Aidan Collins Aidan Collins - 5 months ago 53
Node.js Question

Mongodb find() returns undefined (node.js)

Ive been playing around with mongodb in node.js. I have made a basic collection with some data (i know its there ive checked). When I try to run a find() on the collection it returns undefined. I dont know why this is. The code is below:

function get_accounts(){
var MongoClient = mongodb.MongoClient;
var url = "url";

MongoClient.connect(url, function (err, db) {
if (err) {
console.log('Unable to connect to the mongoDB server. Error:', err);
} else {
//HURRAY!! We are connected. :)
console.log('Connection established to database');
var collection = db.collection('accounts');
collection.find().toArray(function(err, docs) {
console.log("Printing docs from Array")
docs.forEach(function(doc) {
console.log("Doc from Array ");
console.dir(doc);
});
});
console.log("mission complete");
}
db.close();
}
);
}


If you know why this is happening i would like to hear your thoughts. thanks! The database is a mongolab hosted database if that makes any difference.

Answer

You are getting an undefined value because of the asynchronous nature of node.js, nowhere in your code exists logic that tells the console.log statement to wait until the find() statement finishes before it prints out the documents. You have to understand the concept of callbacks in Node.js. There are a few problems here, though, that you could fix. A lot of people getting started with node have the tendency to nest lots of anonymous functions, creating the dreaded "pyramid of doom" or callback hell. By breaking out some functions and naming them, you can make it a lot cleaner and easier to follow:

var MongoClient = require("mongodb").MongoClient

// move connecting to mongo logic into a function to avoid the "pyramid of doom"
function getConnection(cb) {  
    MongoClient.connect("your-mongo-url", function(err, db) {
        if (err) return cb(err);
        var accounts = db.collection("accounts");
        cb(null, accounts);
    })
}    
// list all of the documents by passing an empty selector.
// This returns a 'cursor' which allows you to walk through the documents
function readAll(collection, cb) {  
   collection.find({}, cb);
}

function printAccount(account) {  
    // make sure you found your account!
    if (!account) {
        console.log("Couldn't find the account you asked for!");
    }
    console.log("Account from Array "+ account);
}

// the each method allows you to walk through the result set, 
// notice the callback, as every time the callback
// is called, there is another chance of an error
function printAccounts(accounts, cb) {  
    accounts.each(function(err, account) {
        if (err) return cb(err);
        printAccount(account);
    });
}

function get_accounts(cb) {  
    getConnection(function(err, collection) {
        if (err) return cb(err);    
        // need to make sure to close the database, otherwise the process
        // won't stop
        function processAccounts(err, accounts) {
            if (err) return cb(err);
            // the callback to each is called for every result, 
            // once it returns a null, you know
            // the result set is done
            accounts.each(function(err, account) {
                if (err) return cb(err)  
                if (hero) {  
                    printAccount(account);
                } else {
                    collection.db.close();
                    cb();
                }
            })
        }
        readAll(collection, processAccounts);        
    })
}

// Call the get_accounts function
get_accounts(function(err) {  
     if (err) {
         console.log("had an error!", err);
         process.exit(1);
     }
});