Develop Reference

Is it impossible to wait in javascript?

I have a page with many html forms and I need to go through all of them, submit them and wait for the response, then grab the data.
The natural thing to do is to write a loop like this:
for (i = 0; i < win.document.forms.length; i++)
{
// Submit form i and wait for the response
}
But then the problem dawns on you: How do you wait for the response inside the loop? You can check whether the data is available, and if not? How do you kill time inside the loop? There is no sleep function in js, right?
To run in a loop and check for the data all the time would suck up all the system resources. You can't do that either. My current wisdom is that you need to exit the loop and terminate, but schedule the containing function for execution later on. When the function runs at some later time you must reenter the loop at the point where you left off and once again check for data available.
It's awkward to say the least. Am I missing something? Is there a better solution?

You can use just ajax asynchronous calls with callbacks so you are not blocking thread.
$("form").each(function (index, element) {
var form = $(element);
$.ajax({
url: form.attr('action'),
type: 'POST',
data: form.serialize(),
success: function (result) {
// ... Process the result ...
}
});
});
If you want to use for loop then you will need to also create outer function because variable defined in that loop will not be lexically scoped, so you can end up with submiting same form multiple times.

Function firing additional time for each time it is called - jQuery/AJAX

Scope
We have a function. Inside of it is an ajax request:
function search(){
$.ajax({
url: "url",
dataType: "text",
success: function(result) {
var filter = $(result).find('#myDiv');
// populate a hidden div with this information
$("#ladderInformation").html(filter);
}
});
}
Once this is complete, we iterate over the results of the initial ajax request, and run a new ajax request for each of them:
// yes, this is attached to the above, it's not outside of the first ajax call
.done(function(){
// for every team, we're going to run this ajax call
$('.team').each(function(i){
$.ajax({
url: "(we make use of $(this).text() here)",
dataType: "text",
success: function(teamdata){
$('#teamListing').append($(teamdata).find('#team-sheet'));
// for every time it runs, we'll append a div to the wrapper.
$('.wrapper').append('<div class="teamInfo" id="'+ Team [i] + '"></div>');
}
});
});
});
This is all working completely fine.
Next, I manipulate the elements received:
$(document).ajaxStop(function(){
//this function is irrelevant to the issue
configureTeams();
});
All of the above is wrapped in the first mentioned function, search();. We call this with
$(document).ready(function(){
search();
$('select').on('change', function(){
search();
});
});
On first page load it does everything as expected.
The Issue
When the select listener is fired multiple times, for every time it is used, the app drastically slows down. This was also noticed when I tried to add an "auto refresh" to the results (doing so by adding setTimeout(function(){ search() }, 5000); to the end of the $(document).ajaxStop(){} function).
For every time that search() is called, it will fire an additional time. So, for example, if you've changed select 6 times (or left the app running and auto refreshing for 6 times), it is firing 6 times to match this, and inevitably breaks.
Attempts to Fix
If I were to console.log() in the $(document).ajaxStop() function, it will fire an extra time for each time search() has been called. This can be combated by changing it to $(document).one('ajaxStop', function(){}) but that isn't solving the entire problem, since all that does is prevent the configuring from firing multiple times.
I have got the divs that hold the information emptying at the start of search(), so there are no conflicts with the same data or anything like that, I have tried managing the ajax requests in a queue, I have tried multiple methods of detecting that all of the ajax requests are finished before continuing, I have tried limiting the $.each() function so it doesn't exceed the required amount of iterations, but to no avail.
TL;DR
What I'm trying to accomplish, is that when I run a function multiple times, we completely start over each time. It seems as though it is recording each time it has been called. Could there be a solution with clearing memory, or the AJAX queue, or something like that?
I can provide the full code if that will be more helpful.
Thanks

JQuery - Looping a .load() inside a 'for' statement

I'm not sure if this will actually be possible, since load() is an asynchronous method, but I need some way to basically Load several little bits of pages, one at a time, get some data included in them via JavaScript, and then send that over via Ajax so I can put it on a database I made.
Basically I get this from my page, where all the links I'll be having to iterate through are located:
var digiList = $('.2u');
var link;
for(var i=0;i<digiList.length;i++){
link = "http://www.digimon-heroes.com" + $(digiList).eq(i).find('map').children().attr('href');
So far so good.
Now, I'm going to have to load each link (only a specific div of the full page, not the whole thing) into a div I have somewhere around my page, so that I can get some data via JQuery:
var contentURI= link + ' div.row:nth-child(2)';
$('#single').load('grabber.php?url='+ contentURI,function(){
///////////// And I do a bunch of JQuery stuff here, and save stuff into an object
///////////// Aaaand then I call up an ajax request.
$.ajax({
url: 'insertDigi.php',
type: 'POST',
data: {digimon: JSON.stringify(digimon)},
dataType: 'json',
success: function(msg){
console.log(msg);
}
////////This calls up a script that handles everything and makes an insert into my database.
}); //END ajax
}); //END load callback Function
} //END 'for' Statement.
alert('Inserted!');
Naturally, as would be expected, the loading takes too long, and the rest of the for statement just keeps going through, not really caring about letting the load finish up it's business, since the load is asynchronous. The alert('Inserted!'); is called before I even get the chance to load the very first page. This, in turn, means that I only get to load the stuff into my div before I can even treat it's information and send it over to my script.
So my question is: Is there some creative way to do this in such a manner that I could iterate through multiple links, load them, do my business with them, and be done with it? And if not, is there a synchronous alternative to load, that could produce roughly the same effect? I know that it would probably block up my page completely, but I'd be fine with it, since the page does not require any input from me.
Hopefully I explained everything with the necessary detail, and hopefully you guys can help me out with this. Thanks!

You probably want a recursive function, that waits for one iteration, before going to the next iteration etc.
(function recursive(i) {
var digiList = $('.2u');
var link = digiList.eq(i).find('map').children().attr('href') + ' div.row:nth-child(2)';
$.ajax({
url: 'grabber.php',
data: {
url: link
}
}).done(function(data) {
// do stuff with "data"
$.ajax({
url: 'insertDigi.php',
type: 'POST',
data: {
digimon: digimon
},
dataType: 'json'
}).done(function(msg) {
console.log(msg);
if (i < digiList.length) {
recursive(++i); // do the next one ... when this is one is done
}
});
});
})(0);

Just in case you want them to run together you can use closure to preserve each number in the loop
for (var i = 0; i < digiList.length; i++) {
(function(num) { < // num here as the argument is actually i
var link = "http://www.digimon-heroes.com" + $(digiList).eq(num).find('map').children().attr('href');
var contentURI= link + ' div.row:nth-child(2)';
$('#single').load('grabber.php?url=' + contentURI, function() {
///////////// And I do a bunch of JQuery stuff here, and save stuff into an object
///////////// Aaaand then I call up an ajax request.
$.ajax({
url: 'insertDigi.php',
type: 'POST',
data: {
digimon: JSON.stringify(digimon)
},
dataType: 'json',
success: function(msg) {
console.log(msg);
}
////////This calls up a script that handles everything and makes an insert into my database.
}); //END ajax
}); //END load callback Function
})(i);// <-- pass in the number from the loop
}

You can always use synchronous ajax, but there's no good reason for it.
If you know the number of documents you need to download (you can count them or just hardcode if it's constant), you could run some callback function on success and if everything is done, then proceed with logic that need all documents.
To make it even better you could just trigger an event (on document or any other object) when everything is downloaded (e.x. "downloads_done") and listen on this even to make what you need to make.
But all above is for case you need to do something when all is done. However I'm not sure if I understood your question correctly (just read this again).
If you want to download something -> do something with data -> download another thing -> do something again...
Then you can also use javascript waterfall (library or build your own) to make it simple and easy to use. On waterfall you define what should happen when async function is done, one by one.

I'm new to javascript and I'm fetching JSON data from url, I'm only able to access data in success function, Am I missing something?

Here is the code :-
var quiz;
function startQuiz() {
$.ajax({
url: 'get_quiz/',
cache: 'false',
dataType: 'json',
async: 'false',
success: function(data) {
quiz = data;
alert(quiz[0].q); // I'm able to access quiz here
},
}
);
}
startQuiz();
alert(quiz[0].q); // Not able to access it here.
I'm not able to access quiz here, am I mission something?, Whats wrong with this?

Ajax is assynchronous which can be an unfamiliar concept. Your code will run like this:
1. var quiz;
2. define function startQuiz;
3. call startQuiz;
4. do ajax call (and continue! don't block)
5. alert(quiz[0].q); // Not able to access it here.
-- ajax call comes back
6. quiz = data;
7. alert(quiz[0].q); // I'm able to access quiz here
Ajax is asynchronous, it doesn't block. This means that when you make the ajax call the callback doesn't actually get called until the ajax call returns, it doesn't block and wait. Instead the code will continue on.
Then later when the ajax call returns the data, your callback function will be executed.
Javascript does this by means of an event loop.
See it like this: steps 1-5 are part of the first event. 6-7 are part of the second event.
A cool thing about JavaScript is that in your callback you still have access to anything above it (like the variable quiz) because of scoping. This is called a closure. Your callback function closes around the scope and brings it with him to the next event.

AJAX calls are asynchronous, you should wait for the result to come back from the server. Either do all the work in a callback function or have a look on a promises library (I like Q promises library), which makes waiting for AJAX results very easy.

This is due to the asynchronous nature of JavaScript, once you call startQuiz() it executes and jumps back out and executes your quiz alert().
You have to access your quiz data explicitly after the callback is called to make sure you have access to it.
You also have to worry about scoping, as you may not be actually modifying the same quiz variable.
var quiz,
fetched = false;
$.ajax({
//blah
success : function(data){
fetched = true;
quiz = data;
}
});
setInterval(function(){
if(fetched){
//quiz is populated
}else{
//quiz hasn't be populated yet
}
},50);
Although not a clever example, I'm just trying to get the point across that startQuiz() doesn't wait for the ajax call. The A in AJAX mean asynchronous.

You should give jQuery deferreds a try which ist the preferred way of writing ajax related code since jQuery 1.5: http://javascriptplayground.com/blog/2012/04/jquery-deferreds-tutorial
Once you get the hang of it maybe will be easier to write and understand your code.

How do you make javascript code execute *in order*

Okay, so I appreciate that Javascript is not C# or PHP, but I keep coming back to an issue in Javascript - not with JS itself but my use of it.
I have a function:
function updateStatuses(){
showLoader() //show the 'loader.gif' in the UI
updateStatus('cron1'); //performs an ajax request to get the status of something
updateStatus('cron2');
updateStatus('cron3');
updateStatus('cronEmail');
updateStatus('cronHourly');
updateStatus('cronDaily');
hideLoader(); //hide the 'loader.gif' in the UI
}
Thing is, owing to Javascript's burning desire to jump ahead in the code, the loader never appears because the 'hideLoader' function runs straight after.
How can I fix this? Or in other words, how can I make a javascript function execute in the order I write it on the page...

The problem occurs because AJAX is in its nature asynchronus. This means that the updateStatus() calls are indeed executed in order but returns immediatly and the JS interpreter reaches hideLoader() before any data is retreived from the AJAX requests.
You should perform the hideLoader() on an event where the AJAX calls are finished.

You need to think of JavaScript as event based rather than procedural if you're doing AJAX programming. You have to wait until the first call completes before executing the second. The way to do that is to bind the second call to a callback that fires when the first is finished. Without knowing more about the inner workings of your AJAX library (hopefully you're using a library) I can't tell you how to do this, but it will probably look something like this:
showLoader();
updateStatus('cron1', function() {
updateStatus('cron2', function() {
updateStatus('cron3', function() {
updateStatus('cronEmail', function() {
updateStatus('cronHourly', function() {
updateStatus('cronDaily', funciton() { hideLoader(); })
})
})
})
})
})
});
The idea is, updateStatus takes its normal argument, plus a callback function to execute when it's finished. It's a reasonably common pattern to pass a function to run onComplete into a function which provides such a hook.
Update
If you're using jQuery, you can read up on $.ajax() here: http://api.jquery.com/jQuery.ajax/
Your code probably looks something like this:
function updateStatus(arg) {
// processing
$.ajax({
data : /* something */,
url : /* something */
});
// processing
}
You can modify your functions to take a callback as their second parameter with something like this:
function updateStatus(arg, onComplete) {
$.ajax({
data : /* something */,
url : /* something */,
complete : onComplete // called when AJAX transaction finishes
});
}

I thinks all you need to do is have this in your code:
async: false,
So your Ajax call would look like this:
jQuery.ajax({
type: "GET",
url: "something.html for example",
dataType: "html",
async: false,
context: document.body,
success: function(response){
//do stuff here
},
error: function() {
alert("Sorry, The requested property could not be found.");
}
});
Obviously some of this need to change for XML, JSON etc but the async: false, is the main point here which tell the JS engine to wait until the success call have returned (or failed depending) and then carry on.
Remember there is a downside to this, and thats that the entire page becomes unresponsive until the ajax returns!!! usually within milliseconds which is not a big deals but COULD take longer.
Hope this is the right answer and it helps you :)

We have something similar in one of our projects, and we solved it by using a counter. If you increase the counter for each call to updateStatus and decrease it in the AJAX request's response function (depends on the AJAX JavaScript library you're using.)
Once the counter reaches zero, all AJAX requests are completed and you can call hideLoader().
Here's a sample:
var loadCounter = 0;
function updateStatuses(){
updateStatus('cron1'); //performs an ajax request to get the status of something
updateStatus('cron2');
updateStatus('cron3');
updateStatus('cronEmail');
updateStatus('cronHourly');
updateStatus('cronDaily');
}
function updateStatus(what) {
loadCounter++;
//perform your AJAX call and set the response method to updateStatusCompleted()
}
function updateStatusCompleted() {
loadCounter--;
if (loadCounter <= 0)
hideLoader(); //hide the 'loader.gif' in the UI
}

This has nothing to do with the execution order of the code.
The reason that the loader image never shows, is that the UI doesn't update while your function is running. If you do changes in the UI, they don't appear until you exit the function and return control to the browser.
You can use a timeout after setting the image, giving the browser a chance to update the UI before starting rest of the code:
function updateStatuses(){
showLoader() //show the 'loader.gif' in the UI
// start a timeout that will start the rest of the code after the UI updates
window.setTimeout(function(){
updateStatus('cron1'); //performs an ajax request to get the status of something
updateStatus('cron2');
updateStatus('cron3');
updateStatus('cronEmail');
updateStatus('cronHourly');
updateStatus('cronDaily');
hideLoader(); //hide the 'loader.gif' in the UI
},0);
}
There is another factor that also can make your code appear to execute out of order. If your AJAX requests are asynchronous, the function won't wait for the responses. The function that takes care of the response will run when the browser receives the response. If you want to hide the loader image after the response has been received, you would have to do that when the last response handler function runs. As the responses doesn't have to arrive in the order that you sent the requests, you would need to count how many responses you got to know when the last one comes.

As others have pointed out, you don't want to do a synchronous operation. Embrace Async, that's what the A in AJAX stands for.
I would just like to mention an excellent analogy on sync v/s async. You can read the entire post on the GWT forum, I am just including the relevant analogies.
Imagine if you will ...
You are sitting on the couch watching
TV, and knowing that you are out of
beer, you ask your spouse to please
run down to the liquor store and
fetch you some. As soon as you see
your spouse walk out the front door,
you get up off the couch and trundle
into the kitchen and open the
fridge. To your surprise, there is no
beer!
Well of course there is no beer, your
spouse is still on the trip to the
liquor store. You've gotta wait until
[s]he returns before you can expect
to have a beer.
But, you say you want it synchronous? Imagine again ...
... spouse walks out the door ... now,
the entire world around you stops, you
don't get to breath, answer the
door, or finish watching your show
while [s]he runs across town to
fetch your beer. You just get to sit
there not moving a muscle, and
turning blue until you lose
consciousness ... waking up some
indefinite time later surrounded by
EMTs and a spouse saying oh, hey, I
got your beer.
That's exactly what happens when you insist on doing a synchronous server call.

Install Firebug, then add a line like this to each of showLoader, updateStatus and hideLoader:
Console.log("event logged");
You'll see listed in the console window the calls to your function, and they will be in order. The question, is what does your "updateStatus" method do?
Presumably it starts a background task, then returns, so you will reach the call to hideLoader before any of the background tasks finish. Your Ajax library probably has an "OnComplete" or "OnFinished" callback - call the following updateStatus from there.

move the updateStatus calls to another function. make a call setTimeout with the new function as a target.
if your ajax requests are asynchronous, you should have something to track which ones have completed. each callback method can set a "completed" flag somewhere for itself, and check to see if it's the last one to do so. if it is, then have it call hideLoader.

One of the best solutions for handling all async requests is the 'Promise'.
The Promise object represents the eventual completion (or failure) of an asynchronous operation.
Example:
let myFirstPromise = new Promise((resolve, reject) => {
// We call resolve(...) when what we were doing asynchronously was successful, and reject(...) when it failed.
// In this example, we use setTimeout(...) to simulate async code.
// In reality, you will probably be using something like XHR or an HTML5 API.
setTimeout(function(){
resolve("Success!"); // Yay! Everything went well!
}, 250);
});
myFirstPromise.then((successMessage) => {
// successMessage is whatever we passed in the resolve(...) function above.
// It doesn't have to be a string, but if it is only a succeed message, it probably will be.
console.log("Yay! " + successMessage);
});
Promise
If you have 3 async functions and expect to run in order, do as follows:
let FirstPromise = new Promise((resolve, reject) => {
FirstPromise.resolve("First!");
});
let SecondPromise = new Promise((resolve, reject) => {
});
let ThirdPromise = new Promise((resolve, reject) => {
});
FirstPromise.then((successMessage) => {
jQuery.ajax({
type: "type",
url: "url",
success: function(response){
console.log("First! ");
SecondPromise.resolve("Second!");
},
error: function() {
//handle your error
}
});
});
SecondPromise.then((successMessage) => {
jQuery.ajax({
type: "type",
url: "url",
success: function(response){
console.log("Second! ");
ThirdPromise.resolve("Third!");
},
error: function() {
//handle your error
}
});
});
ThirdPromise.then((successMessage) => {
jQuery.ajax({
type: "type",
url: "url",
success: function(response){
console.log("Third! ");
},
error: function() {
//handle your error
}
});
});
With this approach, you can handle all async operation as you wish.