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How do I store the results of multiple asynchronous $.get requests in an array?

I have a list of items being stored in an array. For each item in the array, I need to make an API request and add some of the data that comes back into another array so that I can perform operations on it later.
I think the issue is that my get request is asynchronous, and as a result the data is not necessarily loaded when I am trying to add it to the array, but I thought that's what .then was supposed to cover.
var cardRequestList = ["Scapeshift","Ghostform", "Daybreak Chaplain"]
var url = "https://api.magicthegathering.io/v1/cards?name=%22";
var cardInfo =[];
for (var cardName in cardRequestList){
var results = getCard(cardRequestList[cardName]);
cardInfo.push(results);
}
console.log(cardInfo); // results should come back here
function getCard(cardName){
var cardUrl = url.concat(cardName,"%22");
$.get(cardUrl).then(
function(data) {
var temp = [data.cards[0].name,data.cards[0].printings]
return temp;
}, function() {
alert( "$.get failed!" );
}
);
}

then() only works for the specific request it's invoked on.
To solve the issue you have you could make your requests in a loop, adding the jqXHR object returned from those calls to an array which you can then apply to $.when() to execute some other logic once all the requests have completed.
Within the requests themselves you need to add the returned data to an array as you cannot return anything from an async call.
With all that said your code would look something like this:
var cardRequestList = ["Scapeshift", "Ghostform", "Daybreak Chaplain"]
var url = "https://api.magicthegathering.io/v1/cards?name=%22";
var cardInfo = [];
var requests = cardRequestList.map(function(cardName) {
return getCard(cardName);
});
function getCard(cardName) {
var cardUrl = url.concat(cardName, "%22");
return $.get(cardUrl).then(function(data) {
cardInfo.push([data.cards[0].name, data.cards[0].printings]);
}, function() {
alert("$.get failed!");
});
}
$.when.apply($, requests).done(function() {
// all requests finished and cardInfo has been populated, place logic here...
console.log(cardInfo);
});

How To Load URL One by One With Javascript

How to touch url one by one via javascript ajax or jquery from array? Because if you touch big process php in one touch will make timeout, so how process one by one?
Example
var myurls = [
"http://example.com/grape.php",
"http://example.com/apple.php",
"http://example.com/orange.php",
"http://example.com/banana.php"];
May be if grape.php is done and then next to apple, if apple is done and then next to orange.
And then if all process finished, show alert success.

You mean this?
var myurls = [
"http://example.com/grape.php",
"http://example.com/apple.php",
"http://example.com/orange.php",
"http://example.com/banana.php"],cnt=0;
function process(data) {
console.log(data);
}
function loadUrl() {
if (cnt>=myurls.length) return;
$.get(myurls[cnt++],function(data) {
process(data);
loadUrl();
});
}
$(function() {
loadUrl();
})

Based on your question and the discussion we had in your comments, I figured that you wanted to perform AJAX calls sequentially based on an array of URLs you have.
Solution 1: Use repeated, self-referencing $.ajax() requests
This is undoubtedly the easier solution. What we have here is that we keep track of the position of the array we are in, and stop making AJAX requests when the array has been iterated through.
$(function() {
// Array of URLs
var myurls = [
"https://jsonplaceholder.typicode.com/posts/1",
"https://jsonplaceholder.typicode.com/posts/2",
"https://jsonplaceholder.typicode.com/posts/3",
"https://jsonplaceholder.typicode.com/posts/4"
];
// Iterate through array, and keep a cursor on which item we are at
var urlCount = 0,
ajaxCall = function() {
if (urlCount < myurls.length) {
console.log('Making AJAX call to url: '+myurls[urlCount]);
$.ajax({
url: myurls[urlCount]
})
.done(function(returnedData) {
console.log(returnedData);
urlCount++;
ajaxCall();
});
}
};
ajaxCall();
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
Solution 2: Using .then() chaining set up using a for loop
I have adapted the code provided in this answer, for your use case, since the example can be quite difficult to understand for those unfamiliar with deferred objects and returned promises.
So the trick is the following:
Set up a master deferred object
Set up a general function that will make AJAX calls for you
Loop through the array, chaining them using .then()
Kick start the first AJAX call on the master deferred object
$(function() {
// Array of URLs
var myurls = [
"https://jsonplaceholder.typicode.com/posts/1",
"https://jsonplaceholder.typicode.com/posts/2",
"https://jsonplaceholder.typicode.com/posts/3",
"https://jsonplaceholder.typicode.com/posts/4"
];
// Set up chain of AJAX requests
var d = $.Deferred(),
_d = d,
ajaxRequest = function(ajaxUrl) {
// Log in browser console that AJAX call is being made
console.log('Making AJAX call to: ' + ajaxUrl);
// Return deferred object for .then() chaining
return $.ajax({
url: ajaxUrl
});
};
// We chain each AJAX call to the next one
for (var i in myurls) {
// Use IIFE so that reference to `i` is fixed
(function(j) {
// Update _d for chaining
_d = _d.then(function() {
// The _request is a defered object returned
// So we can chain deferred methods such as .done()
var _request = ajaxRequest(myurls[j]).done(function(ajaxData) {
// Just to show that data is being returned after each call
console.log(ajaxData);
});
// Return the deferred object for chaining
return _request;
});
})(i);
}
// Kick start sequential ajax call
d.resolve();
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

Multiple jQuery Ajax requests and response handling [duplicate]

Here's an contrived example of what's going on: http://jsfiddle.net/adamjford/YNGcm/20/
HTML:
Click me!
<div></div>
JavaScript:
function getSomeDeferredStuff() {
var deferreds = [];
var i = 1;
for (i = 1; i <= 10; i++) {
var count = i;
deferreds.push(
$.post('/echo/html/', {
html: "<p>Task #" + count + " complete.",
delay: count
}).success(function(data) {
$("div").append(data);
}));
}
return deferreds;
}
$(function() {
$("a").click(function() {
var deferreds = getSomeDeferredStuff();
$.when(deferreds).done(function() {
$("div").append("<p>All done!</p>");
});
});
});
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.

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( ___ );
See http://jsfiddle.net/YNGcm/21/
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.

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(
// the calling function will receive an array of length N, where N is the number of
// deferred objects passed to when.all that succeeded. each element in that array will
// itself be an array of 3 objects, corresponding to the arguments passed to jqXHR.done:
// ( data, textStatus, jqXHR )
function () {
var arrayThis, arrayArguments;
if (Array.isArray(this)) {
arrayThis = this;
arrayArguments = arguments;
}
else {
arrayThis = [this];
arrayArguments = [arguments];
}
def.resolveWith(arrayThis, [Array.prototype.slice.call(arrayArguments)]);
},
// the calling function will receive an array of length N, where N is the number of
// deferred objects passed to when.all that failed. each element in that array will
// itself be an array of 3 objects, corresponding to the arguments passed to jqXHR.fail:
// ( jqXHR, textStatus, errorThrown )
function () {
var arrayThis, arrayArguments;
if (Array.isArray(this)) {
arrayThis = this;
arrayArguments = arguments;
}
else {
arrayThis = [this];
arrayArguments = [arguments];
}
def.rejectWith(arrayThis, [Array.prototype.slice.call(arrayArguments)]);
});
});
}
}
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);
});

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?

When calling multiple parallel AJAX calls, you have two options for handling the respective responses.
Use Synchronous AJAX call/ one after another/ not recommended
Use Promises' array and $.when which accepts promises and its callback .done gets called when all the promises are return successfully with respective responses.
Example
function ajaxRequest(capitalCity) {
return $.ajax({
url: 'https://restcountries.eu/rest/v1/capital/'+capitalCity,
success: function(response) {
},
error: function(response) {
console.log("Error")
}
});
}
$(function(){
var capitalCities = ['Delhi', 'Beijing', 'Washington', 'Tokyo', 'London'];
$('#capitals').text(capitalCities);
function getCountryCapitals(){ //do multiple parallel ajax requests
var promises = [];
for(var i=0,l=capitalCities.length; i<l; i++){
var promise = ajaxRequest(capitalCities[i]);
promises.push(promise);
}
$.when.apply($, promises)
.done(fillCountryCapitals);
}
function fillCountryCapitals(){
var countries = [];
var responses = arguments;
for(i in responses){
console.dir(responses[i]);
countries.push(responses[i][0][0].nativeName)
}
$('#countries').text(countries);
}
getCountryCapitals()
})
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div>
<h4>Capital Cities : </h4> <span id="capitals"></span>
<h4>Respective Country's Native Names : </h4> <span id="countries"></span>
</div>

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);
e.g.
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,
$.ajax({
type: "POST",
url: '/echo/html/',
data: {
html: "<p>Task #" + count + " complete.",
delay: count / 2
},
success: function (data) {
$("div").append(data);
}
}));
}
return promise;
}
$(function () {
$("a").click(function () {
var promise = GetSomeDeferredStuff();
promise.then(function () {
$("div").append("<p>All done!</p>");
});
});
});
Notes:
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.

I want to propose other one with using $.each:
We may to declare ajax function like:
function ajaxFn(someData) {
this.someData = someData;
var that = this;
return function () {
var promise = $.Deferred();
$.ajax({
method: "POST",
url: "url",
data: that.someData,
success: function(data) {
promise.resolve(data);
},
error: function(data) {
promise.reject(data);
}
})
return promise;
}
}
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]);
arrayOfFn.push(ajaxFnForArray);
}
And calling functions with sending ajax:
$.when(
$.each(arrayOfFn, function(index, value) {
value.call()
})
).then(function() {
alert("Cheer!");
}
)

If you're transpiling and have access to ES6, you can use spread syntax which specifically applies each iterable item of an object as a discrete argument, just the way $.when() needs it.
$.when(...deferreds).done(() => {
// do stuff
});
MDN Link - Spread Syntax

I had a case very similar where I was posting in an each loop and then setting the html markup in some fields from numbers received from the ajax. I then needed to do a sum of the (now-updated) values of these fields and place in a total field.
Thus the problem was that I was trying to do a sum on all of the numbers but no data had arrived back yet from the async ajax calls. I needed to complete this functionality in a few functions to be able to reuse the code. My outer function awaits the data before I then go and do some stuff with the fully updated DOM.
// 1st
function Outer() {
var deferreds = GetAllData();
$.when.apply($, deferreds).done(function () {
// now you can do whatever you want with the updated page
});
}
// 2nd
function GetAllData() {
var deferreds = [];
$('.calculatedField').each(function (data) {
deferreds.push(GetIndividualData($(this)));
});
return deferreds;
}
// 3rd
function GetIndividualData(item) {
var def = new $.Deferred();
$.post('#Url.Action("GetData")', function (data) {
item.html(data.valueFromAjax);
def.resolve(data);
});
return def;
}

If you're using angularJS or some variant of the Q promise library, then you have a .all() method that solves this exact problem.
var savePromises = [];
angular.forEach(models, function(model){
savePromises.push(
model.saveToServer()
)
});
$q.all(savePromises).then(
function success(results){...},
function failed(results){...}
);
see the full API:
https://github.com/kriskowal/q/wiki/API-Reference#promiseall
https://docs.angularjs.org/api/ng/service/$q

jQuery Deferred/Promises dynamic array not executing callbacks in correct order

Grateful for any insight into what I'm misunderstanding here. My requirement is as follows:
I have an array of URLs. I want to fire off an AJAX request for each URL simultaneously, and as soon as the first request completes, call the first callback. Then, if and when the second request completes, call that callback, and so on.
Option 1:
for (var i = 0; i < myUrlArray.length; i++) {
$.ajax({
url: myUrlArray[i]
}).done(function(response) {
// Do something with response
});
}
Obviously this doesn't work, as there is no guarantee the responses will complete in the correct order.
Option 2:
var promises = [];
for (var i = 0; i < myUrlArray.length; i++) {
promises.push($.ajax({
url: myUrlArray[i]
}));
}
$.when.apply($, promises).then(function() {
// Do something with each response
});
This should work, but the downside is that it waits until all AJAX requests have completed, before firing any of the callbacks.
Ideally, I should be able to call the first callback as soon as it's complete, then chain the second callback to execute whenever that response is received (or immediately if it's already resolved), then the third, and so on.
The array length is completely variable and could contain any number of requests at any given time, so just hard coding the callback chain isn't an option.
My attempt:
var promises = [];
for (var i = 0; i < myUrlArray.length; i++) {
promises.push($.ajax({
url: myUrlArray[i] // Add each AJAX Deferred to the promises array
}));
}
(function handleAJAX() {
var promise;
if (promises.length) {
promise = promises.shift(); // Grab the first one in the stack
promise.then(function(response) { // Set up 'done' callback
// Do something with response
if (promises.length) {
handleAJAX(); // Move onto the next one
}
});
}
}());
The problem is that the callbacks execute in a completely random order! For example, if I add 'home.html', 'page2.html', 'page3.html' to the array, the order of responses won't necessarily be 'home.html', 'page2.html', 'page3.html'.
I'm obviously fundamentally misunderstanding something about the way promises work. Any help gratefully appreciated!
Cheers
EDIT
OK, now I'm even more confused. I made this JSFiddle with one array using Alnitak's answer and another using JoeFletch's answer and neither of them work as I would expect! Can anyone see what is going on here?
EDIT 2
Got it working! Based on JoeFletch's answer below, I adapted the solution as follows:
var i, responseArr = [];
for (i = 0; i < myUrlArray.length; i++) {
responseArr.push('0'); // <-- Add 'unprocessed' flag for each pending request
(function(ii) {
$.ajax({
url: myUrlArray[ii]
}).done(function(response) {
responseArr[ii] = response; // <-- Store response in array
}).fail(function(xhr, status, error) {
responseArr[ii] = 'ERROR';
}).always(function(response) {
for (var iii = 0; iii < responseArr.length; iii++) { // <-- Loop through entire response array from the beginning
if (responseArr[iii] === '0') {
return; // As soon as we hit an 'unprocessed' request, exit loop
}
else if (responseArr[iii] !== 'done') {
$('#target').append(responseArr[iii]); // <-- Do actual callback DOM append stuff
responseArr[iii] = 'done'; // <-- Set 'complete' flag for this request
}
}
});
}(i)); // <-- pass current value of i into closure to encapsulate
}
TL;DR: I don't understand jQuery promises, got it working without them. :)

Don't forget that you don't need to register the callbacks straight away.
I think this would work, the main difference with your code being that I've used .done rather than .then and refactored a few lines.
var promises = myUrlArray.map(function(url) {
return $.ajax({url: url});
});
(function serialize() {
var def = promises.shift();
if (def) {
def.done(function() {
callback.apply(null, arguments);
serialize();
});
}
})();

Here's my attempt at solving this. I updated my answer to include error handling for a failed .ajax call. I also moved some code to the complete method of the .ajax call.
var urlArr = ["url1", "url2"];
var responseArr = [];
for(var i = 0; i < length; i++) {
responseArr.push("0");//0 meaning unprocessed to the DOM
}
$.each(urlArr, function(i, url){
$.ajax({
url: url,
success: function(data){
responseArr[i] = data;
},
error: function (xhr, status, error) {
responseArr[i] = "Failed Response";//enter whatever you want to place here to notify the end user
},
complete: function() {
$.each(responseArr, function(i, element){
if (responseArr[i] == "0") {
return;
}
else if (responseArr[i] != "done")
{
//do something with the response
responseArr[i] = "done";
}
});
}
});
})

Asynchronous requests aren't guaranteed to finish in the same order that they are sent. some may take longer than others depending on server load and the amount of data being transferred.
The only options are either to wait until they are all done, only send one at a time, or just deal with them being called possibly out of order.

How can jQuery deferred be used?

jQuery 1.5 brings the new Deferred object and the attached methods .when, .Deferred and ._Deferred.
For those who haven't used .Deferred before, I've annotated the source for it.
What are the possible usages of these new methods, how do we go about fitting them into patterns?
I have already read the API and the source, so I know what it does. My question is how can we use these new features in everyday code?
I have a simple example of a buffer class that calls AJAX requests in order. (Next one starts after the previous one finishes).
/* Class: Buffer
* methods: append
*
* Constructor: takes a function which will be the task handler to be called
*
* .append appends a task to the buffer. Buffer will only call a task when the
* previous task has finished
*/
var Buffer = function(handler) {
var tasks = [];
// empty resolved deferred object
var deferred = $.when();
// handle the next object
function handleNextTask() {
// if the current deferred task has resolved and there are more tasks
if (deferred.isResolved() && tasks.length > 0) {
// grab a task
var task = tasks.shift();
// set the deferred to be deferred returned from the handler
deferred = handler(task);
// if its not a deferred object then set it to be an empty deferred object
if (!(deferred && deferred.promise)) {
deferred = $.when();
}
// if we have tasks left then handle the next one when the current one
// is done.
if (tasks.length > 0) {
deferred.done(handleNextTask);
}
}
}
// appends a task.
this.append = function(task) {
// add to the array
tasks.push(task);
// handle the next task
handleNextTask();
};
};
I'm looking for demonstrations and possible uses of .Deferred and .when.
It would also be lovely to see examples of ._Deferred.
Linking to the new jQuery.ajax source for examples is cheating.
I am particularly interested in what techniques are available when we abstract away whether an operation is synchronously or asynchronously done.

The best use case I can think of is in caching AJAX responses. Here's a modified example from Rebecca Murphey's intro post on the topic:
var cache = {};
function getData( val ){
// return either the cached value or jqXHR object wrapped Promise
return $.when(
cache[ val ] ||
$.ajax('/foo/', {
data: { value: val },
dataType: 'json',
success: function( resp ){
cache[ val ] = resp;
}
})
);
}
getData('foo').then(function(resp){
// do something with the response, which may
// or may not have been retrieved using an
// XHR request.
});
Basically, if the value has already been requested once before it's returned immediately from the cache. Otherwise, an AJAX request fetches the data and adds it to the cache. The $.when/.then doesn't care about any of this; all you need to be concerned about is using the response, which is passed to the .then() handler in both cases. jQuery.when() handles a non-Promise/Deferred as a Completed one, immediately executing any .done() or .then() on the chain.
Deferreds are perfect for when the task may or may not operate asynchronously, and you want to abstract that condition out of the code.
Another real world example using the $.when helper:
$.when($.getJSON('/some/data/'), $.get('template.tpl')).then(function (data, tmpl) {
$(tmpl) // create a jQuery object out of the template
.tmpl(data) // compile it
.appendTo("#target"); // insert it into the DOM
});

Here is a slightly different implementation of an AJAX cache as in ehynd's answer.
As noted in fortuneRice's follow-up question, ehynd's implementation didn't actually prevent multiple identical requests if the requests were performed before one of them had returned. That is,
for (var i=0; i<3; i++) {
getData("xxx");
}
will most likely result in 3 AJAX requests if the result for "xxx" has not already been cached before.
This can be solved by caching the request's Deferreds instead of the result:
var cache = {};
function getData( val ){
// Return a promise from the cache (if available)
// or create a new one (a jqXHR object) and store it in the cache.
var promise = cache[val];
if (!promise) {
promise = $.ajax('/foo/', {
data: { value: val },
dataType: 'json'
});
cache[val] = promise;
}
return promise;
}
$.when(getData('foo')).then(function(resp){
// do something with the response, which may
// or may not have been retreived using an
// XHR request.
});

A deferred can be used in place of a mutex. This is essentially the same as the multiple ajax usage scenarios.
MUTEX
var mutex = 2;
setTimeout(function() {
callback();
}, 800);
setTimeout(function() {
callback();
}, 500);
function callback() {
if (--mutex === 0) {
//run code
}
}
DEFERRED
function timeout(x) {
var dfd = jQuery.Deferred();
setTimeout(function() {
dfd.resolve();
}, x);
return dfd.promise();
}
jQuery.when(
timeout(800), timeout(500)).done(function() {
// run code
});
When using a Deferred as a mutex only, watch out for performance impacts (http://jsperf.com/deferred-vs-mutex/2). Though the convenience, as well as additional benefits supplied by a Deferred is well worth it, and in actual (user driven event based) usage the performance impact should not be noticeable.

This is a self-promotional answer, but I spent a few months researching this and presented the results at jQuery Conference San Francisco 2012.
Here is a free video of the talk:
https://www.youtube.com/watch?v=juRtEEsHI9E

Another use that I've been putting to good purpose is fetching data from multiple sources. In the example below, I'm fetching multiple, independent JSON schema objects used in an existing application for validation between a client and a REST server. In this case, I don't want the browser-side application to start loading data before it has all the schemas loaded. $.when.apply().then() is perfect for this. Thank to Raynos for pointers on using then(fn1, fn2) to monitor for error conditions.
fetch_sources = function (schema_urls) {
var fetch_one = function (url) {
return $.ajax({
url: url,
data: {},
contentType: "application/json; charset=utf-8",
dataType: "json",
});
}
return $.map(schema_urls, fetch_one);
}
var promises = fetch_sources(data['schemas']);
$.when.apply(null, promises).then(
function () {
var schemas = $.map(arguments, function (a) {
return a[0]
});
start_application(schemas);
}, function () {
console.log("FAIL", this, arguments);
});

Another example using Deferreds to implement a cache for any kind of computation (typically some performance-intensive or long-running tasks):
var ResultsCache = function(computationFunction, cacheKeyGenerator) {
this._cache = {};
this._computationFunction = computationFunction;
if (cacheKeyGenerator)
this._cacheKeyGenerator = cacheKeyGenerator;
};
ResultsCache.prototype.compute = function() {
// try to retrieve computation from cache
var cacheKey = this._cacheKeyGenerator.apply(this, arguments);
var promise = this._cache[cacheKey];
// if not yet cached: start computation and store promise in cache
if (!promise) {
var deferred = $.Deferred();
promise = deferred.promise();
this._cache[cacheKey] = promise;
// perform the computation
var args = Array.prototype.slice.call(arguments);
args.push(deferred.resolve);
this._computationFunction.apply(null, args);
}
return promise;
};
// Default cache key generator (works with Booleans, Strings, Numbers and Dates)
// You will need to create your own key generator if you work with Arrays etc.
ResultsCache.prototype._cacheKeyGenerator = function(args) {
return Array.prototype.slice.call(arguments).join("|");
};
Here is an example of using this class to perform some (simulated heavy) calculation:
// The addingMachine will add two numbers
var addingMachine = new ResultsCache(function(a, b, resultHandler) {
console.log("Performing computation: adding " + a + " and " + b);
// simulate rather long calculation time by using a 1s timeout
setTimeout(function() {
var result = a + b;
resultHandler(result);
}, 1000);
});
addingMachine.compute(2, 4).then(function(result) {
console.log("result: " + result);
});
addingMachine.compute(1, 1).then(function(result) {
console.log("result: " + result);
});
// cached result will be used
addingMachine.compute(2, 4).then(function(result) {
console.log("result: " + result);
});
The same underlying cache could be used to cache Ajax requests:
var ajaxCache = new ResultsCache(function(id, resultHandler) {
console.log("Performing Ajax request for id '" + id + "'");
$.getJSON('http://jsfiddle.net/echo/jsonp/?callback=?', {value: id}, function(data) {
resultHandler(data.value);
});
});
ajaxCache.compute("anID").then(function(result) {
console.log("result: " + result);
});
ajaxCache.compute("anotherID").then(function(result) {
console.log("result: " + result);
});
// cached result will be used
ajaxCache.compute("anID").then(function(result) {
console.log("result: " + result);
});
You can play with the above code in this jsFiddle.

1) Use it to ensure an ordered execution of callbacks:
var step1 = new Deferred();
var step2 = new Deferred().done(function() { return step1 });
var step3 = new Deferred().done(function() { return step2 });
step1.done(function() { alert("Step 1") });
step2.done(function() { alert("Step 2") });
step3.done(function() { alert("All done") });
//now the 3 alerts will also be fired in order of 1,2,3
//no matter which Deferred gets resolved first.
step2.resolve();
step3.resolve();
step1.resolve();
2) Use it to verify the status of the app:
var loggedIn = logUserInNow(); //deferred
var databaseReady = openDatabaseNow(); //deferred
jQuery.when(loggedIn, databaseReady).then(function() {
//do something
});

You can use a deferred object to make a fluid design that works well in webkit browsers. Webkit browsers will fire resize event for each pixel the window is resized, unlike FF and IE which fire the event only once for each resize. As a result, you have no control over the order in which the functions bound to your window resize event will execute. Something like this solves the problem:
var resizeQueue = new $.Deferred(); //new is optional but it sure is descriptive
resizeQueue.resolve();
function resizeAlgorithm() {
//some resize code here
}
$(window).resize(function() {
resizeQueue.done(resizeAlgorithm);
});
This will serialize the execution of your code so that it executes as you intended it to. Beware of pitfalls when passing object methods as callbacks to a deferred. Once such method is executed as a callback to deferred, the 'this' reference will be overwritten with reference to the deferred object and will no longer refer to the object the method belongs to.

You can also integrate it with any 3rd-party libraries which makes use of JQuery.
One such library is Backbone, which is actually going to support Deferred in their next version.

I've just used Deferred in real code. In project jQuery Terminal I have function exec that call commands defined by user (like he was entering it and pressing enter), I've added Deferreds to the API and call exec with arrays. like this:
terminal.exec('command').then(function() {
terminal.echo('command finished');
});
or
terminal.exec(['command 1', 'command 2', 'command 3']).then(function() {
terminal.echo('all commands finished');
});
the commands can run async code, and exec need to call user code in order. My first api use pair of pause/resume calls and in new API I call those automatic when user return promise. So user code can just use
return $.get('/some/url');
or
var d = new $.Deferred();
setTimeout(function() {
d.resolve("Hello Deferred"); // resolve value will be echoed
}, 500);
return d.promise();
I use code like this:
exec: function(command, silent, deferred) {
var d;
if ($.isArray(command)) {
return $.when.apply($, $.map(command, function(command) {
return self.exec(command, silent);
}));
}
// both commands executed here (resume will call Term::exec)
if (paused) {
// delay command multiple time
d = deferred || new $.Deferred();
dalyed_commands.push([command, silent, d]);
return d.promise();
} else {
// commands may return promise from user code
// it will resolve exec promise when user promise
// is resolved
var ret = commands(command, silent, true, deferred);
if (!ret) {
if (deferred) {
deferred.resolve(self);
return deferred.promise();
} else {
d = new $.Deferred();
ret = d.promise();
ret.resolve();
}
}
return ret;
}
},
dalyed_commands is used in resume function that call exec again with all dalyed_commands.
and part of the commands function (I've stripped not related parts)
function commands(command, silent, exec, deferred) {
var position = lines.length-1;
// Call user interpreter function
var result = interpreter.interpreter(command, self);
// user code can return a promise
if (result != undefined) {
// new API - auto pause/resume when using promises
self.pause();
return $.when(result).then(function(result) {
// don't echo result if user echo something
if (result && position === lines.length-1) {
display_object(result);
}
// resolve promise from exec. This will fire
// code if used terminal::exec('command').then
if (deferred) {
deferred.resolve();
}
self.resume();
});
}
// this is old API
// if command call pause - wait until resume
if (paused) {
self.bind('resume.command', function() {
// exec with resume/pause in user code
if (deferred) {
deferred.resolve();
}
self.unbind('resume.command');
});
} else {
// this should not happen
if (deferred) {
deferred.resolve();
}
}
}

The answer by ehynds will not work, because it caches the responses data. It should cache the jqXHR which is also a Promise.
Here is the correct code:
var cache = {};
function getData( val ){
// return either the cached value or an
// jqXHR object (which contains a promise)
return cache[ val ] || $.ajax('/foo/', {
data: { value: val },
dataType: 'json',
success: function(data, textStatus, jqXHR){
cache[ val ] = jqXHR;
}
});
}
getData('foo').then(function(resp){
// do something with the response, which may
// or may not have been retreived using an
// XHR request.
});
The answer by Julian D. will work correct and is a better solution.