我是node.js的新手，正在学习教程，但是当我刷新localhost：3000/books时，代码返回错误
错误：Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
产品代码：app.js

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

console.log('starting');
// init app & middleware
const app = express();
console.log('init complete');

// db connection
let db;
connectToDb((err) => {
    console.log('connecting to db');
    if (!err) {
        app.listen(3000, () => {
            console.log('app listening on port 3000');
        });
        db = getDb();
    } else {
        console.log(err);
    }
});

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

    db.collection('books')
        .find() // cursor toArray forEach
        .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"});
});

和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
};

我用compass托管mongodb服务器：我有一个以bookstore命名的数据库和一个以books命名的集合在我拥有的书籍中：

[{
  "_id": {
    "$oid": "636faa18a918cf889958ea12"
  },
  "title": "Name of wind",
  "author": {
    "firstName": "Patrick",
    "lastName": "Rothfuss"
  },
  "pages": 500,
  "genres": [
    "fantassy",
    "magical"
  ],
  "rating": 9,
  "reviews": [
    {
      "name": "Ludwig",
      "body": "AWESOME!"
    }
  ]
},{
  "_id": {
    "$oid": "636facd8a918cf889958ea16"
  },
  "title": "Book of fire?",
  "author": {
    "firstName": "Willem",
    "lastName": "Johnson"
  },
  "pages": 350,
  "genres": [
    "fantassy",
    "comedy"
  ],
  "rating": 6,
  "reviews": [
    {
      "name": "Ludwig",
      "body": "BAD!"
    }
  ]
},{
  "_id": {
    "$oid": "636facd8a918cf889958ea17"
  },
  "title": "Thrones of destiny",
  "author": {
    "firstName": "Jonas",
    "lastName": "Adams"
  },
  "pages": 689,
  "genres": [
    "magical",
    "fantassy"
  ],
  "rating": 10,
  "reviews": [
    {
      "name": "Ludwig",
      "body": "The best!"
    }
  ]
},{
  "_id": {
    "$oid": "636facd8a918cf889958ea18"
  },
  "title": "Idiots!",
  "author": {
    "firstName": "Jack",
    "lastName": "Phiers"
  },
  "pages": 486,
  "genres": [
    "action",
    "comedy"
  ],
  "rating": 8,
  "reviews": [
    {
      "name": "Ludwig",
      "body": "Great!"
    }
  ]
},{
  "_id": {
    "$oid": "636facd8a918cf889958ea19"
  },
  "title": "Where are we?",
  "author": {
    "firstName": "Jone",
    "lastName": "Johnson"
  },
  "pages": 685,
  "genres": [
    "magical",
    "sci-fi"
  ],
  "rating": 7.6,
  "reviews": [
    {
      "name": "Ludwig",
      "body": "Good!"
    }
  ]
},{
  "_id": {
    "$oid": "63737fae2ea0cb2ca699eec7"
  },
  "title": "Colour Magic",
  "author": {
    "firstName": "Terry",
    "lastName": "Pretchett"
  },
  "pages": 768,
  "rating": 7,
  "genres": [
    "fantassy",
    "comedy"
  ]
}]

型
当注销我在app.js中创建的books数组时，它是一个空数组