Call back issue in forloop in node js - node.js

Question

here is my code snippet.
I am trying to get details of my friends.friendsList is in reside within the user collection itself and there i am inserting users's id. So first i am fetching userId,then fetching each of their details.but my problem is i am not getting the values of friendsDetails outside the fetchDetailsfunction. I tried many time. i am new to node and sails js. i think its problem of asynchronous execution. How can i solve this problem?
getFriendsDetails:function(req,res){
var userId = req.param('id');
var friendsDetails=[];
User.findOne({
id: userId
}).exec(function(err,user){
var friendsIds=user.friends;
friendsIds.forEach(function(id){
User.findOne({
id: id
}).exec(function fetchDetails(err,Userdetails){
var obj={
id:Userdetails.id,
name:Userdetails.name,
pro_pic:Userdetails.profile_pic
}
friendsDetails.push(obj);
console.log(friendsDetails);//Here consoling pushed data correctly.
});
console.log(friendsDetails);//here i am getting null array
});
});

Answer 1

You can use mongoDB $in clause to get an array of friend documents.
getFriendsDetails:function(req,res){
var userId = req.param('id');
var friendsDetails=[];
User.findOne({
id: userId
}).exec(function(err,user){
var friendsIds = user.friends;
User.find({id : { $in : friendsIds}}).toArray(function(err, data){
console.log(data);
data.forEach(function(friendObj){
var obj={
id: friendObj.id,
name: friendObj.name,
pro_pic: friendObj.profile_pic
}
friendsDetails.push(obj);
});
console.log(friendsDetails);
});
});
}

Answer 2

Your variable is not in the same scope. Therefore, it is null outside the exec function. You are right, it is due to the async nature of javascript.
What you can do is pass a callback to the exec function and pass your variable in.
.exec(function(err, user, callback)
and then call it like this
callback(friendsDetails);
Your second console.log won't wait until your function is finished because it is async.