I'm doing mongodb database after tutorial from NetNinja. Since I'm kinda bad with js, I'm just following his actions. I'm stuck with:

我正在做一个又一个的MongoDB数据库教程。因为我和js的关系不好，所以我只是跟着他的动作走。我被困住了：

node:internal/errors:496
    ErrorCaptureStackTrace(err);
    ^
...
Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

I'm having only 2 files:
app.js

我只有两个文件：app.js

const express = require('express')
const { connectToDb, getDb} = require('./db')

const app = express()

let db

connectToDb((err) => {
    if(!err) {
        app.listen(3000, () => {
            console.log('app listening on port 3000')
        })
        db = getDb()
    }
})

app.get('/books/books', (req, res) => {
    let books = []

    db.collection('books')
        .find()
        .sort({ author: 1 })
        .forEach(book => books.push(book))
        .then(() => {
            res.status(200).json(books)
        })
        .catch(() => {
            res.status(500).json({error : 'Could not fetch the documents'})
        })

    res.json({mssg: "welcome to the api"})
})

db.js

Db.js

const { MongoClient } = require('mongodb')

let dbConnection

module.exports = {
    connectToDb: (cb) => {
        MongoClient.connect('mongodb://0.0.0.0:27017/bookstore')
            .then((client) => {
                dbConnection = client.db()
                return cb()
            })
            .catch(err => {
                console.log(err)
                return cb(err)
            })
    },
    getDb: () => dbConnection
}

I found the place that throws me this error, when I'm removing it, there's no error:
app.js, app.get block

我找到抛出这个错误的位置，当我删除它时，没有错误：app.js，app.get块

.then(() => {
            res.status(200).json(books)
        })

Since NetNinja has no problems, error could be triggered by different versions of Node, but I dunno for sure.
Is there a replacement to '.then', that'll let me use database?

由于网忍者没有问题，错误可能会被不同版本的Node触发，但我不确定。有没有替代‘.Then’的方法，让我可以使用数据库？

The error in the code you provided is that you're sending a response (res.json({mssg: "welcome to the api"})) after making a database query. In Node.js and Express, the response should only be sent once. When you send a response, you can't send another one.

您提供的代码中的错误在于您在进行数据库查询之后发送了一个响应(res.json({msg：“欢迎使用API”}))。在Node.js和Express中，响应应该只发送一次。当您发送响应时，您不能再发送另一个响应。

This error does not occur because of the node version or .then its occuing because you are sending a response 2 time

出现此错误不是因为节点版本或。而是因为您正在发送响应2次

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

First:

首先：

res.json({mssg: "welcome to the api"});

Second:

第二：

res.status(200).json(books);

Fix:

修复：

just remove the second response

删除第二个响应

app.get('/books/books', (req, res) => {
    let books = []

    db.collection('books')
        .find()
        .sort({ author: 1 })
        .forEach(book => books.push(book))
        .then(() => {
            res.status(200).json(books)
        })
        .catch(() => {
            res.status(500).json({error : 'Could not fetch the documents'})
        })

    
})