Pass in an array of Deferreds to $.when()

Pass in an array of Deferreds to $.when()

Here’s an contrived example of what’s going on:
Click me!

function getSomeDeferredStuff() {
var deferreds = [];

var i = 1;
for (i = 1; i <= 10; i++) { var count = i; deferreds.push( $.post('/echo/html/', { html: "

Task #” + count + ” complete.”,
delay: count
}).success(function(data) {

return deferreds;

$(function() {
$(“a”).click(function() {
var deferreds = getSomeDeferredStuff();

$.when(deferreds).done(function() {

All done!


I want “All done!” to appear after all of the deferred tasks have completed, but $.when() doesn’t appear to know how to handle an array of Deferred objects. “All done!” is happening first because the array is not a Deferred object, so jQuery goes ahead and assumes it’s just done.
I know one could pass the objects into the function like $.when(deferred1, deferred2, …, deferredX) but it’s unknown how many Deferred objects there will be at execution in the actual problem I’m trying to solve.


Solution 1:

To pass an array of values to any function that normally expects them to be separate parameters, use Function.prototype.apply, so in this case you need:

$.when.apply($, my_array).then( ___ );


In ES6, you can use the ... spread operator instead:

$.when(...my_array).then( ___ );

In either case, since it’s unlikely that you’ll known in advance how many formal parameters the .then handler will require, that handler would need to process the arguments array in order to retrieve the result of each promise.

Solution 2:

The workarounds above (thanks!) don’t properly address the problem of getting back the objects provided to the deferred’s resolve() method because jQuery calls the done() and fail() callbacks with individual parameters, not an array. That means we have to use the arguments pseudo-array to get all the resolved/rejected objects returned by the array of deferreds, which is ugly:

$.when.apply($,deferreds).then(function() {
     var objects=arguments; // The array of resolved objects as a pseudo-array

Since we passed in an array of deferreds, it would be nice to get back an array of results. It would also be nice to get back an actual array instead of a pseudo-array so we can use methods like Array.sort().

Here is a solution inspired by when.js‘s when.all() method that addresses these problems:

// Put somewhere in your scripting environment
if (typeof jQuery.when.all === 'undefined') {
    jQuery.when.all = function (deferreds) {
        return $.Deferred(function (def) {
            $.when.apply(jQuery, deferreds).then(
                function () {
                    def.resolveWith(this, []);
                function () {
                    def.rejectWith(this, []);

Now you can simply pass in an array of deferreds/promises and get back an array of resolved/rejected objects in your callback, like so:

$.when.all(deferreds).then(function(objects) {
    console.log("Resolved objects:", objects);

Solution 3:

You can apply the when method to your array:

var arr = [ /* Deferred objects */ ];

$.when.apply($, arr);

How do you work with an array of jQuery Deferreds?

Solution 4:

When calling multiple parallel AJAX calls, you have two options for handling the respective responses.

  1. Use Synchronous AJAX call/ one after another/ not recommended
  2. Use Promises' array and $.when which accepts promises and its callback .done gets called when all the promises are return successfully with respective responses.


function ajaxRequest(capitalCity) {
   return $.ajax({
        url: ''+capitalCity,
        success: function(response) {
        error: function(response) {
   var capitalCities = ['Delhi', 'Beijing', 'Washington', 'Tokyo', 'London'];

   function getCountryCapitals(){ //do multiple parallel ajax requests
      var promises = [];   
      for(var i=0,l=capitalCities.length; i<l; i++){
            var promise = ajaxRequest(capitalCities[i]);
      $.when.apply($, promises)
   function fillCountryCapitals(){
        var countries = [];
        var responses = arguments;
        for(i in responses){
<script src=""></script>
  <h4>Capital Cities : </h4> <span id="capitals"></span>
  <h4>Respective Country's Native Names : </h4> <span id="countries"></span>

Solution 5:

As a simple alternative, that does not require $.when.apply or an array, you can use the following pattern to generate a single promise for multiple parallel promises:

promise = $.when(promise, anotherPromise);


function GetSomeDeferredStuff() {
    // Start with an empty resolved promise (or undefined does the same!)
    var promise;
    var i = 1;
    for (i = 1; i <= 5; i++) {
        var count = i;

        promise = $.when(promise,
            type: "POST",
            url: '/echo/html/',
            data: {
                html: "<p>Task #" + count + " complete.",
                delay: count / 2
            success: function (data) {
    return promise;

$(function () {
    $("a").click(function () {
        var promise = GetSomeDeferredStuff();
        promise.then(function () {
            $("div").append("<p>All done!</p>");


  • I figured this one out after seeing someone chain promises sequentially, using promise = promise.then(newpromise)
  • The downside is it creates extra promise objects behind the scenes and any parameters passed at the end are not very useful (as they are nested inside additional objects). For what you want though it is short and simple.
  • The upside is it requires no array or array management.

Solution 6:

I want to propose other one with using $.each:

  1. We may to declare ajax function like:

    function ajaxFn(someData) {
        this.someData = someData;
        var that = this;
        return function () {
            var promise = $.Deferred();
                method: "POST",
                url: "url",
                data: that.someData,
                success: function(data) {
                error: function(data) {
            return promise;
  2. Part of code where we creating array of functions with ajax to send:

    var arrayOfFn = [];
    for (var i = 0; i < someDataArray.length; i++) {
        var ajaxFnForArray = new ajaxFn(someDataArray[i]);
  3. And calling functions with sending ajax:

        $.each(arrayOfFn, function(index, value) {
    ).then(function() {