Firebase邮箱/密码登录失败：Cannot read property 'then' of undefined

核心语法错误（直接触发报错） #

你的代码存在明显的逻辑语法问题：在async函数中，你先用await调用signInWithEmailAndPassword，随后执行console.log语句，紧接着链式调用.then——这相当于在console.log的返回值（undefined）上调用.then，必然会抛出"Cannot read property 'then' of undefined"错误。

修复后的代码示例 #

二选一即可，不要混用await和.then/.catch：

写法1：用await + try/catch（适配async函数风格）

const addRating = async (id, rating) => { 
    try {
        console.log('auth: ', auth)
        console.log('email: ', email)
        console.log('password: ', password)
        
        const userCredential = await signInWithEmailAndPassword(auth, email, password)
        console.log('userCredential: ', userCredential)
        const user = userCredential.user;             
        // 执行后续Realtime Database读写逻辑
    } catch (error) {
        const errorCode = error.code;
        const errorMessage = error.message;
        console.log("addReviewRequest failing to log to firebase with: " + errorCode + " - " + errorMessage);
    }
};

写法2：用.then/.catch（不使用await）

const addRating = (id, rating) => { 
    console.log('auth: ', auth)
    console.log('email: ', email)
    console.log('password: ', password)
    
    signInWithEmailAndPassword(auth, email, password)
    .then((userCredential) => {     
        console.log('userCredential: ', userCredential)
        const user = userCredential.user;             
        // 执行后续Realtime Database读写逻辑
    })
    .catch((error) => {
        const errorCode = error.code;
        const errorMessage = error.message;
        console.log("addReviewRequest failing to log to firebase with: " + errorCode + " - " + errorMessage);
    });
};

额外排查点（解决潜在深层问题） #

1. 绑定Auth实例到Firebase App
   在firebase.js中，明确将初始化的app传入getAuth，避免隐式实例导致的异常：

   // 原代码
   export const auth = getAuth();
   // 修改为
   export const auth = getAuth(app);
   

2. 清理依赖缓存

   • 执行expo r -c清除Expo缓存
   • 删除node_modules和package-lock.json/yarn.lock，重新执行npm install/yarn install

3. 监听Auth状态变化
   通过onAuthStateChanged排查是否存在未预期的状态切换：

   onAuthStateChanged(auth, (user) => {
     console.log("Auth state changed: ", user);
   });
   

内容的提问来源于stack exchange，提问作者Butri