For loop in async function and timing issue

I am having the absolute worst time with this issue of trying to add data to an array via a for loop. I am building out my very first api and I so don't have a ton of experience so bare with me please. My gut is telling me that this is a synchronous/asynchronous issue but I don't have the knowledge of how to fix it either way.

What I am doing is querying my mongodb for the user's email that is being passed through a session. When that email is found it looks for that user's friends, and those friends are supposed to be passed to the usersFriends array which is then sent in the chunk to the browser. I included all the code in this block even though the transaction block isn't really pertinent or so I think.

The Problem: Basically the problem boils down to usersFriends array outputting an empty array everywhere except when the console.log is inside the for loop. Thoughts? 

app.get('/api/chunk', function(req, res){

var last5;
var usersFriends = [];

Transaction.find().sort({$natural:-1}).limit(5).exec(function(err, docs){
    if (err) {
        console.log(err);
    } else {
        last5 = docs;           
    }
});

User.findOne({ email: req.user.email }, function(err, user){
    if (!user) {
        console.log(err);
    } else {
        for (var i = 0; i < user.friends.length; i++) {
            (function(cntr){
            User.findOne({ email: user.friends[cntr].email}, function(err, result) {
                result.password = "Sneaky sneaky"
                var name = result.firstName + " " + result.lastName;                    
                usersFriends.push({
                    name: name,
                    index: cntr
                });
            });                 
            })(i);

            var chunk = {
                "friends": usersFriends,
                "transactions": last5
            };
        }       }console.log(usersFriends); // empty array 
});

});

Combining all the various things we've discussed in comments for this list of changes:

  1. Pass the cntr in an IIFE so it is uniquely captured for each separate .findOne() request.
  2. In each response, check if this is the last one done so we know that all results have arrived.
  3. Start the .findOne() operations from the completion of the Transaction.find() operation (since I don't know that operation, I'm guessing how this particular aspect should be implemented, but you should be able to see the general idea).

Though would result in this code:

app.get('/api/chunk', function(req, res) {

    var last5;
    var usersFriends = [];

    Transaction.find().sort({
        $natural: -1
    }).limit(5).exec(function(err, docs) {
        if (err) {
            console.log(err);
        } else {
            last5 = docs;
            User.findOne({
                email: req.user.email
            }, function(err, user) {
                if (!user) {
                    console.log(err);
                } else {
                    var totalCnt = user.friends.length;
                    for (var i = 0; i < totalCnt; i++) {
                        (function(cntr) {
                            User.findOne({
                                email: user.friends[cntr].email
                            }, function(err, result) {
                                result.password = "Sneaky sneaky"
                                var name = result.firstName + " " + result.lastName;
                                usersFriends.push({
                                    name: name,
                                    index: cntr
                                });
                                if (usersFriends.length === totalCnt) {
                                    // all results are done here
                                    // you can create the final response
                                    var chunk = {
                                        "friends": usersFriends,
                                        "transactions": last5
                                    };
                                }
                            });
                        })(i);

                    }
                }
            });
        }
    });
});