NodeJS + MongoDB ASYNC结果出现较晚

Question

I have a simple function which returns a list of connected users in a channel, I was using it in a initial stage with a simple array, but know, I want to retrieve some user details from the mongodb database and add it to that array.

The problem that I'm having is that the return is giving an empty result, I can clearly see later on in the console log that is because the queries are running late and the return is running before having the results back.

I know it is because the asynchronous functions, I've tried with Q and others but I just simply cant get it working. :(

//GET USERS FOR SPECIFIED ROOM
function get_room_users( room ){

    // create an array to hold all the usernames of the poeple in a specific room
    var roomusers = new Array();        

    // get all the clients in ‘room1′
    var clients = io.sockets.clients( room );

    var i = 0;

    for(var i = 0; i < clients.length; i++) {

        db.users.findOne( { email: clients[i].username }, function(err, userdata) {

            if( err || ! userdata ){

                console.log('no user found');

            } else { 
                console.log("DISPLAYING USER DATA: " + userdata.nickname);
                roomusers[roomusers.length] = userdata.nickname;


                console.log("HERE THE CONSOLE SHOWS ROOMUSERS FILLED IN");
                console.log(roomusers);

                i++;

            }

        });

    }

    console.log("HERE THE ARRAY LOOKS EMPTY BEFORE SENDING BACK");
    console.log(roomusers);

    return roomusers;


}

var users = get_room_users(room1);

console.log("HERE THE ARRAY LOOKS EMPTY!")
console.log(users);

CONSOLE LOG RESULT

HERE THE ARRAY LOOKS EMPTY BEFORE SENDING BACK
[]
HERE THE ARRAY LOOKS EMPTY!
[]
HERE THE CONSOLE SHOWS ROOMUSERS FILLED IN
['user@user.com']

FINALLY RESOLVED: Ok, so as one of the users commented I was thinking in a synchronous mode, I changed the emit method up after the bucle got resolved in order to emit when everything has been done. Keep in mind that I'm using Q to create promisses :)

//GET USERS FOR SPECIFIED ROOM function get_room_users( room ){

var the_promises = [];

// create an array to hold all the usernames of the poeple in a specific room
var roomusers = new Array();        

// get all the clients in ‘room1′
var clients = io.sockets.clients( room );

clients.forEach(function (socket) {

    var deferred = Q.defer();

    db.usuarios.findOne( { email: socket.username }, function(err, userdata) {

        if( err || ! userdata ){

            console.log('no user found');
            deferred.reject();

        } else { 
            console.log("DISPLAYING USER DATA: " + socket.username);
            roomusers[roomusers.length] = socket.username;

            console.log("DENTRO DEL BUCLE");
            console.log(roomusers);

            deferred.resolve();

        }

    });

    the_promises.push(deferred.promise);    


});


Q.all(the_promises).done( function(){   
    console.log("ALL THINGS DONE!"); 
    // broadcast to everyone in room 1 the usernames of the clients connected.
    io.sockets.to( room ).emit('updateroomusers', roomusers);   
 } );

}

Answer 1

I suggest you to use Q Promises ( https://npmjs.org/package/q ). Promises is a very powerfull tool for scheduling asynchronous operations.

Answer 2

Yes, this is a problem of using a synchronous style on an asynchronous environment. Execution runs ahead of io.sockets.clients( room ) before a response comes back.

The following should work:

io.sockets.clients(room).forEach(function (socket) {
    // Your for-loop code
    db.users.findOne( ... );
});