Develop Reference

Issue with jQuery deferred

I'm facing an issue with deferred usage where 2 nested function that should wait for each other actually run in the wrong order silently.
I cant' figure out where I mix return promise.
So here is what I try to achieve. In a mobile Cordova app, when user enter the Game view, I got a function that download question in WebSql, and I want then to retrieve one question, and then my slider function load the content.
So I nested the getQuestion function in the .done() event.
router.addRoute('game', function () {
'use strict';
//Reload Question List when User enter the Game view.
questionService.initialize().done(
//Now we got question, initialize the Game View
questionService.getQuestion().done(
function (data) {
console.log(data);
slider.slidePage(new GameView(data).render().$el);
})
);
Here is how I use the $.Deferred() in both function. First I declare my $.Deferred() and at the end of the function I return the promise.
But my getQuestion() in code below does not wait for the initialize() function to end before start.
Where did I mixed up my promise return?
var getQuestions = function(param) {
var deferred = $.Deferred();
param = param;
$.ajax({
type: 'POST',
url: 'myserver',
data: {
region: uRegion
},
success: function(value, status) {
//do something with value
this.db = window.openDatabase('database details');
this.db.transaction(function(tx) {
storeQuestion(tx);
}, function(error) {
deferred.reject('Transaction error: ' + error);
}, function() {
//Transaction success
deferred.resolve();
});
},
error: function(textStatus, exception) {}
});
return deferred.promise();
};

You are not passing the success callback handler. As per current implementation your are invoking getQuestion() immediately.
Use anonymous function, rest its fine
//Reload Question List when User enter the Game view.
questionService.initialize().done(function(){
//Now we got question, initialize the Game View
questionService.getQuestion().done(
function (data) {
console.log(data);
slider.slidePage(new GameView(data).render().$el);
})
});

Function Keep Triggering Before JSON Done Processing

Function 1: Get JSON Data & Store
I am creating a script where an array of twitch channels will go through the JSON function loop to be processed and then stored using "localStorage.setItem" as temporary storage. I'm saving them in name,viewer and url.
Function 2: Sort Data
Stored data can later be used to display the information without having to use function 1 again.
Problem
The sortdata function keeps on firing before function 1 is complete. Resorting in error because the data is undefined. This error popped before the console displays all the information stored from function 1.
My code:
$(window).load(function(){
$.when(getData()).promise().done(function(){
getStoredObj();
});
});
function getData(){
var streamArray=[];
jQuery.each (channels, function (i, channel) {
channelId = channel.id;
channelUrl = channel.url;
var promise = $.ajax({
dataType: "jsonp",
url: twitchApi + channelId,
success: 1,
}).done(function ( data ) {
if (data.stream == null) {
} else {
var displayName = data.stream.channel.display_name;
var viewerCount = data.stream.viewers;
streamArray.push({name: displayName, views: viewerCount, url: channelUrl});
localStorage.setItem("storedStreamArray", JSON.stringify(streamArray));
console.log(JSON.stringify(streamArray));
}
});
});
}
function getStoredObj () {
var retrievedObject = localStorage.getItem('storedStreamArray');
var parsedObject = JSON.parse(retrievedObject);
<sorting codes here>
}
Some help here really appreciated. :)

You're calling $.when with the result of getData, but getData doesn't return anything, let alone a deferred that when can use. As a result, there's nothing to wait for and your done callback calls getStoredObj immediately.
In getData, you need to collect all the deferreds returned by your ajax calls and pass them back to the caller. That would look like:
function getData(){
return jQuery.map (channels, function (i, channel) {
return $.ajax(...).done(function ( data ) {
// Do work
});
});
}
Each iteration returns its ajax deferred, which are aggregated by map and returned to the caller. Then you can run when on the result and wait for loading to finish before you sort anything.

Multiple Promise Chains in Single Function

I have some code that will dynamically generate an AJAX request based off a scenario that I'm retrieving via an AJAX request to a server.
The idea is that:
A server provides a "Scenario" for me to generate an AJAX Request.
I generate an AJAX Request based off the Scenario.
I then repeat this process, over and over in a Loop.
I'm doing this with promises here: http://jsfiddle.net/3Lddzp9j/11/
However, I'm trying to edit the code above so I can handle an array of scenarios from the initial AJAX request.
IE:
{
"base": {
"frequency": "5000"
},
"endpoints": [
{
"method": "GET",
"type": "JSON",
"endPoint": "https://api.github.com/users/alvarengarichard",
"queryParams": {
"objectives": "objective1, objective2, objective3"
}
},
{
"method": "GET",
"type": "JSON",
"endPoint": "https://api.github.com/users/dkang",
"queryParams": {
"objectives": "objective1, objective2, objective3"
}
}
]
This seems like it would be straight forward, but the issue seems to be in the "waitForTimeout" function.
I'm unable to figure out how to run multiple promise chains. I have an array of promises in the "deferred" variable, but the chain only continues on the first one--despite being in a for loop.
Could anyone provide insight as to why this is? You can see where this is occuring here: http://jsfiddle.net/3Lddzp9j/10/

The main problems are that :
waitForTimeout isn't passing on all the instructions
even if waitForTimeout was fixed, then callApi isn't written to perform multiple ajax calls.
There's a number of other issues with the code.
you really need some data checking (and associated error handling) to ensure that expected components exist in the data.
mapToInstruction is an unnecessary step - you can map straight from data to ajax options - no need for an intermediate data transform.
waitForTimeout can be greatly simplified to a single promise, resolved by a single timeout.
synchronous functions in a promise chain don't need to return a promise - they can return a result or undefined.
Sticking with jQuery all through, you should end up with something like this :
var App = (function ($) {
// Gets the scenario from the API
// sugar for $.ajax with GET as method - NOTE: this returns a promise
var getScenario = function () {
console.log('Getting scenario ...');
return $.get('http://demo3858327.mockable.io/scenario2');
};
var checkData = function (data) {
if(!data.endpoints || !data.endpoints.length) {
return $.Deferred().reject('no endpoints').promise();
}
data.base = data.base || {};
data.base.frequency = data.base.frequency || 1000;//default value
};
var waitForTimeout = function(data) {
return $.Deferred(function(dfrd) {
setTimeout(function() {
dfrd.resolve(data.endpoints);
}, data.base.frequency);
}).promise();
};
var callApi = function(endpoints) {
console.log('Calling API with given instructions ...');
return $.when.apply(null, endpoints.map(ep) {
return $.ajax({
type: ep.method,
dataType: ep.type,
url: ep.endpoint
}).then(null, function(jqXHR, textStatus, errorThrown) {
return textStatus;
});
}).then(function() {
//convert arguments to an array of results
return $.map(arguments, function(arg) {
return arg[0];
});
});
};
var handleResults = function(results) {
// results is an array of data values/objects returned by the ajax calls.
console.log("Handling data ...");
...
};
// The 'run' method
var run = function() {
getScenario()
.then(checkData)
.then(waitForTimeout)
.then(callApi)
.then(handleResults)
.then(null, function(reason) {
console.error(reason);
})
.then(run);
};
return {
run : run
}
})(jQuery);
App.run();
This will stop on error but could be easily adapted to continue.

I'll try to answer your question using KrisKowal's q since I'm not very proficient with the promises generated by jQuery.
First of all I'm not sure whether you want to solve the array of promises in series or in parallel, in the solution proposed I resolved all of them in parallel :), to solve them in series I'd use Q's reduce
function getScenario() { ... }
function ajaxRequest(instruction) { ... }
function createPromisifiedInstruction(instruction) {
// delay with frequency, not sure why you want to do this :(
return Q.delay(instruction.frequency)
.then(function () {
return this.ajaxRequest(instruction);
});
}
function run() {
getScenario()
.then(function (data) {
var promises = [];
var instruction;
var i;
for (i = 0; i < data.endpoints.length; i += 1) {
instruction = {
method: data.endpoints[i].method,
type: data.endpoints[i].type,
endpoint: data.endpoints[i].endPoint,
frequency: data.base.frequency
};
promises.push(createPromisifiedInstruction(instruction));
}
// alternative Q.allSettled if all the promises don't need to
// be fulfilled (some of them might be rejected)
return Q.all(promises);
})
.then(function (instructionsResults) {
// instructions results is an array with the result of each
// promisified instruction
})
.then(run)
.done();
}
run();
Ok let me explain the solution above:
first of all assume that getScenario gets you the initial json you start with (actually returns a promise which is resolved with the json)
create the structure of each instruction
promisify each instruction, so that each one is actually a promise whose
resolution value will be the promise returned by ajaxRequest
ajaxRequest returns a promise whose resolution value is the result of the request, which also means that createPromisifiedInstruction resolution value will be the resolution value of ajaxRequest
Return a single promise with Q.all, what it actually does is fulfill itself when all the promises it was built with are resolved :), if one of them fails and you actually need to resolve the promise anyways use Q.allSettled
Do whatever you want with the resolution value of all the previous promises, note that instructionResults is an array holding the resolution value of each promise in the order they were declared
Reference: KrisKowal's Q

Try utilizing deferred.notify within setTimeout and Number(settings.frequency) * (1 + key) as setTimeout duration; msg at deferred.notify logged to console at deferred.progress callback , third function argument within .then following timeout
var App = (function ($) {
var getScenario = function () {
console.log("Getting scenario ...");
return $.get("http://demo3858327.mockable.io/scenario2");
};
var mapToInstruction = function (data) {
var res = $.map(data.endpoints, function(settings, key) {
return {
method:settings.method,
type:settings.type,
endpoint:settings.endPoint,
frequency:data.base.frequency
}
});
console.log("Instructions recieved:", res);
return res
};
var waitForTimeout = function(instruction) {
var res = $.when.apply(instruction,
$.map(instruction, function(settings, key) {
return new $.Deferred(function(dfd) {
setTimeout(function() {
dfd.notify("Waiting for "
+ settings.frequency
+ " ms")
.resolve(settings);
}, Number(settings.frequency) * (1 + key));
}).promise()
})
)
.then(function() {
return this
}, function(err) {
console.log("error", err)
}
, function(msg) {
console.log("\r\n" + msg + "\r\nat " + $.now() + "\r\n")
});
return res
};
var callApi = function(instruction) {
console.log("Calling API with given instructions ..."
, instruction);
var res = $.when.apply(instruction,
$.map(instruction, function(request, key) {
return request.then(function(settings) {
return $.ajax({
type: settings.method,
dataType: settings.type,
url: settings.endpoint
});
})
})
)
.then(function(data) {
return $.map(arguments, function(response, key) {
return response[0]
})
})
return res
};
var handleResults = function(data) {
console.log("Handling data ..."
, JSON.stringify(data, null, 4));
return data
};
var run = function() {
getScenario()
.then(mapToInstruction)
.then(waitForTimeout)
.then(callApi)
.then(handleResults)
.then(run);
};
return {
// This will expose only the run method
// but will keep all other functions private
run : run
}
})($);
// ... And start the app
App.run();
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js">
</script>
jsfiddle http://jsfiddle.net/3Lddzp9j/13/

You have a return statement in the loop in your waitForTimeout function. This means that the function is going to return after the first iteration of the loop, and that is where you are going wrong.
You're also using the deferred antipattern and are using promises in places where you don't need them. You don't need to return a promise from a then handler unless there's something to await.
The key is that you need to map each of your instructions to a promise. Array#map is perfect for this. And please use a proper promise library, not jQuery promises (edit but if you absolutely must use jQuery promises...):
var App = (function ($) {
// Gets the scenario from the API
// NOTE: this returns a promise
var getScenario = function () {
console.log('Getting scenario ...');
return $.get('http://demo3858327.mockable.io/scenario');
};
// mapToInstructions is basically unnecessary. each instruction does
// not need its own timeout if they're all the same value, and you're not
// reshaping the original values in any significant way
// This wraps the setTimeout into a promise, again
// so we can chain it
var waitForTimeout = function(data) {
var d = $.Deferred();
setTimeout(function () {
d.resolve(data.endpoints);
}, data.base.frequency);
return d.promise();
};
var callApi = function(instruction) {
return $.ajax({
type: instruction.method,
dataType: instruction.type,
url: instruction.endPoint
});
};
// Final step: call the API from the
// provided instructions
var callApis = function(instructions) {
console.log(instructions);
console.log('Calling API with given instructions ...');
return $.when.apply($, instructions.map(callApi));
};
var handleResults = function() {
var data = Array.prototype.slice(arguments);
console.log("Handling data ...");
};
// The 'run' method
var run = function() {
getScenario()
.then(waitForTimeout)
.then(callApis)
.then(handleResults)
.then(run);
};
return {
run : run
}
})($);
App.run();

Synchronous request in Windows Azure?

So in my server code, variable invites is undefined outside of the success function.
function getInvites(id){
var InvitesTable = tables.getTable("Invites").where({"PlanID": id}).select("UserID","Attending");
var invites;
InvitesTable.read({ success: function(resultss) {
invites = resultss;
console.log(invites); //works here
}});
console.log(invites); //undefined here
}
From similar questions, I realize its because of it being asynchronous. So the success function call is run after the console.log(invites); //undefined here call.
My question is how do I stop that in Windows Azure?
Added code
function read(query, user, request) {
request.execute({
success: function(results) {
results.forEach(function(r) {
getInvites(r.id, function(invites) {
r.invites = invites;
});
});
request.respond();
}
});
}
function getInvites(id, cb){
var InvitesTable = tables.getTable("Invites").where({"PlanID": id}).select("UserID","Attending");
InvitesTable.read({ success: function(results) {
if (cb) cb(results);
}});
}

You don't "stop that," you design your application around the async nature of whatever environment you're using.
I assume you're trying to do something like this:
function getInvites(id){
var InvitesTable = tables.getTable("Invites").where({"PlanID": id}).select("UserID","Attending");
var invites;
InvitesTable.read({ success: function(resultss) {
invites = resultss;
}});
return invites;
}
// later...
var invites = getInvites(someId);
//do something with `invites`
This obviously won't work, since you return the value of invites before the async call completes.
Instead, you write your app in async style:
function getInvites(id, cb){
var InvitesTable = tables.getTable("Invites").where({"PlanID": id}).select("UserID","Attending");
InvitesTable.read({ success: function(resultss) {
if (cb) cb(resultss);
}});
}
// later...
getInvites(someId, function(invites) {
//do something with `invites`
});
This leaves out error handling code for the sake of simplicity, so you'd have to add that as well.
After seeing your full code, it looks like you have a simple problem of managing many parallel asynchronous operations. Consider what happens: your loop runs, iterating over an array of n objects. For each, you call getInvites, which begins a database request and returns.
This means your loop runs very quickly, but now you have n outstanding database requests that you must wait on before you can call request.respond().
An extremely basic solution would be to do something like count the number of times your getInvites callback is called, and then finally complete the request when that number reaches n.
However, it is time-consuming and mistake-prone to manage this bookkeeping manually every time you make async requests. This is a situation where flow control libraries are extremely useful. I will use jQuery's Deferred in this example, since it may already be familiar to you (even if you don't know you've actually used it before — if you've ever used jQuery's XHR API, you've used Deferreds).
Given that you're in a server environment, you obviously don't have jQuery; however, there are people who have extracted only the code necessary for Deferred for you.
Once we have Deferreds for every pending request, we can use when to register a callback that gets called only after all pending Deferreds complete.
function read(query, user, request) {
request.execute({
success: function(results) {
var dfds = [];
for (var i = 0; i < results.length; i++) {
dfds.push(getInvites(results[i].id)); // Makes an array of Deferreds representing
// each of our pending requests.
}
Deferred.when.apply(Deferred, dfds) // see details below
.done(function() {
for (var i = 0; i < results.length; i++) {
results[i].invites = arguments[i]; // Copy each set of invites to each result
}
request.respond(); // We're done!
})
.fail(function() {
// Handle errors here
});
}
});
}
function getInvites(id){
var dfd = new Deferred(); // Create a new Deferred, which automatically starts in the 'pending' state
var InvitesTable = tables.getTable("Invites").where({"PlanID": id}).select("UserID","Attending");
InvitesTable.read({ success: function(results) {
dfd.resolve(results); // When we get data back, we 'resolve' the Deferred --
// in other words, say its operation is done,
// and pass along the operation's results.
},
error: function(err) { // TODO: Not sure if this is how the API you're using handles errors
dfd.reject(err); // Marks the Deferred as failed.
}});
return dfd.promise(); // We (synchronously) return the Promise. The caller can attach event handlers
// to the Promise, which are invoked when we eventually resolve or reject the Deferred.
}
Notes:
jQuery.when (or in this server-side case, Deferred.when) normally expects you to pass a fixed number of Deferreds as arguments:
$.when(dfd1, dfd2).done(function(result1, result2) { ... });
However, we have a variable number of Deferreds, so we must apply an array of Deferreds to when and then in the done handler, access each result via the implicit arguments object.
Array.forEach(...) is slow. In most cases, it is better to use a regular for loop.

I've stumbled on same need for synchronous DB access, so I wrote small module called query-synchronizer.
Idea was to count how many times query was started and ended. If all started count was equal to ended count, other part of code would be executed. Your code would look like this:
var synchronizer = require('query-synchronizer');
function read(query, user, request) {
request.execute({
success: function(results) {
results.forEach(function(r) {
var InvitesTable = tables.getTable("Invites").where({"PlanID": r.id}).select("UserID","Attending");
synchronizer.read(InvitesTable, function(results){
r.invites = invites;
});
});
synchronizer.done(function(){
request.respond();
});
}
});
}

How to chain ajax requests?

I have to interact with a remote api that forces me to chain requests. Thats a callback-hell in asynchronous mode:
// pseudocode: ajax(request_object, callback)
ajax(a, function() {
ajax(b(a.somedata), function() {
ajax(c(b.somedata), function() {
c.finish()
}
})
})
It would be much more readable in sync mode:
sjax(a)
sjax(b(a.somedata))
sjax(c(b.somedata))
c.finish()
But Sjax is evil :) How do I do that in a nice not-so-evil and readable way?

You could have a single function which is passed an integer to state what step the request is in, then use a switch statement to figure out what request needs to be make next:
function ajaxQueue(step) {
switch(step) {
case 0: $.ajax({
type: "GET",
url: "/some/service",
complete: function() { ajaxQueue(1); }
}); break;
case 1: $.ajax({
type: "GET",
url: "/some/service",
complete: function() { ajaxQueue(2); }
}); break;
case 2: $.ajax({
type: "GET",
url: "/some/service",
complete: function() { alert('Done!'); }
}); break;
}
}
ajaxQueue(0);
Hope that helps!

Don't use anonymous functions. Give them names. I don't know if you're able to do what I wrote below though:
var step_3 = function() {
c.finish();
};
var step_2 = function(c, b) {
ajax(c(b.somedata), step_3);
};
var step_1 = function(b, a) {
ajax(b(a.somedata), step_2);
};
ajax(a, step_1);

This function should chain together a list of ajax requests, if the callbacks always return the parameters necessary for the next request:
function chainajax(params, callbacks) {
var cb = shift(callbacks);
params.complete = function() {
var newparams = cb(arguments);
if (callbacks)
chainajax(newparams, callbacks);
};
$.ajax(params);
};
You can define these callback functions separately and then chain them together:
function a(data) {
...
return {type: "GET", url: "/step2.php?foo"}
};
// ...
function d(data) { alert("done!"); };
chainajax({type: "GET", url: "/step1.php"},
[a, b, c, d]);
You could also declare the functions "inline" in the call to chainajax, but that might get a little confusing.

Maybe what you can do is write a server-side wrapper function. That way your javascript only does a single asynchronous call to your own web server. Then your web server uses curl (or urllib, etc.) to interact with the remote API.

Update: I've learn a better answer for this if you are using jQuery, see my update under the title: Using jQuery Deffered
Old answer:
You can also use Array.reduceRight (when it's available) to wrap the $.ajax calls and transform a list like: [resource1, resource2] into $.ajax({url:resource1,success: function(...) { $ajax({url: resource2... (a trick that I've learn from Haskell and it's fold/foldRight function).
Here is an example:
var withResources = function(resources, callback) {
var responses = [];
var chainedAjaxCalls = resources.reduceRight(function(previousValue, currentValue, index, array) {
return function() {
$.ajax({url: currentValue, success: function(data) {
responses.push(data);
previousValue();
}})
}
}, function() { callback.apply(null, responses); });
chainedAjaxCalls();
};
Then you can use:
withResources(['/api/resource1', '/api/resource2'], function(response1, response2) {
// called only if the ajax call is successful with resource1 and resource2
});
Using jQuery Deffered
If you are using jQuery, you can take advantage of jQuery Deffered, by using the jQuery.when() function:
jQuery.when($.get('/api/one'), $.get('/api/two'))
.done(function(result1, result2) {
/* one and two is done */
});

Check out this FAQ item on the jQuery site. Specially the callback reference and the complete method.
What you want is data from A to be passed to B and B's data passed to C. So you would do a callback on complete.
I haven't tried this though.

I believe that implementing a state machine will make the code more readable:
var state = -1;
var error = false;
$.ajax({success: function() {
state = 0;
stateMachine(); },
error: function() {
error = true;
stateMachine();
}});
function stateMachine() {
if (error) {
// Error handling
return;
}
if (state == 0) {
state = 1;
// Call stateMachine again in an ajax callback
}
else if (state == 1) {
}
}

I made a method using Promises
// How to setup a chainable queue method
var sequence = Promise.resolve();
function chain(next){
var promise = new Promise(function(resolve){
sequence.then(function(){
next(resolve);
});
});
sequence = promise;
}
// How to use it
chain(function(next){
document.write("<p>start getting config.json</p>");
setTimeout(function(){
document.write("<p>Done fetching config.json</p>");
next();
}, 3000);
});
chain(function(next){
document.write("<p>start getting init.js</p>")
setTimeout(function(){
document.write("<p>starting eval scripting</p>");
next();
}, 3000);
});
chain(function(next){
document.write("<p>Everything is done</p>");
});
Bonus: A ultraligth 138 byte limited A- Promise (that can only resolve - without parameters, and only call the last then-method )
Background:
I made this for node.js at the point where it dose not have promises ATM. I didn't want a complete full blown Promise library that I was dependent on and had to include in my package.json, I needed it to be fast and light and do mostly one thing only. I only needed it for one thing (chaining things like you want to)
function Q(a,b){b=this;a(function(){b.then&&b.then();b.then=i});return b}function i(a){a&&a()}Q.prototype={then:function(a){this.then=a}};
How?
// Start with a resolved object
var promise = new Q(function(a){a()});
// equal to
// var promise = Promise.resolve();
// example usage
new Q(function(resolve){
// do some async stuff that takes time
// setTimeout(resolve, 3000);
}).then(function(){
// its done
// can not return a new Promise
}); // <- can not add more then's (it only register the last one)
and for the chainable queue method
// How to setup a chainable queue method with ultraligth promise
var sequence = new Q(function(a){a()});
function chain(next){
var promise = new Q(function(resolve){
sequence.then(function(){
next(resolve);
});
});
sequence = promise;
}

The complete callback is what you're looking for:
$.ajax({
type: 'post',
url: "www.example.com",
data: {/* Data to be sent to the server. It is converted to a query string, if not already a string. It's appended to the url for GET-requests. */},
success:
function(data) {
/* you can also chain requests here. will be fired if initial request is successful but will be fired before completion. */
},
complete:
function() {
/* For more a more synchronous approach use this callback. Will be fired when first function is completed. */
}
});