要检查用户是否已登录，您可以使用'onAuthStateChanged'方法。

将此信息存储在组件的状态中，或者使用它来有条件地渲染组件的部分。

import React, { useEffect, useState } from 'react'; import firebase from 'firebase/app'; import 'firebase/auth'; export default function Home() { const [user, setUser] = useState(null); useEffect(() => { const unsubscribe = firebase.auth().onAuthStateChanged(user => { setUser(user); }); return () => unsubscribe(); }, []); return ( 
 
Hello World
 {user ? ( 
authenticated user
 ) : null} 
 ); }

要在服务器端执行用户身份验证，Next.js提供了'getServerSideProps'来获取用户的身份验证状态

import firebase from 'firebase/app'; import 'firebase/auth'; export default function Home({ user }) { return ( 
 
Hello World
 {user ? ( 
authenticated user
 ) : null} 
 ); } export async function getServerSideProps(context) { const user = await new Promise((resolve, reject) => { firebase.auth().onAuthStateChanged(user => { resolve(user); }); }); return { props: { user, }, }; }

更新的解决方案：

创建一个服务器端路由

import firebase from 'firebase/app'; import 'firebase/auth'; import express from 'express'; const app = express(); app.get('/api/user', async (req, res) => { const user = await new Promise((resolve, reject) => { firebase.auth().onAuthStateChanged(user => { resolve(user); }); }); res.send(user); }); app.listen(3000, () => { console.log('Server started at http://localhost:3000'); });

客户端代码

import React, { useState, useEffect } from 'react'; import axios from 'axios'; export default function Home() { const [user, setUser] = useState(null); useEffect(() => { const fetchUser = async () => { const res = await axios.get('http://localhost:3000/api/user'); const user = res.data; setUser(user); }; fetchUser(); }, []); return ( 
 
Hello World
 {user ? ( 
authenticated user
 ) : null} 
 ); }