使用Moongose处理node.js中重定向的then()块链

我有以下代码，但收到一条错误消息，指出我正在try 在发送重定向后修改标头.我认为这就是我处理if陈述和返回的方式.我已经修改了代码，现在重定向可以工作了，但我想我误解了逻辑.请参阅此处的第一个代码:

exports.postSignup = (req, res) => {
  const email = req.body.email;
  const password = req.body.password;
  const confirmPassword = req.body.confirmPassword;
  // check if user exist
  User.findOne({ emailAddress: email })
    .then((user) => {
      if (user) {
        console.log("user exists please log in!");
        return res.redirect("/auth/login");
      }
      const newUser = new User({
        emailAddress: email,
        password: password,
        cart: {
          products: [],
        },
      });
      return newUser.save();
    })
    .then((result) => {
        console.log("added user");
        res.redirect("/auth/login");
    })
    .catch((e) => {
      console.log(e);
      throw e;
    });
};

有效的固定版本:

exports.postSignup = (req, res) => {
  const email = req.body.email;
  const password = req.body.password;
  const confirmPassword = req.body.confirmPassword;
  // check if user exist
  User.findOne({ emailAddress: email })
    .then((user) => {
      if (user) {
        console.log("user exists please log in!");
        return res.redirect("/auth/login");
      }
      const newUser = new User({
        emailAddress: email,
        password: password,
        cart: {
          products: [],
        },
      });
      return newUser.save();
    })
    .then((result) => {
      // this makes the redirect only if the user is created
      if (result) {
        console.log("added user");
        res.redirect("/auth/login");
      }
    })
    .catch((e) => {
      console.log(e);
      throw e;
    });
};

有人能帮我弄清楚为什么早期版本不起作用吗？我只能想象我调用了两次重定向().