我在这里看到两个问题：
1.最简单的代码修复方法是更改以下行：

.then(firebaseAuth.sendPasswordResetEmail(auth, user.attr.email, null))

到

.then(() => firebaseAuth.sendPasswordResetEmail(auth, user.attr.email, null))

请注意，在您的代码相当于要求firebaseAuth返回一个处理承诺链中该步骤的函数之前，arrow函数 Package 了对firebaseAuth的调用。
1.如果return new Promise()的唯一目的是进行验证并获取用户，则可以简单地。

.then((userCredential) => {
   if(userCredential == undefined) throw Error("createUserWithEmailAndPassword failed");
    return new User(userCredential.user.uid, req.body.params.userAttributes);
  })

型
并且用户将在下一个链中可用，

.then(user => {
  // do stuff with user
})

您可以使用async函数和try块来awaituserCredential值，如下所示：

app.post('/create_user', async(req, res) => {
  user_uid = req.body.params.uid;
  newUserEmail = req.body.params.email;
  newUserPassword = req.body.params.password;
  let user;

  try {
    const userCredential =
      await firebaseAuth.createUserWithEmailAndPassword(
        auth,
        newUserEmail,
        newUserPassword
      );

    if (userCredential == undefined)
      return res
        .status(400)
        .send('❌ CreateUserWithEmailAndPassword failed 🙁');

    user = new User(
      userCredential.user.uid,
      req.body.params.userAttributes
    );

    await firebaseAuth.sendPasswordResetEmail(auth, user.attr.email, null);

    return res.status(201).send('Created!! 😀');
  } catch (error) {
    console.log('❌ Error:', error);
    return res.status(400).json({
      message: error,
    });
  }
});

我们永远不会发现new Promise() Package 了一个已经创造了承诺的库。
还要记住，链的形式是......

// promiseA, promiseB & promiseC are expressions that evaluate to promises
return promiseA.then(resolutionOfPromiseA => {
  // synchronous work
  return promiseB;
}).then(resolutionOfPromiseB => {
  // synchronous work
  return promiseC;
}).catch(error => {})

在更新的语法中：

async function myFunction() {
  try {
    let resolutionOfPromiseA = await promiseA;
    // synchronous work
    let resolutionOfPromiseB = await promiseB;
    // synchronous work
    return promiseC;
  } catch(error) {
  }

坚持OP的旧语法风格（只要保持一致就很好）

let user;
 firebaseAuth.createUserWithEmailAndPassword(auth, newUserEmail, newUserPassword)
 .then(userCredential => {
     // note that we don't create a promise
     if(userCredential == undefined) throw Error("createUserWithEmailAndPassword failed");
     user = new User(userCredential.user.uid, req.body.params.userAttributes);
     // note that we return this one. It's good form, even if the next then doesn't use the result
     return firebaseAuth.sendPasswordResetEmail(auth, user.attr.email, null)
  })
 .then(res => {   // this res is the result of firebaseAuth.sendPasswordResetEmail
   // user is in scope here because it's in the containing scope
   /* other functions using User object */
 })
 .then(() => {                  // finished promise chaining
    res.status(200).send();
 })
 .catch((e) => {
    console.log(e)
    res.status(403).send();
})

型

edit如果你真的希望user在sendPasswordReset之后的块中，并且你真的不想在包含作用域中保留用户变量，你可以说...

// move let declaration here
  let user = new User(//...

  // string an extra return user here
  // some linters think this is a faux pas
  return firebaseAuth.sendPasswordResetEmail(auth, user.attr.email, null)
    .then(() => return user)
)}.then(user =>