Nodejs and Expressjs avoid nested callback queries

Question

I'm using:

• nodejs (0.6.8)
• expressjs (2.5.2)
• socket.io (0.8.7)
• node-db-mysql (0.7.6)

Currently the code works correctly, but I wonder if there is a way to avoid callbacks nested functions:

The queries:

/************************* 
 Queries 
**************************/
var sql_states = 'SELECT ...';
var sql_colors = 'SELECT ...';
var sql_languages = 'SELECT ...';
/* ... more queries */

The execution of nested queries:

/*************************     
 Run queries
**************************/
db.query(sql_states).execute(function(error, r) {
    if (error) {
        req.session.error = 'Operation failed States';
        res.redirect('back');
    }
    else if (r.length  > 0) 
    {
        for(var i in r){
            states += '<option value="'+r[i]['id_state']+'">'+r[i]['name']+'</option>';
        }

        var colors = '';

        db.query(sql_colors).execute(function(error, r) {
            if (error) {
                req.session.error = 'Operation failed Colors';
                res.redirect('back');
            }
            else if (r.length  > 0) 
            {
                for(var i in r){
                    colors += '<option value="'+r[i]['id_color']+'">'+r[i]['name']+'</option>';
                }

                var languages = '';

                db.query(sql_languages).execute(function(error, r) {
                    if (error) {
                        req.session.error = 'Operation failed Languages';
                        res.redirect('back');
                    }
                    else if (r.length  > 0) 
                    {
                        for(var i in r){
                            languages += '<option value="'+r[i]['id_language']+'">'+r[i]['name']+'</option>';
                        }
                        ...

Any suggestions are welcome.

Thanks.

Answer 1

This is a typical problem in asynchronous JavaScript development. To be able to create a kind of serial execution path for you asynchronous functions there are various solutions that you should use for any medium to large project.

https://github.com/caolan/async

https://www.npmjs.com/package/promise

I do personally use async since I find it very well documented. You can easily solve your nested callbacks with an async.each constructs.

async.each(openFiles, function(file, callback) {
     // do processing for every file and return callback when async call is completed 
}, function(err){
     // execute this piece of code when all your processing is complete.
});

Answer 2

You can use the async module to cleanly manage just about any asynchronous flow control scenario you can dream up.