Node.js - object loses values after function - node.js

Question

I have a REST API in Node.js using Mongoose. I have the following function that does something specific for my application. The problem is that I set test.questions value and after a particular loop, I find it is losing scope of those variables. What is the problem here? Here is my code:
randomizeTest = (req, res) => {
const test = new Test;
let questions: String[] = [];
let num = 5;
while (num >= 1) {
var self = this;
this.quesmodel.count().exec(function (err, count) {
var random = Math.floor(Math.random() * count)
self.quesmodel.findOne().skip(random).exec(
function (err, result) {
questions.push(result._id);
test.questions = questions;
console.log(test.questions); // prints data
});
});
num--;
}
console.log(test.questions); // prints nothing
test.save(function (err, test) {
if (err) {
res.sendStatus(400);
console.error(err);
} else {
res.status(200).json({ test });
}
});
}
After Navid's answer, I tried this now:
let questions: String[] = [];
let num = 5;
var self = this;
asyncLoop(questions, function (item, next) {
while (num >= 1) {
self.quesmodel.count().exec(function (err, count) {
var random = Math.floor(Math.random() * count)
self.quesmodel.findOne().skip(random).exec(
function (err, result) {
questions.push(result._id);
test.questions = questions;
next();
});
});
num--;
}
}, function () {
console.log(test.questions);
console.log('Finished!');
});

Answer 1

In nodejs functions with some I/O job run asynchronously so when one thread is looping inside while loop and executing database I/O jobs, another one is running the rest of your code and executing console.log(test.questions); with its previous values which is empty.
PS. one good way to handle these type of problems is using some async libraries to run your loops sequentially like node-async-loop.
var asyncLoop = require('node-async-loop');
var array = ['item0', 'item1', 'item2'];
asyncLoop(array, function (item, next)
{
do.some.action(item, function (err) //database operations come here
{
if (err)
{
next(err);
return;
}
next();
});
}, function (err)
{
if (err)
{
console.error('Error: ' + err.message);
return;
}
// the rest of your code like console.log(test.questions); goes here
console.log('Finished!');
});
this might help you but its not the best way to do it:
let questions: String[] = [];
let num = Array.from(Array(5).keys()); //this makes num = [0, 1, 2, 3, 4]
var self = this;
asyncLoop(num, function (item, next) {
self.quesmodel.count().exec(function (err, count) {
var random = Math.floor(Math.random() * count)
self.quesmodel.findOne().skip(random).exec(function (err, result) {
questions.push(result._id);
test.questions = questions;
next();
});
});
}, function (err) {
if(err)
throw err;
else {
console.log(test.questions);
console.log('Finished!');
}
});

Answer 2

Here's a solution using the promises built into mongodb (which are a much better way to manage multiple asynchronous operations) and then simplifying things a bit with async/await so your loop actually runs serially:
randomizeTest = async (req, res) => {
const test = new Test();
let questions: String[] = [];
test.questions = questions;
try {
for (let num = 5; num >= 1; --num) {
let count = await this.quesmodel.count();
let random = Math.floor(Math.random() * count);
let result = await this.quesmodel.findOne().skip(random).exec();
questions.push(result._id);
}
console.log(test.questions); // prints final results
await test.save();
res.status(200).json({ test });
} catch(e) {
res.sendStatus(500);
console.error(e);
}
}
FYI, your scheme for picking a random record from a collection is subject to a race condition if there are other processes modifying that collection while you are selecting the random record. This is because there's a time period between when you do .count() and .skip(random) and the collection could be changed in that time window. There are multiple other techniques for picking a random item that each have their own situations where they are best.

Answer 3

I am definitely sure this is problem with your loop. Please use async in place of while.