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How to get data from back end side, to use it in the browser side?

I am new to programming, and I heard that some guys on this website are quite angry, but please don't be. I am creating one web app, that has a web page and also makes som ecalculations and works with database (NeDB). I have an index.js
const selects = document.getElementsByClassName("sel");
const arr = ["Yura", "Nairi", "Mher", "Hayko"];
for (let el in selects) {
for (let key in arr) {
selects[el].innerHTML += `<option>${arr[key]}</option>`;
}
}
I have a function which fills the select elements with data from an array.
In other file named: getData.js:
var Datastore = require("nedb");
var users = new Datastore({ filename: "players" });
users.loadDatabase();
const names = [];
users.find({}, function (err, doc) {
for (let key in doc) {
names.push(doc[key].name);
}
});
I have some code that gets data from db and puts it in array. And I need that data to use in the index.js mentioned above, but the problem is that I don't know how to tranfer the data from getData.js to index.js. I have tried module.exports but it is not working, the browser console says that it can't recognize require keyword, I also can't get data directly in index.js because the browse can't recognize the code related to database.

You need to provide a server, which is connected to the Database.
Browser -> Server -> DB
Browser -> Server: Server provides endpoints where the Browser(Client) can fetch data from. https://expressjs.com/en/starter/hello-world.html
Server -> DB: gets the Data out of the Database and can do whatever it want with it. In your case the Data should get provided to the Client.
TODOs
Step 1: set up a server. For example with express.js (google it)
Step 2: learn how to fetch Data from the Browser(Client) AJAX GET are the keywords to google.
Step 3: setup a Database connection from you Server and get your data
Step 4: Do whatever you want with your data.

At first I thought it is a simple method, but them I researched a little bit and realized that I didn't have enough information about how it really works. Now I solved the problem, using promises and templete engine ejs. Thank you all for your time. I appreciate your help)

MongoDB Atlas - Create and db from a script

i'm trying to create collection from a script in mongodb, i found on this link a useful script to do that Make a script to create MongoDB collections from Mongo shell? (thanks for that script).
I'm using now this kind of script and it work fine:
db = db.getSiblingDB('emanueledb');
var collectionList = ["collection1", "collection2", "collection3"]
collectionList.forEach(function(collName) {
db.createCollection(collName, {autoIndexId: true});
});
I'm trying to cycle also some db but with this kind of code:
var dblist = ["emanueledb", "db_ema_dv"];
var collectionList = ["ab111", "bc111"];
dblist.forEach(function(dbname) { db.getSiblingDB(dbname)
collectionList.forEach(function(collName) {
db.createCollection(collName, {autoIndexId: true});
})
});
it works but create only the two collection on db_ema_dv. What is wrong?
Is also possible to get collection and db from an external csv to automate everything??
CSV example:
dbname,collectionname
emanueledb,collection1
emanueledb,collection2
testdb,testcollection
Thanks

In the code you posted this expression does nothing (it selects the database but you don't do anything with the database):
db.getSiblingDB(dbname)
Try:
db.getSiblingDB(dbname).createCollection(collName, {autoIndexId: true});

Perfect, thank you! Now works!
var dblist = ["emanueledb", "db_ema_dv"];
var collectionList = ["ab111111", "bc111111"];
dblist.forEach(function(dbname) {
collectionList.forEach(function(collName) {
db.getSiblingDB(dbname).createCollection(collName, {autoIndexId: true});
})
});
as requested before, is possibile to get db and collection from a csv?
CSV Example:
dblist,collectionList
emanueledb,collection1
emanueledb,collection2
testdb,testcollection

To get a database from a .csv you can use the mongoimport command. This will read a CSV file into a collection.

Express with JSON Data Control

I use lowDB dependency to control the JSON Data with Express and actually it works. But there is a bug and I cannot find how to solve it.
I create /create page to add information in JSON file and it contains 4 form and submit button.
And In express I code like this. each forms data will save it in variable and push with lowdb module.
router.post('/post', function (req, res) {
let pjName = req.body.projectName;
let pjURL = req.body.projectURL;
let pjtExplanation = req.body.projectExplanation;
let pjImgURL = req.body.projectImgURL;
console.log(pjName);
db.get('project').push({
name: pjName,
url: pjURL,
explanation: pjtExplanation,
imgurl: pjImgURL
}).write();
console.log(db.get('project'));
console.log(db.get('project').value());
res.redirect('/');
})
And it works well. But when I modify the JSON file myself (ex. reset the JSON file) and execute again. It shows the data that I reset before. I think in this app somewhere saves the all data and show save it in array again.
And When I shutdown the app in CMD and execute again, the Array is initialized.

As you may already know the lowdb persist the data into your secondary memory (hdd), and may return a promise depending on your environment when you call write method.As mentioned in the doc
Persists database using adapter.write (depending on the adapter, may return a promise).
So the data may be still getting write when you read them, so the old data is queried. Try this,
db.get('project').push({
name: pjName,
url: pjURL,
explanation: pjtExplanation,
imgurl: pjImgURL
}).write().then(() => {
console.log(db.get('project'));
console.log(db.get('project').value());
});

Is it safe to use a single Mongoose database from two files/processes?

I've been working on a server and a push notification daemon that will both run simultaneously and interact with the same database. The idea behind this is that if one goes down, the other will still function.
I normally use Swift but for this project I'm writing it in Node, using Mongoose as my database. I've created a helper class that I import in both my server.js file and my notifier.js file.
const Mongoose = require('mongoose');
const Device = require('./device'); // This is a Schema
var uri = 'mongodb://localhost/devices';
function Database() {
Mongoose.connect(uri, { useMongoClient: true }, function(err) {
console.log('connected: ' + err);
});
}
Database.prototype.findDevice = function(params, callback) {
Device.findOne(params, function(err, device) {
// etc...
});
};
module.exports = Database;
Then separately from both server.js and notifier.js I create objects and query the database:
const Database = require('./db');
const db = new Database();
db.findDevice(params, function(err, device) {
// Simplified, but I edit and save things back to the database via db
device.token = 'blah';
device.save();
});
Is this safe to do? When working with Swift (and Objective-C) I'm always concerned about making things thread safe. Is this a concern? Should I be worried about race conditions and modifying the same files at the same time?
Also, bonus question: How does Mongoose share a connection between files (or processes?). For example Mongoose.connection.readyState returns the same thing from different files.

The short answer is "safe enough."
The long answer has to do with understanding what sort of consistency guarantees your system needs, how you've configured MongoDB, and whether there's any sharding or replication going on.
For the latter, you'll want to read about atomicity and consistency and perhaps also peek at write concern.
A good way to answer these questions, even when you think you've figured it out, is to test scenarios: Hammer a duplicate of your system with fake data and events and see if what happen is OK or not.

pulling information from a remote mongoDB

I have the following code that return 0 or no result. I have no running errors
var Users_Collection = new Meteor.Collection("bp_qstat_jobs_monitor_temp");
if (Meteor.isServer) {
Meteor.startup(function () {
// code to run on server at startup
});
}
if (Meteor.isClient) {
var cursor = Users_Collection.find('j_owner');
var info = cursor.count();
console.log(cursor, info);
}
so the question is what I'm doing wrong??? (the collection has 200 records)

Usually mongodb will not give error if you are finding the collection which is not available. It will give empty only. Please make sure that you are connected to the correct db, and find the collection.
Check this link for more details.