JavaScript Code does not run in wanted order (Node.js, MongoDB)

Question

I know about Node.js's non blocking I/O and what are async functions but I'm having trouble to point down why this code runs like it runs.

I'm connecting to a MongoDB collection, searching for duplicates, putting the first values that repeat themselves in an array object (dupIndex). I see 2 values when the array prints (console.log(dupIndex);), but 0 when I use the .length property later (console.log(dupIndex.length);) -- when I'm actually expecting 2.

I would like to continue and manipulate the collection with the data I have in dupIndex (like use deleteMany method), but if it shows 0, I can't, at least not like this. Can someone explain this and help me with this?

Thanks!

//connect us to a running MongoDB server
const {MongoClient, ObjectID} = require('mongodb');

var dataBase = "TodoApp";
//connecting to a database
//connects to the database /TodoApp, if no such db exists it auto creates it
MongoClient.connect(`mongodb://localhost:27017/${dataBase}`, (err, db)=>{
    if(err){
        //return or 'else' - so if an error happens, it won't continue
        return console.log(`Unable to connect. Error: ${err}`);
    }

    console.log(`Connected to MongoDB server. Database: ${dataBase}.`);
        var dupIndex = [];
    //find all
    db.collection('Todos')
    .find()
    .toArray().then((docs) => {
        for ( var i =0; i< docs.length -1; i++){
            for(var j =i+1; j< docs.length; j++){
                    if(docs[i].text === docs[j].text)
                    {   
                        console.log(`in`);
                        dupIndex.push(docs[i].text);
                        i++;
                    }
            }
        }
        console.log(dupIndex);

    }, (err)=> {
        console.log(`unable t o fetch`);
    });

    console.log(dupIndex.length);
    // for(var i = 0; dupIndex)
    //close the connection to the db
    db.close();
});

Answer 1

Because console.log(dupIndex.length); runs before your nested loops.

db.collection('Todos')
.find()
.toArray()

This is an async call and control is passed to console.log(dupIndex.length); try writing console.log(dupIndex.length); next to console.log(dupIndex);

For Example:

 db.collection('Todos')
.find()
.toArray().then((docs) => {
    for ( var i =0; i< docs.length -1; i++){
        for(var j =i+1; j< docs.length; j++){
                if(docs[i].text === docs[j].text)
                {   
                    dupIndex.push(docs[i].text);
                    i++;
                }
        }
    }
    return dupIndex;
}, (dupIndexRecieved)=> {
    console.log(dupIndexRecieved.length); data recieved here
}, (err)=> {
    console.log(`unable t o fetch`);
});

Answer 2

//connect us to a running MongoDB server

const {MongoClient, ObjectID} = require('mongodb');

var dataBase = "TodoApp";
//connecting to a database
//connects to the database /TodoApp, if no such db exists it auto creates it
MongoClient.connect(`mongodb://localhost:27017/${dataBase}`, (err, db)=>{
if(err){
    //return or 'else' - so if an error happens, it won't continue
    return console.log(`Unable to connect. Error: ${err}`);
}

console.log(`Connected to MongoDB server. Database: ${dataBase}.`);
    var dupIndex = [];
//find all
db.collection('Todos')
.find()
.toArray().then((docs) => {
    for ( var i =0; i< docs.length -1; i++){
        for(var j =i+1; j< docs.length; j++){
                if(docs[i].text === docs[j].text)
                {   
                    console.log(`in`);
                    dupIndex.push(docs[i].text);
                    i++;
                }
        }
    }

    //Mongo db find returns a promise. The value you are accessing with the then function. So whatever you want to do with the db query return value you have to do it here inside the then function.

    console.log(dupIndex.length);

    console.log(dupIndex);
    // for(var i = 0; dupIndex)
    //close the connection to the db
    db.close();

}, (err)=> {
    console.log(`unable t o fetch`);
});

//when you make the call here its get called before the then function. Thats why the length is zero. 

});