Stuck on Node + MongoDB asynchronous query issue

Question

A little preface: I am very new to working with Node so please bear with my ignorance

I am trying to pass some info from an array in Node.js, and check whether it exists in a MongoDB document. I am still struggling to wrap my head around Node and how to work with databases asynchronously.

I have the following code

for (i in articleTitle) {

    console.log(articleTitle[i]);

    // Use connect method to connect to the Server 
    MongoClient.connect(mongoUrl, function(err, db) {
        if (err) throw err; // Throw error
        var query = { title: articleTitle[i] }; // Query Parameter

        // Perform Query
        db.collection(mongoCollection).find(query).toArray(function(err, result) {
            if (err) throw err; // Throw error
            if (result == '') {
                console.log('No results found for title:', articleTitle[i]);
            } else {
                console.log('Found an entry');
            }

            db.close(); // Close connection
        });
    });
}

In the above code, I have an array of strings called articleTitle (for example: ['Title1', 'Title2', 'Title3']), I then run through each of those titles in the array (using the for() loop) to check if each title exists in the database.

The output I get is as follows:

> Title1
> Title2
> Title3
> No results found for title: Title 3
> No results found for title: Title 3
> No results found for title: Title 3

As evident above it seems to be checking for the last object in the array three times. I have also tried to implement the async package but struggled with that as well.

Any help would be appreciated.

Zac Webb · Accepted Answer · 2017-08-08 23:41:44Z

2

The issue you have is scope of the variable i in the callback function.

Use for (let i in articleTitle) instead. This creates a new variable i for every iteration and scope is restricted to that iteration.

The answers to this question JavaScript closure inside loops – simple practical example explain in detail about why this happens and about scope and closure in JavaScript. The above question is an exact duplicate of this question.

edited Aug 8, 2017 at 23:41

Zac Webb

6636 silver badges22 bronze badges

answered Aug 8, 2017 at 23:35

Sriraghavan Subramanian

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1 Comment

Zac Webb Over a year ago

That worked perfectly. Thanks for the further information. (Will mark as answer in 7 minutes ;p)

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