We Keep Coding

Javascript return reference to jqXHR from function

Recently I abstracted a bunch of AJAX calls into a function using the $.post() method. My idea was to use the jqXHR object returned by $.post() to add standard error handlers and such (via .fail() .done() .always()) to remove code duplication. I then thought that I could return the jqXHR object from the method to the original calling function so I could add additional handlers to do more context-specific things.
However, the jqXHR object returned seems to be a copy of the original object, not the object itself. As such, its state never gets updated by the original call so none of my additional .done() functions get executed.
Is it possible to return a reference to an object in JS? I have a C++ background, so is there a way to mimic the "return pointer to object" functionality in JS?
I know there have been discussions on pass-by-reference/value on here, but I couldn't find one dealing specifically with Deferred objects
EDIT: example code
function AJAX(url,data,onSuccess) {
var jqxhr = $.post({url,data});
jqxhr.done(onSuccess);
jqxhr.fail(displayError());
return jqxhr;
}
...
function example() {
var dfrd = AJAX("example.php",data,successFunc);
dfrd.done(alert("Hello, World!"));
}
The .done() & .fail() within the AJAX function get executed normally, but the .done() within example() never executes because dfrd.state() is always pending

So this turned out to be a timing issue.
It appears that, with multiple .done() functions, there is a slight delay when the function receives a response from the $.post() call. So my onSuccess(resp) {} that was passed to the AJAX function would execute after the alert(). The particular onSuccess function I was using to test had a page refresh call in it, which would close the alert before I could see it.
I have a C++ background, so I guess I need to upgrade my JS testing procedures.
Thanks to all who commented!

Updating parameters to JavaScript callback before it returns, without using globals

I'm working on making a modification to a node.js module to improve error handling for one of my uses cases. The specifics of the module aren't really relevant, but one of the things I want to do is trigger a delayed retry when receiving a certain status code from the response to an API request. To do this I'm using the timeOut function to trigger a new call to the function after a period of time.
The structure of the code looks like this:
Outer function (parameters specified by client application)
——API request (using parameters)
——Callback from API request (response with status code)
——If error, set timeout to call outer function after delay
However, I also want to handle the case that the outer function is called again while waiting for the timeout. I don't want any calls to trigger a new API request while a timeout is pending, but I do want the parameters from the most recent call to be used when the timeout finishes.
I've been able to get this working using variables that are global to the module. Each time a new call comes in to the outer function it updates a global object with the new parameters then, if a timeout is pending, returns without calling the API request. The timeout function uses the parameters from the global object to call the outer function, rather than the parameters from when it was set. This way it always uses the most recent values that were passed into the outer function, but doesn't make duplicate API calls.
Here's a simplified example of how I've achieved this behavior with global variables: JSFiddle. Hit run a few times until you get a "failure response" which then triggers the timeout.
This works, but I would prefer not add these global variables into the module if there's a better way.
Is there any way to get this same behavior but have all of the state encapsulated in the outer function without using globals? I'm also open to completely rethinking the way I'm handling this if anyone has ideas.

You're not going to be able to do this without creating variables outside of your outer function, however it's still possible to create those variables without polluting your global scope.
To do so, wrap your outer function in another function that gets executed immediately, sometimes known as an IIFE:
mainFunction: (function() {
var savedParam1;
var savedParam2;
var savedParam3;
var pendingTimeout = false;
return function(param1, param2, param3) {
savedParam1 = param1;
savedParam2 = param2;
savedParam3 = param3;
if (pendingTimeout) {
log('pending timeout -- ignoring new call');
return;
}
/* and so on */
}
})(); // the () causes the outer function to run immediately,
// which creates a scope for the savedParam / pendingTimeout variables,
// and then returns the inner function (your old outer function)
// to be used for mainFunction

variable is undefined when attached to callback function (Javascript / Dojo)

I have this variable declared at global scope:
var wait;
then, inside of an event listener, I assign a function to this variable, then attach a callback function: (I am using Dojo, via the ArcGIS Javascript API)
wait = doThis();
wait.addCallback(function (){
doNextThing();
});
doThis removes some layers from a map:
doThis(){
var layer = map.getLayer("mapLayer");
if (layer) {
map.removeLayer(layer);
}
..but when I run it, I get an error saying 'wait' is undefined...
I have similar syntax elsewhere in my code that works...is it because I am assigned the callback within an event listener? If so, is there a workaround? I really need doThis() to be completed before doNextThing() begins.

In JavaScript function always returns value and if you skip return keyword inside function then it returns undefined automatically. And in your doThis code there is no return statement. This is why wait gets undefined and fails on next step.
What should be returned? From shown example we can only deduce that object returned from doThis() offers addCallback function. Since ArcGIS is built upon Dojo Toolkit it is probably the Deferred object. What returns Deferred is open question having no other clues from your example.

When to assign functions to variable, using jquery

so I've been messing around with some Jquery Ajax promises/deffers etc... and i've come across something I don't completely understand, not strictly related to the Jquery Ajax.
I have always declared and called functions like so:
function foo1() { //sets function
alert('foo1');
}
foo1(); //calls function
But it seems the more I see different code a lot of people are declaring functions like the following, I just copied and pasted an example I saw so I would't miss anything:
var promise = $.ajax({
url: "/myServerScript"
});
promise.done(myStopAnimationFunction);
I understand what the above does, just an example.
The question is, is it better to assign functions to variables? What are the pros/cons, and in what situations is this way used?
At what point in this code is the actual function called. Does
promise.done(myStopAnimationFunction);
call both the ajax function, and then the callback, or just the callback?
Thanks

In your example, you're assigning your promise variable to what $.ajax returns (which is a jqXHR object)
var promise = $.ajax({
url: "/myServerScript"
});
Your then saying that once it's done, you want to call myStopAnimationFunction. Because $.ajax is async by default, the browser will skip right over this and only call your myStopAnimationFunction when the request is complete.
promise.done(myStopAnimationFunction);
Now, with your myStopAnimationFunction; you could always just do the following:
promise.done(function(){
$('.loader').hide();
});
but if you have code which you'll be using a lot, put it in a function so you don't need to repeat yourself (see DRY) - this has nothing to do with jQuery, however.
Your example is exactly the same as doing:
$.ajax({
url: "/myServerScript"
}).done(function(){
$('.loader').hide();
});

Those are two very different things! The first one is a function declaration. The second one is a function invocation, and what is assigned to the promise variable is the value returned by the function you're calling ($.ajax).
In any case, it is possible to assign functions to variables too (but I'm not sure if that's what you're really asking – if it is, this is a duplicate of var functionName = function() {} vs function functionName() {}).
Does promise.done(myStopAnimationFunction);
call both the ajax function, and then the callback, or just the callback?
Neither. That line is a call to done on the promise object, to register a callback to be called when the ajax response arrives. At that point you call done, the ajax request may have already fired, and the response even might already be available (if that's the case, the callback will be called immediately).

How does one use $.deferred properly with non-observable functions?

Let's say for example that I have two functions with random code inside and also that based on the user's system (slow, medium, or fast) there is no way to tell how long the two functions will take to complete, so the use of setTimeout is not practical when trying to fire function2 only after function1 is complete.
How can you use jQuery.deferred to make function2 fire only after function1 no matter what the time requirements are, and considering that both functions are 100% non-jQuery functions with no jQuery code inside them and therefore completely un-observable by jQuery? At the very most, the functions might include jQuery methods like .css() which do not have a time association and can run slower on old computers.
How do I assure that function2 is not executing at the same time as function1 if I call them like this:
function1(); function2();
using $.deferred? Any other answers besides those regarding $.deferred are also welcome!
ADDED March 20:
What if function1() is a lambda function where, depending on user input, the function may or may not have asynchronous calls and it is not possible to tell how many operations the function will do? It'd be a function where you wouldn't have any clue as to what would happen next, but no matter what, you'd still want function2 to execute only after everything from the lambda function (function1) is done, no matter how long it takes but as long as the asynchronous aspects are completed. How can this be achieved?
ADDED March 22:
So I guess the only way to do what I'm asking is to pass anonymous functions as callbacks to asynchromous functions that execute the callbacks after they are done, or to create event listeners that will do execute what you want when the event is finally triggered.
There's not really any way to just execute to asynchronous calls on two seperate lines and have them fire in order without manually constructing mechanisms (event handlers) within the frame containing the said functions to handle the actual execution of their actions.
A good example of these types of mechanisms would be jQuery's .queue() method and $.Defferred object.
The answers below along with reading up on jQuery's API on .queue()ing and using $.Deferred helped clarify this.
Tgr gave a great example below on how to create custom chainable functions using jQuery's $.Deferred object, and the custom functions themselves don't necessarily have to have any jQuery code inside them, which is exactly what I was looking for.

function first(deferred) {
// do stuff
deferred.resolve();
}
function second() {
// do stuff
}
$.Deferred(first).then(second);
But as Tomalak pointed out, this is unnecessary, unless you do something very tricky in first (like utilising web workers).
Update:
The basic idea is that whenever you do something that is not immediate, you create a Deferred object, and return that. (jQuery's AJAX calls already do this.) You can then use Deferred.then to delay follow-up operations.
function first() {
var deferred = $.Deferred();
var callback = function() {
deferred.resolve();
}
// do immediate stuff
someAsyncOperation(callback);
return deferred.promise(); // turns the Deferred into a Promise, which
// means that resolve() will not be accessible
}
function second() {
// do stuff
}
first().then(second); // or: $.when(first).then(second)
If second is also an asynchronous operation, you can use $.when's merging capabilities:
function second() {
var anotherDeferred = $.Deferred();
// do stuff with anotherDeferred
return anotherDeferred.promise();
}
$.when(first(), second()).then(third); // third will run at the moment when
// both first and second are done

JavaScript itself is not asynchronous. It is single-threaded, synchronous.
function1();
function2();
will execute one after another unless they contain asynchronous calls. In that case, there will always be a callback you can pass (like onSuccess for XmlHttpRequest). Place the second function there.
To say the truth, they strictly execute one after another even if they contain asynchronous bits. It's just that the asynchronous bits might not yet be finished when the rest of the function is.
EDIT Your jsFiddle example, fixed (see it):
function foo() {
$('#foo')
.html('<span>foo1</span>')
.animate(
{ /* properties */
left: '100px'
},
360, /* duration */
'swing', /* easing */
function () { /* "complete" callback */
$('#foo').append('<span>foo2</span>');
bar();
}
);
}
As I said. There will always be a callback you can pass.