javascript - 与 mongoDB(和 Mongoose )的 promise 链接。如果已发送响应，如何在 .then() 之后使用 .save() 并正确突破 promise 链？

Question

我有以下用于注册用户的代码。我首先验证用户输入的地方。其次，我检查用户是否已经存在，如果存在，它应该返回响应 400。如果不存在，则转到第 3 步并添加新用户。最后在第 4 步中返回新创建的条目。从逻辑上讲，它可以正常工作并将数据正确地添加到数据库中，但是它总是在 postman (从第 2 步开始)上以 'User already exists' 响应，即使它是一个新用户已将用户正确添加到D b。这让我觉得第三步是在发送第 2 步中的响应之前完成的，这意味着我没有正确链接 promise 。此外，新用户永远不会作为响应发回，我认为这是因为我没有正确使用 Promise.then() 和 user.save() 。我还收到以下错误(在代码之后发布)，我理解这意味着我在发送第一个响应后尝试发送第二个响应。我可以用 async 和 await 解决这个问题，但我想学习如何用这种方式来解决这个问题。谢谢，任何帮助表示赞赏。

const { User, validateUser } = require('../models/userModel');
const mongoose = require('mongoose');
const express = require('express');
const router = express.Router();

router.post('/', (req, res) => {
    return Promise.resolve()
        .then(() => {
            //Step 1: validae the user input and if there is an error, send 400 res and error message
            console.log('My user post body req::', req.body);
            const { error } = validateUser(req.body); //this is using Joi.validate() which has a error property if errors are found
            if (error) {
                return res.status(400).send(error.details[0].message);
            }
        })
        .then(() => {
            //step 2: check if user already exists, if yes send res 400
            let user = User.findOne({ email: req.body.email });
            if (user) {
                return res.status(400).send('User already exists');
            }
        })
        .then(() => {
            //Step 3: enter new user into the database
            user = new User({
                name: req.body.name,
                email: req.body.email,
                password: req.body.password
            });
            return user.save();
        })
        .then((result) => {
            //step 4: return the newly added user
            return res.status(200).send(result);
        })
        .catch((error) => {
            console.log('Error Adding new User', error);
        });
});

module.exports = router;

我从 catch 中收到以下错误消息。即使我是，我也会带着每一个回应回来

Error Adding new User Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader (_http_outgoing.js:494:11)
    at ServerResponse.header (/home/ssaquif/WebDevProjects/movie-reviews-backend/node_modules/express/lib/response.js:771:10)
    at ServerResponse.send (/home/ssaquif/WebDevProjects/movie-reviews-backend/node_modules/express/lib/response.js:170:12)
    at ServerResponse.json (/home/ssaquif/WebDevProjects/movie-reviews-backend/node_modules/express/lib/response.js:267:15)
    at ServerResponse.send (/home/ssaquif/WebDevProjects/movie-reviews-backend/node_modules/express/lib/response.js:158:21)
    at /home/ssaquif/WebDevProjects/movie-reviews-backend/routes/users.js:35:27
    at processTicksAndRejections (internal/process/task_queues.js:93:5) {
  code: 'ERR_HTTP_HEADERS_SENT'

Answer 1

您不需要在您的 route 使用 Promise.resolve。您只需要一个 then block 链，您需要在其中将一个值返回给下一个 block 。

我像这样重构了你的代码:

router.post("/", (req, res) => {
    //Step 1: validate the user input and if there is an error, send 400 res and error message
    console.log("My user post body req::", req.body);
    const { error } = validateUser(req.body);
    if (error) {
      return res.status(400).send(error.details[0].message);
    }

    //step 2: check if user already exists, if yes send res 400
    User.findOne({ email: req.body.email })
      .then(user => {
        if (user) {
          return res.status(400).send("User already exists");
        }
        return;
      })
      .then(() => {
        //Step 3: enter new user into the database
        let user = new User({
          name: req.body.name,
          email: req.body.email,
          password: req.body.password
        });
        return user.save();
      })
      .then(result => {
        //step 4: return the newly added user
        return res.status(200).send(result);
      })
      .catch(error => {
        console.log("Error Adding new User", error);
        res.status(500).send("Error");
      });
  });

用户注册成功后会出现这样的结果:

{
    "_id": "5dd65df52f7f615d8067150d",
    "name": "ssaquif",
    "email": "test@test.com",
    "password": "123123",
    "__v": 0
}

当使用现有的电子邮件时，响应将像这样，状态码为 400。

User already exists