mongodb showing array of null when printing the outside of query - mongodb

Question

i am trying to push the resultant of the count to an array in mogodb query, while pushing it showing the array after that if print it outside of query it is showing empty array.
collection1 in db is like below
[{title:Home,
date:24-10-2016},
{title:Accesories,
date:13-02-2016}
]
my code
exports.listOfCategories=function(req,res){
collection1.find().exec(function (err, categories) {
if (err) {
return res.status(400).send({
message: errorHandler.getErrorMessage(err)
});
} else {
var categoryList = categories;
var catTitle;
var allCat = [];
// console.log(categoryList);
for (var i = 0; i < categoryList.length; i++) {
catTitle = categoryList[i].title;
contentCounts(catTitle);
function contentCounts(content, callback) {
var catName = new RegExp(content, 'i');
var mongoQuery = {
"ProCategory.title": catName
}
collection2.find(mongoQuery).count(function (err, count) {
generateContentArr(content, count)
});
}
function generateContentArr(content, count) {
allCat.push({
name: content,
count: count
});
console.log(JSON.stringify(allCat));
// Here it is showing the array what i pushed
}
}
console.log(JSON.stringify(allCat));
// Here it not showing the total array, it showing an empty array
res.json(allCat);
}
});
}
Thanks in advance

Answer 1

You are not waiting for the result of an async operation, in your case in the for loop you need to wait for the result of mongo operation, but as for loop is synchronous, you are just making calls to mongo but don't wait for the results, and print the empty array right after the loop.
I would suggest you to use promises instead of callbacks, I don't know which version of mongoose you are using but the last version have promise support for mongo methods like find and count. Here is an example for your case:
var Promise = require("bluebird");
function countByTitle(catTitle){
var mongoQuery = {"ProCategory.title": new RegExp(catTitle, 'i')}
return collection2.count(mongoQuery).then(function(count) {
return {
name: catTitle,
count: count
};
});
}
collection1.find().then(function (categories) {
var categoryList = categories;
var promises = [];
for (var i = 0; i < categoryList.length; i++) {
promises.push(countByTitle(categoryList[i].title));
}
return Promise.all(promises).then(results => {
console.log(JSON.stringify(results));
})
}).catch(function (err) {
//if there is any error while resolving the promises, this block will be called
return res.status(400).send({
message: errorHandler.getErrorMessage(err)
});
});