Question

I am currently working on a project with 3 friends using nodeJs, expressJs, MongoDB, html5,…
Since we’re fairly new to these technologies we bumped into some problems.
A big problem that I can’t find a solution for is the asynchronous execution of certain code.

I want a for each loop to finish, so that I have an updated online friends list, and than execute the res.render (in which I pass the online friends list), because currently it does the res.render before it finishes the loop.
Code:

function onlineFriends(req, res) {
var onlinefriends = new Array();
onlinefriends.push("mark");
FriendList.findOne({
    owner: req.session.username
}, function (err, friendlist) {
    friendlist.friends.forEach(function (friend) { // here forEach starts
        OnlineUser.findOne({
            userName: friend
        }, function (err, onlineFriend) {
            if (onlineFriend != null) {
                onlinefriends.push(onlineFriend.userName);
                console.log("a loop");
            }
        });

    });  
        console.log("online friends: " + onlinefriends);
        console.log("redirecting");
        res.render('index', { // this is still inside the forEach function
            friendlist: friendlist.friends,
            onlinefriendlist: onlinefriends,
            username: req.session.username
        });// and here it ends
});

}

output will be as follows:

online friends: mark
redirecting
a loop
a loop
a loop
a loop
a loop
a loop
a loop

As discussed here ( JavaScript, Node.js: is Array.forEach asynchronous? ) , the answer is that the for-each is blocking, but in my example it seems to be non-blocking because it executes the res.render before it has finished looping?
How can I make sure that the for each is finished so I have an up to date onlinefriends list (and friendlist) which I can than pass to the res.render instead of the res.render happening way before the for -each loop finishes (which gives me an incorrect list of online users) ?

Thanks very much!

Answer 1

The following console log:

console.log("a loop");

is inside a callback

I believe that the callback of the function OnlineUser.findOne() is called asynchronously, that is why the code will log “a loop” after the redirect log

You should put the redirection after all the loop callbacks have been executed

Something like:

var count = 0;
friendlist.friends.forEach(function (friend) { // here forEach starts
    OnlineUser.findOne({
        userName: friend
    }, function (err, onlineFriend) {
        count++;
        if (onlineFriend != null) {
            onlinefriends.push(onlineFriend.userName);
            console.log("a loop");
        }
        if(count == friendlist.friends.length) { // check if all callbacks have been called
            redirect();
        }
    });
}); 

function redirect() {
    console.log("online friends: " + onlinefriends);
    console.log("redirecting");
    res.render('index', { // this is still inside the forEach function
        friendlist: friendlist.friends,
        onlinefriendlist: onlinefriends,
            username: req.session.username
    });// and here it ends
}