRoute
                        exact
                        path="/:option"
                        component={AppNav}
                    />

                    <Route component={PageNotFound} />
                </Switch>
            </Router>
        );
    }
}

export default App;

由于某些原因，如果没有找到令牌，则localStorage.getItem返回undefined。我期待null符合预期here

为什么我的getItem返回undefined以及如何解决？