javascript - 当我尝试在下面的代码中访问 db.collection 时，为什么会出现未定义的情况？

Question

var express = require('express');
var GoogleUrl = require('google-url');
var favicon = require('serve-favicon');
var mongo = require('mongodb').MongoClient;
var app = express();
var db;
var googleUrl = new GoogleUrl({key: '********'});
var PORT = 8080;


mongo.connect('mongodb://localhost:27017/url-shortener', function(err, db){
   if(err){
       throw new Error('Database failed to connect');
   } else{
       console.log('Successfully connected to MongoDB on port 27017');
   }
   db.createCollection('sites', {
      autoIndexID: true 
   });
   db.close();
});

app.use(favicon(__dirname+'/public/favicon.ico'));


app.get('/new/*', function(req, res){
   console.log('This is the url: '+req.params[0]);
   googleUrl.shorten(req.params[0], function(err, shortUrl){
       if(err){
           console.log(err);
       }else{
           console.log(shortUrl);
       }
       check_db(req.params[0], shortUrl, db);
   });
});


app.listen(PORT, function(){
    console.log('Express listening on: '+PORT);
});

//When I try to call my db.collection method here, I get an error statement saying that TypeError: Cannot read property 'collection' of undefined.

function check_db(longUrl, shortUrl, db){
    db.collection('sites').findOne({
        'longUrl': longUrl, 
        'shortUrl': shortUrl
    }, function(err, result){
        if(err){
            throw new Error(err);
        }if(result){
            console.log('This site already exists on the database');
        }else{
            console.log('This site does not exist on the database');
        }
    });
}

I have made sure to declare my db variable globally and I even passed it to the check_db function to make sure.

Following is the error statement in full

TypeError: Cannot read property 'collection' of undefined at check_db (/home/ubuntu/workspace/urlshortener/server.js:45:7) at /home/ubuntu/workspace/urlshortener/server.js:35:8 at /home/ubuntu/workspace/urlshortener/node_modules/google-url/lib/google.js:18:7 at Immediate._onImmediate (/home/ubuntu/workspace/urlshortener/node_modules/google-url/lib/google.js:163:11) at processImmediate [as _immediateCallback] (timers.js:383:17)

Answer 1

您的数据库未定义。您必须将其分配给连接回调中创建的数据库。

mongo.connect('mongodb://localhost:27017/url-shortener', function(err, newDb){
   if(err){
       throw new Error('Database failed to connect');
   } else{
       console.log('Successfully connected to MongoDB on port 27017');
   }
   db = newDb; // ADD THIS
   db.createCollection('sites', {
      autoIndexID: true 
   });
   // db.close();
});

编辑:

正如@notionquest的回答所指出的，您还必须保持连接打开，直到您的请求完成为止。