Question

我尝试使用Node和MongoDB执行CRUD操作。

我得到以下错误。

TypeError：无法读取属性＆＃39;集合＆＃39;未定义的

const express = require('express')
const bodyParser= require('body-parser')
const MongoClient = require('mongodb').MongoClient
const app = express()
app.use(bodyParser.urlencoded({extended: true}))

const url = 'mongodb://localhost:27017/testdb';

var db
MongoClient.connect(url, function(err, client) {
  if (err) return console.log(err)
  var db = client.db('testdb');

})

app.post('/quotes', (req, res) => {
    var collectionName = 'quotes';
    var collection = db.collection(collectionName);
    collection.save(req.body, (err, result) => {
    if (err) return console.log(err)

    console.log('saved to database')
    res.redirect('/')
  })
})

错误跟踪：

TypeError：无法读取属性＆＃39;集合＆＃39;未定义的在app.post（/var/www/html/Express/SimpleCrud/server.js:20:21）在Layer.handle [as handle_request]（/var/www/html/Express/SimpleCrud/node_modules/express/lib/router/layer.js:95:5）在下一个（/var/www/html/Express/SimpleCrud/node_modules/express/lib/router/route.js:137:13）在Route.dispatch（/var/www/html/Express/SimpleCrud/node_modules/express/lib/router/route.js:112:3）在Layer.handle [as handle_request]（/var/www/html/Express/SimpleCrud/node_modules/express/lib/router/layer.js:95:5） at /var/www/html/Express/SimpleCrud/node_modules/express/lib/router/index.js:281:22 在Function.process_params（/var/www/html/Express/SimpleCrud/node_modules/express/lib/router/index.js:335:12）在下一个（/var/www/html/Express/SimpleCrud/node_modules/express/lib/router/index.js:275:10） at /var/www/html/Express/SimpleCrud/node_modules/body-parser/lib/read.js:130:5 在invokeCallback（/var/www/html/Express/SimpleCrud/node_modules/raw-body/index.js:224:16）

我正在使用Mongodb 3.1

Answer 1

这是一个简单的关闭问题 var是本地/全局范围，您有var db

的多个声明

var db  <====== first declaration which is undefined 
MongoClient.connect(url, function(err, client) {
 if (err) return console.log(err)
 var db = client.db('testdb');   <======= second declaration where u connected to 
 the db which is undefined out of this scope

})

我个人更喜欢猫鼬，只要它不是打字稿：D

几个例子：

const MongoClient = require('mongodb').MongoClient,
Server = require('mongodb').Server;

const mongoClient = new MongoClient(new Server('localhost', 27017));
mongoClient.open(function(err, mongoClient) {
const db1 = mongoClient.db("mydb");

mongoClient.close();
});

或

const MongoClient = require('mongodb').MongoClient;
const assert = require('assert');

// Connection URL
const url = 'mongodb://localhost:27017';

// Database Name
const dbName = 'myproject';

//Use connect method to connect to the server
MongoClient.connect(url, function(err, client) {
  assert.equal(null, err);
  console.log("Connected successfully to server");

  const db = client.db(dbName);

  client.close();
});

source

Answer 2

代码中存在简单的错误。在与mongodb服务器建立连接之前，您已启动了http服务器。这就是当你的post api加载到内存中时变量db未定义的原因。由于与mongodb的连接是异步的。为了利用mongodb实例，您必须首先连接mongdb服务器，然后启动http服务器并将数据库实例全局存储或存储在您需要的某个模块中，并获取数据库实例以执行查询。

无法阅读财产＆＃39;收集＆＃39;未定义的MongoDB

2 个答案: