I know there are plenty of examples on how to make synchronous AJAX calls (using deferred promises) but my issue is a little different. Here is what I am trying to do
Make an AJAX call
If the response contains a certain value then loop around again and make another AJAX call with different parameters. Value of Parameters will be dependent upon the response from the first AJAX call so have to wait before making the second AJAX call.
If the response contains (exit = true) then exit the loop.
I understand that if the server never returns condition 3 we would be in an endless loop.
How do I continue looping on an AJAX call until a the response from the AJAX call contains a certain value?
Put the code to make the AJAX call in a named function. The callback function of the AJAX checks whether the response contains exit = true. If not, it calls the named function again to make the next AJAX call with the updated function.
function call_ajax(params ...) {
$.ajax({
url: "url",
data: { whatever },
dataType: 'json'
success: function(response) {
if (!response.exit) {
// calculate new parameters
call_ajax(new_params ...);
}
}
})
}
If there is jquery ajax loading and I fire another ajax by quickly clicking the button, it kind of gets stuck. How can I handle multiple requests fired together?
How do I do following?
Discard/abort all previous requests and only process the latest one.
Do not allow new request until previous request completes (variation: can be same ajax request or any new ajax request from the page).
AJAX is Asynchronous. So you can fire them at the same time.
Or in the success callback (or .done() callback), you can call one request after another. So it will be easy to manage your issue (you click the button but get stucked), because you can control.
$.ajax({
url: "http://..."
})
.done(function( data ) {
// Other AJAX call
// or restore disabled elements
// while you were receiving the response.
});
If you want a work-around, just tell me.
you can use ajax "beforeSend" to lock the current request.So that user can send a new request only if the previous one is done. As for the process sequence, you can use a global value to store data and always assign it with the new response value.
function request(callback){
if(!$btn.hasClass('disabled')){
$.ajax({
type:'...',
url:'...',
beforeSend:function(){
$btn.addClass('disabled');//so that user cannot send a new request
},
success:function(data){
window.g_data = data;
callback && callback()//success callback
$btn.removeClass('disabled');
}
})
}
}
function callback(){
//process window.g_data
}
Have a look at this library:
Async is a utility module which provides straight-forward, powerful functions for working with asynchronous JavaScript.
Async
I have a webApi 2 application. I used Ajax several time in this way async:false , because sometimes
The statement I am calling has to be complete before the next statement in my function can be called
I get a browser warning then
Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience.
I understand that calling a web service synchronisly can make problems such as a long waiting, browser crush ... etc. But I need to get the result before continuing the next statement.
I need to know
How can I avoid this warning and amke the call asynchrous?
Why Synchronous XMLHttpRequest on the main thread is deprecated? what is the meaning of this statement?
You could move your sync call to a web worker. But again, you will make it async.
Its better to make the current sync call to server as async call.
Example(pseudo sync call code):
var dataFromServer = makeSyncCallToServer();
//use dataFromServer
Example(pseudo async call code):
makeAsyncCallToServer(function(dataFromServer) {
//use dataFromServer here
});
If you want to make multiple calls to server, then async comes handy here.
For example, you could use async.series
async.series([yourAsnycFunctionOne, yourAsnycFunctionTwo, yourAsnycFunctionThree], finalFunctionToCall)
So yourAsnycFunctionOne, yourAsnycFunctionTwo, yourAsnycFunctionThree will be called in series and finalFunctionToCall will be called in the end of the series.
In your case, you could do something like:
function getCustomerList(callback) {
//make the async http call and call "callback" with the customer data
}
function postFormDataToServer(formdata, callback) {
//send form data server and call "callback" after successful post
}
function hideGrid() {}
function hideForm() {}
function showForm() {}
function showGrid() {}
$('#add-button').on('click', function() {
showForm();
hideGrid();
});
$('#form-id').on('submit', function(e) {
e.preventDefault();
var formdata = {/**populate form data**/};
hideForm();
postFormDataToServer(formdata, function() {
getCustomerList(function(customerList) {
//populate grid with new data
showGrid();
});
});
});
I've tried Googling this but could not reslove it. It may seem like a really simple issue to others but I'm baffled by it. I have the below code in which I get undefined for the first alert but I still get the correct values in the 2nd alert. BUT if I comment out the first alert (just the line with alert) then the 2nd alert output becomes undefined. Can any one explain why this is and how I may output the 2nd alert correctly without the first one, any Help is greatly appreciated.
function getDetails(ID){
var qArray = [];
$.get('get_Question', {"Id":ID}, function(){})
.success(function(data){
var json = $.parseJSON(data);
qArray.push(json.value1);
qArray.push(json.value2);
});
//First Alert
alert("-> "+qArray[0]);
return qArray;
}
This is the 2nd alert which calls the above method:
var myArray = getDetails(4);
alert("myArray [0]: "+myArray[0]);
You can't return a value, the $.get() call is asynchronous.
You need to defer any operations on qArray until the AJAX call has completed, i.e. inside the callback.
Better yet, use deferred callbacks:
function getDetails(ID) {
return $.get('get_Question', {"Id":ID})
.pipe(function(json) {
return [json.value1, json.value2];
});
}
The .pipe deferred function creates a new promise which will ultimately return the desired array, but only once the AJAX call has completed.
You would then use this like this:
getDetails(ID).done(function(qArray) {
alert("-> " + qArray[0]);
});
Note that $.get() doesn't directly support error callbacks, but with deferred objects you can get access to them:
getDetails(ID).done(function(qArray) {
alert("-> " + qArray[0]);
}).fail(function(jqXHR, textStatus, errorThrown)) {
alert("The AJAX request failed:" + errorThrown);
});
Without this you'd need to build the error handling directly into the getDetails() function and then require some mechanism to tell the rest of the application logic about the error.
NB I've assumed that you don't really need to call JSON.parse() manually - if your web server returns the right Content-Type header then jQuery will do that for you automatically.
Ajax calls happens asynchroniusly, meaning you can't wait for the call to return and get the value. The way to do it is to employ a callback. Your example will become something similar to this:
function getDetails(ID, callback){
$.get('get_Question', {"Id":ID}, function(){})
.success(function(data){
var qArray = [];
var json = $.parseJSON(data);
qArray.push(json.value1);
qArray.push(json.value2);
callback(qArray)
});
}
Calling it will change a bit:
getDetails(4, function (myArray) {
alert("myArray [0]: "+myArray[0]);
});
The First Alert is called before the ajax call is finished, so the variable is still undefined.
This is because the $.get() is done asynchronously. There is no option for $.get() to pass parameter for async calls, so you should use $.ajax() instead and pass a param async: false
The $.get call creates a new asynchronous request for the resource in question.
When you call the first alert it is undefined because the request hasn't been completed yet. Also since you are forced to pause on the alert the request has time to be completed in the background. Enough time for it to be available by the second alert.
The same thing happens when you comment out the first alert. This time the second alert is called before the request is completed and the value is undefined.
You need to either make your requests synchronous or consider continuing execution after receiving the response by using a callback function within the success callback function you have already defined in $.get.
As several others have said, ajax-request are asynchronous. You could however set the async property to false to get a synchronous request.
Example:
function getDetails(ID) {
var result = $.ajax('get_Question', {
async : false,
data : { 'Id' : ID }
});
// do something with the result
return result;
}
I myself would have use a callback function instead beacuse async:false is bad practice and is also deprecated.
You'll need to rewrite $.get to use $.ajax and specify async: false
AJAX is asynchronous: you can't tell when the request will complete. This usually means you need to pass callback methods that will be called with the result of the request when it completes. In your case this would look something like:
function getDetails(ID, callbackFunc){
$.get('get_Question', {"Id":ID}, function(){})
.success(function(data){
var qArray = [];
var json = $.parseJSON(data);
qArray.push(json.value1);
qArray.push(json.value2);
callbackFunc(qarray);
});
}
getDetails(4, function(qArray) {
alert("myArray [0]: "+qArray[0]);
};
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.