Getting empty array in nodejs - node.js

Question

I am posting value But I am getting empty array . I know its node asynchronous problem . But I don't know how do i solve this. I have refer this following link:
How do I return the response from an asynchronous call?
But I could not able to understand . Kindly help me to understand promises and how do i use that in my code.
router.post('/inspection_list', function (req, res) {
var id = req.body.project_id;
console.log(id)
// res.send("ok")
db.inspection.findOne({'_id':id},(err,response)=>{
if(err){
console.log("error");
}
else{
console.log("Data")
var inspection = [];
var data = response.inspection_data;
var f = data.map(function (item) {
var fielduser = item.fielduser_id
db.fielduser.findOne({'_id': mongoose.Types.ObjectId(fielduser)},(err,user)=>{
console.log(user.owner_name);
console.log(item.inspection_name)
inspection.push({inspection_name:item.inspection_name,field_user_name : user.owner_name})
})
});
console.log(inspection) // Here am getting empty value
// setTimeout(function(){ console.log(inspection) }, 5000); my timeout code
}
})
});

Answer 1

router.post('/inspection_list', async function (req, res) {
var id = req.body.project_id;
try{
var response = await db.inspection.findOne({'_id':id})
var inspection = [];
var data = response.inspection_data;
for ( var i = 0; i<data.length; i++){
var item = data[i]
var fielduser = item.fielduser_id
var user = await db.fielduser.findOne({'_id': mongoose.Types.ObjectId(fielduser)})
inspection.push({inspection_name:item.inspection_name,field_user_name : user.owner_name})
}
}
catch(err){
throw err
}
})
This uses async and await, you can use it if you are using node version >=7.6
Also note the following:
router.post('/inspection_list', async function (req, res)
Handling each error seperately
router.post('/inspection_list', async function (req, res) {
var id = req.body.project_id;
try{
var response = await db.inspection.findOne({'_id':id})
}
catch(err){
// handle error here
throw err
}
var inspection = [];
var data = response.inspection_data;
for ( var i = 0; i<data.length; var item = data[i]
var fielduser = item.fielduser_id
try{
var user = await db.fielduser.findOne({'_id': mongoose.Types.ObjectId(fielduser)})
}
catch(err){
// handle error
}
inspection.push({inspection_name:item.inspection_name,field_user_name : user.owner_name})
}
})

Answer 2

Using mongoose would be the easy way out, it returns Promises for all query and save functions, so you'd simply do:
YourModel.findOne({params}).then(() => {...})
If you're unable to do that, in your case, a 'promisified' example would be:
var findAndFillArray = (project_id) => new Promise((resolve) => {
.... your previous code here ....
inspection.push({inspection_name:item.inspection_name,field_user_name :
user.owner_name})
if (data.length === inspection.length){ // Or some other preferred condition
resolve(inspection);
}
})
Then you'd call this function after you get the id, like any other function:
var id = req.body.project_id;
findAndFillArray(id).then((inspection_array) => {
res.send(inspection_array) // Or whatever
})
Now, map and all list functions are synchronous in JS, are you sure the error is due to that?