TypeError in Node.js application - node.js

Question

I've copied this example program from Node.js textbook:
var MongoClient = require('mongodb').MongoClient;
var website = {
url: 'http://www.google.com',
visits: 0
};
var findKey = {
url: 'www.google.com'
}
MongoClient.connect('mongodb://127.0.0.1:27017/demo', { useNewUrlParser: true }, function(err, client) {
var db = client.db('demo');
if(err) throw err;
var collection = db.collection('websites');
collection.insert(website, function(err, docs) {
var done = 0;
function onDone(err) {
done++;
if(done < 4) return;
collection.find(findKey).toArray(function(err, results) {
console.log('Visits:', results[0].visits);
//cleanup
collection.drop(function() {
client.close();
});
});
}
var incrementVisits = {
'$inc': {
'visits': 1
}
};
collection.update(findKey, incrementVisits, onDone);
collection.update(findKey, incrementVisits, onDone);
collection.update(findKey, incrementVisits, onDone);
collection.update(findKey, incrementVisits, onDone);
});
});
It throws this error when I run it:
/Users/me/Documents/Beginning NodeJS/node_modules/mongodb/lib/utils.js:132
throw err;
^
TypeError: Cannot read property 'visits' of undefined
at /Users/me/Documents/Beginning NodeJS/update/2update.js:26:43
at result (/Users/me/Documents/Beginning NodeJS/node_modules/mongodb/lib/utils.js:414:17)
at executeCallback (/Users/me/Documents/Beginning NodeJS/node_modules/mongodb/lib/utils.js:406:9)
at handleCallback (/Users/me/Documents/Beginning NodeJS/node_modules/mongodb/lib/utils.js:128:55)
at self.close (/Users/me/Documents/Beginning NodeJS/node_modules/mongodb/lib/cursor.js:905:60)
at handleCallback (/Users/me/Documents/Beginning NodeJS/node_modules/mongodb/lib/utils.js:128:55)
at completeClose (/Users/me/Documents/Beginning NodeJS/node_modules/mongodb/lib/cursor.js:1044:14)
at Cursor.close (/Users/me/Documents/Beginning NodeJS/node_modules/mongodb/lib/cursor.js:1057:10)
at /Users/me/Documents/Beginning NodeJS/node_modules/mongodb/lib/cursor.js:905:21
at handleCallback (/Users/me/Documents/Beginning NodeJS/node_modules/mongodb-core/lib/cursor.js:199:5)
I can't see whats wrong here but the textbook is a few years old and I've already had issues where the code was out of date and wouldn't work so I want to check if that is the case here.

Answer 1

It's a pretty horrible example you are following, but basically there are errors there essentially compounded from http:///www.google.com which is created as the value in the document is different to www.google.com, therefore you don't get a result and it's undefined when trying to read a property from an empty array.
The basic corrections would be to fix that, and actually use findOneAndUpdate() in all cases, since that will atomically return a document.
var MongoClient = require('mongodb').MongoClient;
var website = {
url: 'http://www.google.com',
visits: 0
};
var findKey = {
url: 'http://www.google.com'
}
MongoClient.connect('mongodb://127.0.0.1:27017/demo', { useNewUrlParser: true }, function(err, client) {
var db = client.db('demo');
if(err) throw err;
var collection = db.collection('websites');
collection.findOneAndUpdate(
findKey, website, { upsert: true },function(err, doc) {
var done = 0;
function onDone(err,doc) {
done++;
console.log("Visits: %s", doc.value.visits);
if (done >= 4) {
collection.drop(function(err) {
client.close();
});
}
}
var incrementVisits = {
'$inc': {
'visits': 1
}
};
var options = { returnOriginal: false };
collection.findOneAndUpdate(findKey, incrementVisits, options, onDone);
collection.findOneAndUpdate(findKey, incrementVisits, options, onDone);
collection.findOneAndUpdate(findKey, incrementVisits, options, onDone);
collection.findOneAndUpdate(findKey, incrementVisits, options, onDone);
});
});
Note those "four" calls at the end do not resolve immediately. These simply queue up async functions and there is no guaranteed order to their execution.
But the script will return:
Visits: 1
Visits: 2
Visits: 3
Visits: 4
A much better and "modern" example would instead be:
const { MongoClient } = require("mongodb");
const uri = "mongodb://localhost:27017/";
const options = { useNewUrlParser: true };
const website = {
url: 'http://www.google.com',
visits: 0
};
const findKey = { url: 'http://www.google.com' };
(async function() {
try {
const client = await MongoClient.connect(uri,options);
const db = client.db('demo');
const collection = db.collection('websites');
await collection.insertOne(website);
var times = 4;
while (times--) {
let doc = await collection.findOneAndUpdate(
findKey,
{ $inc: { visits: 1 } },
{ returnOriginal: false },
);
console.log("Visits: %s", doc.value.visits);
}
await collection.drop();
client.close();
} catch(e) {
console.error(e);
} finally {
process.exit();
}
})()
Since we actually await each call executed in the while loop, we guarantee that these are actually executed sequentially. We also await everything, so the code is clean and ordered and we can just hang up the database connection when everything is done, without waiting on callbacks to resolve or other methods.

Answer 2

It seems you Mongo instance returns some kind of error, which makes the results parameter undefined. So, check for errors in the line before (which you should do anyway, but maybe with a more sophisticated error handling):
collection.find(findKey).toArray(function(err, results) {
// this is added
if( err ) {
console.log( err );
return;
}
console.log('Visits:', results[0].visits);
//cleanup
collection.drop(function() {
client.close();
});
});

Answer 3

Instead of
console.log('Visits:', results[0].visits);
Try printing out :
console.log('Visits:', results[0]);
so that from results[0] you can check if there exits a property 'visits'