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JavaScript [Q.JS] Are you stuck in nested callback functions?

If you have seen situation when one asynchronous call result used in another asynchronous call. And you tried to make that code look less scary and more readable then you must go through this library and understand it’s nice tricks and syntax.

Just want to give the same example from its site which clearly shows the problem it is trying to solve.


step1(function (value1) {
    step2(value1, function(value2) {
        step3(value2, function(value3) {
            step4(value3, function(value4) {
                // Do something with value4


With QJS library 


.then(function (value4) {
    // Do something with value4
}, function (error) {
    // Handle any error from step1 through step4


If you want to see its potential uses then you should see one of the WCF data API  breezejs.

To test this library i have created a dummy Ajax function which simulate the Ajax asynchronous behaviour.


var Ajax = function(config){
    var emptyFn = function(){},
        xhr = { 
            success : config.success || emptyFn,
            failure: config.failure || emptyFn,
            call: function(id){
                //random response delay
                var randomNum = Math.floor((Math.random()*3000)+1);
                    xhr.success('['+config.url+' call]');
                }, randomNum);
    return xhr;


Next step is to wrap Ajax function in QJS promise object. And we are ready to play with QJS features.


var ajaxPromise = function(url){
     var deferred = Q.defer(),
        projectMgr = Ajax({
            url: url,
            success: function(res){
            failure: function(res){
    return deferred.promise;


Feed all Ajax call response to one function


.spread(function(responseA, responseB, responseC, responseD){
 //handle data


Waterfall Ajax call where response data passes to the next Ajax call.


.then(function (responseD) {
 // Do something with value4
}, function (error) {
 // Handle any error from step1 through step4


There are couple more useful functions

fail:   Shorthand function of ‘then’ where you just want to handle the error

fin:   It is a ‘finally’ function which get call without parameter.

Example handler


var handlers = {
    mainTask: function(){
            console.debug('main task - ', arguments);
            return 'maintask';
    task1: function(){
            console.debug('then task1 - ', arguments);
            return 'task1';
    task2: function(){
            console.debug('then task2 - ', arguments);
            return 'task2';
    throwError: function(){
            throw new Error('some error');
    handleError: function(){
            console.log('handle error - ', arguments); 
    finallyTask: function(){
            console.debug('then finallyTask - ', arguments);
            return 'finallyTaskDone';    


Handling the error through ‘then’ function and ‘finally’


.then(handlers.task2, handlers.handleError)


Same behaviour with ‘fail’ shorthand function




There are more stuff you can explore. 

If you are a big fan of one liner code then you can write your code something like below with new array functions


var find = function(){ console.log('find'); },
 validate = function() { console.log('validate'); },
 update = function() { console.log('update'); },
 show = function() { console.log('show'); },
 funcs = [find, validate, update, show];


With function chaining




Or with new ‘forEach’ function


var result = Q.resolve();
    result = result.then(func);


Or with ‘reduce’


funcs.reduce(function(funca, funcb){
    return funca.then(funcb);
}, Q.resolve())


Ok above two were not really one liner but these are pretty compact and expressive.


Ajax call retry with some delay

There was a scenario in our project where we wanted to show error message after 5 recursive fail to get the data. And i found much easier to do with this library.


function readRetry(url, retryDelay, times){
    return projectMgrPromise(url)
            console.log('got the data.');
        }, function(error){
            if(times == 0){
                throw new Error('Error while reading the data.');
            return Q.delay(retryDelay)
                    return readRetry(url, retryDelay, times-1);

//function call with 5 retry and 200 delay
readRetry('testUrl', 200, 5);


Asynchronous call handing are really hard specially if you are creating single page web application. So it is always better to give some thought on design before writing the code. I still remember battling with async callback issues in our one of the project and we created event system to overcome from it.


PS: JQuery and DOJO also support these kind of functions.


Some useful links


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