Develop Reference

Why is my Ajax callback being processed too soon?

I have a general ajax function which I'm calling from loads of places in my code. It's pretty standard except for some extra debugging stuff I've recently added (to try to solve this issue), with a global 'ajaxworking' variable:
rideData.myAjax = function (url, type, data, successfunc) {
var dataJson = JSON.stringify(data),
thisurl = quilkinUrlBase() + url;
if (ajaxworking.length > 0) {
console.log(thisurl + ": concurrent Ajax call with: " + ajaxworking);
}
ajaxworking = thisurl;
$.ajax({
type: type,
data: dataJson,
url: thisurl,
contentType: "application/json; charset=utf-8",
dataType: "json",
async: true,
success: function (response) {
ajaxworking = '';
successfunc(response);
},
error: webRequestFailed
});
};
Now, there's one section of my code where a second ajax call is made depending on the result of the first:
getWebRides = function (date) {
var rideIDs = [];
var intdays = bleTime.toIntDays(date);
rideData.myAjax("GetRidesForDate", "POST", intdays, function (response) {
rides = response;
if (rides.length === 0) {
$('#ridelist').empty(); // this will also remove any handlers
qPopup.Alert("No rides found for " + bleTime.DateString(date));
return null;
}
$.each(rides, function (index) {
rideIDs.push(rides[index].rideID);
});
GetParticipants(rideIDs);
});
},
'GetParticipants' (which also calls 'myAjax') works fine - most of the time. But in another part of my code, 'GetWebRides' is itself called directly after another ajax call - i.e. there are 3 calls, each successive one depending on the previous. The 'top-level' call is as follows:
rideData.myAjax("SaveRide", "POST", ride, function (response) {
// if successful, response should be just a new ID
if (response.length < 5) {
// document re-arrangement code snipped here for brevity
getWebRides(date);
}
else {
qPopup.Alert(response);
}
});
so, only when there are three successive calls like this, I'm getting the 'concurrent' catch in the third one:
GetParticipants: concurrent call with GetRidesForDate
and (if allowed to proceed) this causes a nasty probem at the server with datareaders already being open. But why is this only occurring when GetParticipants is called as the third in the chain?
I see, after some research. that there are now other ways of arranging async calls, e.g. using 'Promises', but I'd like to understand what's going on here.

Solved this.
Part of the 'document re-arrangement code' that I had commented out for this post, was in fact calling another Ajax call indirectly (very indirectly, hence it took a long time to find).

How to create callback function using Ajax?

I am working on the jquery to call a function to get the return value that I want to store for the variable email_number when I refresh on a page.
When I try this:
function get_emailno(emailid, mailfolder) {
$.ajax({
url: 'getemailnumber.php',
type: 'POST',
data : {
emailid: emailid,
mailfolder: mailfolder
},
success: function(data)
{
email_number = data;
}
});
return email_number;
}
I will get the return value as 6 as only when I use alert(email_number) after the email_number = data;, but I am unable to get the value outside of a function.
Here is the full code:
var email_number = '';
// check if page refreshed or reloaded
if (performance.navigation.type == 1) {
var hash = window.location.hash;
var mailfolder = hash.split('/')[0].replace('#', '');
var emailid = 'SUJmaWg4RTFRQkViS1RlUzV3K1NPdz09';
get_emailno(emailid, mailfolder);
}
function get_emailno(emailid, mailfolder) {
$.ajax({
url: 'getemailnumber.php',
type: 'POST',
data : {
emailid: emailid,
mailfolder: mailfolder
},
success: function(data)
{
email_number = data;
}
});
return email_number;
}
However, I have been researching and it stated that I would need to use callback via ajax but I have got no idea how to do this.
I have tried this and I still don't get a return value outside of the get_emailno function.
$.ajax({
url: 'getemailnumber.php',
type: 'POST',
async: true,
data : {
emailid: emailid,
mailfolder: mailfolder
},
success: function(data)
{
email_number = data;
}
});
I am getting frustrated as I am unable to find the solution so I need your help with this. What I am trying to do is I want to call on a get_emailno function to get the return value to store in the email_number variable.
Can you please show me an example how I could use a callback function on ajax to get the return value where I can be able to store the value in the email_number variable?
Thank you.

From the jquery documentation, the $.ajax() method returns a jqXHR object (this reads fully as jquery XMLHttpRequest object).
When you return data from the server in another function like this
function get_emailno(emailid, mailfolder) {
$.ajax({
// ajax settings
});
return email_number;
}
Note that $.ajax ({...}) call is asynchronous. Hence, the code within it doesn't necessarily execute before the last return statement. In other words, the $.ajax () call is deferred to execute at some time in the future, while the return statement executes immediately.
Consequently, jquery specifies that you handle (or respond to) the execution of ajax requests using callbacks and not return statements.
There are two ways you can define callbacks.
1. Define them within the jquery ajax request settings like this:
$.ajax({
// other ajax settings
success: function(data) {},
error: function() {},
complete: function() {},
});
2. Or chain the callbacks to the returned jqXHR object like this:
$.ajax({
// other ajax settings
}).done(function(data) {}).fail(function() {}).always(function() {});
The two methods are equivalent. success: is equivalent to done(), error: is equivalent to fail() and complete: is equivalent to always().
On when it is appropriate to use which function: use success: to handle the case where the returned data is what you expect; use error: if something went wrong during the request and finally use complete: when the request is finished (regardless of whether it was successful or not).
With this knowledge, you can better write your code to catch the data returned from the server at the right time.
var email_number = '';
// check if page refreshed or reloaded
if (performance.navigation.type == 1) {
var hash = window.location.hash;
var mailfolder = hash.split('/')[0].replace('#', '');
var emailid = 'SUJmaWg4RTFRQkViS1RlUzV3K1NPdz09';
get_emailno(emailid, mailfolder);
}
function get_emailno(emailid, mailfolder) {
$.ajax({
url: 'getemailnumber.php',
type: 'POST',
data : {
emailid: emailid,
mailfolder: mailfolder
},
success: function(data)
{
// sufficient to get returned data
email_number = data;
// use email_number here
alert(email_number); // alert it
console.log(email_number); // or log it
$('body').html(email_number); // or append to DOM
}
});
}

Set variables in JavaScript once function finishes

I have two functions that makes Ajax calls: getData and getMoreData. getMoreData requires a url variable that is dependent on the url variable getData. This questions continues from: String append from <select> to form new variable.
By appending an item obtained from the received from getData onto a base URL, I create a new variable (Let's call this NewDimensionURL) that I use for getMoreData url. However, NewDimensionURL will show error because the original list (from getData) has yet to be populated and will append nothing onto the base URL.
An idea that I have is to set NewDimensionalURL once getData finishes populating the combobox, so that getMoreData can run after.
JavaScript
var GetDimensions = 'SomeURL1';
//--Combines URL of GetDimensionValues with #dimensionName (the select ID)
var UrlBase = "Required URL of getMoreData";
var getElement = document.getElementById("dimensionName");
var GetDimensionValues = UrlBase + getElement.options[getElement.selectedIndex].text;
function handleResults(responseObj) {
$("#dimensionName").html(responseObj.DimensionListItem.map(function(item) {
return $('<option>').text(item.dimensionDisplayName)[0];
}));
}
function handleMoreResults (responseObj) {
$("#dimensionId").html(responseObj.DimensionValueListItem.map(function(item) {
return $('<option>').text(item.dimensionValueDisplayName)[0];
}));
}
function getData() {
debugger;
jQuery.ajax({
url: GetDimensions,
type: "GET",
dataType: "json",
async: false,
success: function (data) {
object = data;
handleResults(data);
}
});
}
function getMoreData() {
debugger;
jQuery.ajax({
url: GetDimensionValues,
type: "GET",
dataType: "json",
async: false,
success: function (data) {
object = data;
handleMoreResults (data);
}
});
}
Answered
Reordered as:
var GetDimensionValues;
function handleResults(responseObj) {
$("#dimensionName").html(responseObj.DimensionListItem.map(function(item) {
return $('<option>').text(item.dimensionDisplayName)[0];
}));
GetDimensionValues = UrlBase + getElement.options[getElement.selectedIndex].text;
}
Created onchange function Repopulate() for getMoreData() to parse and for handleMoreResults() to populate.

I'm guessing you just do getData(); getMoreData() back to back? If so, then you're running getmoreData BEFORE getData has ever gotten a response back from the server.
You'll have to chain the functions, so that getMoreData only gets executed when getData gets a response. e.g.
$.ajax($url, {
success: function(data) {
getMoreData(); // call this when the original ajax call gets a response.
}
});

Without seeing your code it's hard to say if this is the right solution, but you should try chaining the functions:
$.ajax({url: yourUrl}).then(function (data) {
// deal with the response, do another ajax call here
}).then(function () {
// or do something here
});

how to use properties and methods withing a JavaScript class to exchange data?

i have small issue with exchanging data in between methods in a JavaScript object (class):
var TEST = (function () {
var TEST = function() {
};
TEST.prototype.get = function() {
$.ajax({
type: "GET",
url: "http://test.com/getall",
dataType: "json",
success: function (data) {
return data; // if i console log this i will get a json obj
}
});
};
TEST.prototype.parse = function(data) {
$.each(this.get(), function(k, v){
console.log(v);
});
};
return TEST;
})();
so i am trying to call one method in the each statement in another method. the issue is that
the response is undefined.
i also tried it like this, but with he same result
var testing = new TEST();
var get = testing.get();
testing.parse(get);
What am i missing? how can i return the data from this.get to be used in this.parse.
thanks

$.ajax() per default is asynchronous. That means, that the execution of your function get() wont wait until the request is finished. Hence you return no value from it, which results in undefined being returned.
In order to have your get() function be able to return a value, you would have to do the request in a synchronous way and set a variable in the outer function (as success itself is just another function, whose return value is not caught):
TEST.prototype.get = function() {
var result;
$.ajax({
type: "GET",
url: "http://test.com/getall",
async: false, // this is the important part!
dataType: "json",
success: function (data) {
result = data;
}
});
return result;
};
EDIT
As mentioned by #pebbl, this will halt the execution of all your scripts, until the request is done. Hence your whole page will be blocked for the time being.
The general approach is to use callbacks in such cases, which will be executed once the requests finished. So in your case something like this:
TEST.prototype.get = function( cb ) {
$.ajax({
type: "GET",
url: "http://test.com/getall",
dataType: "json",
success: function (data) {
cb( data );
}
});
};
with later on calling like this:
var testing = new TEST();
testing.get( function( data ) {
testing.parse( data );
});

You can't construct your function this way as you are relying on an asyncronous call, which will return it's result outside of the normal execution flow. The only way you can actually receive the result of your .get function is to use a callback.
Put simply your value isn't being returned from the .get function, it's being returned from the callback you are passing into jQuery's .ajax method.
You'd be far better off redesigning your code so as to still support the asyncronous call -- rather than disabling async.
A rough idea is to change your parse function like so:
TEST.prototype.parse = function(data) {
this.get(function(result){
$.each(result, function(k, v){
console.log(v);
});
});
};
And to change your get function accordingly:
TEST.prototype.get = function(callback) {
$.ajax({
type: "GET",
url: "http://test.com/getall",
dataType: "json",
success: callback
});
};
The above is just a quick example, you'd be wise reading up on the following jQuery topics:
http://api.jquery.com/promise/
http://api.jquery.com/category/deferred-object/
If you design your code around the promise pattern you'll find it complicated at first, but it gives you a lot of power in your code -- and gets around the whole callback stacking madness you can end up with when dealing in ajax calls.
Whilst it's not entirely clear from the jQuery.ajax documentation, this function returns a jqXHR object which implements the promise interface. So this means you can use the promise methods done, always and fail.
http://api.jquery.com/jQuery.ajax/

Javascript value is returned from webservice but will not show unless a breakpoint is used

I have a javascript function that calls a web service. The data comeback (I see the Jason return in FireBug) the value is blank when I attempt to use it unless I set a break point. With a break point set the value can be used, without it is not available.
Here is a snippet of the offending call.
function getTheNote(noteCode){
var _myNote = "";
var theID = $('#CustNo').val();
var myDTO = { 'theID': theID, 'noteCode': noteCode, };
var toPass = JSON.stringify(myDTO);
$.ajax({
type: 'POST',
contentType: "application/json; charset=utf-8",
dataType: "json",
url: "AR_Cust_Mgt.aspx/getNote",
data: toPass,
success: function (data) {
_myNote = data.d;
}
});
//setTimeout(_myNote += _myNote, 120000);
//for(var x = 0; x < 200000; x++){}
//return _myNote;
alert(_myNote);
}
Originally I was sending the value back to a calling function the return statement is where I would set my break point and the data would be returned, without nothing. Now you can see I attempted to use an alert inside the function with the same results.
With a break point I get a value without I get nothing, I have even attempted to use some delays.
Please help.

The ajax call is asynchronous. Anything you want to do with the result needs to be in your anonymous function success: function(data) { ... or the anonymous function needs to call other functions to do stuff.
As it is coded now, $.ajax will be called, the script execution continues on before the ajax call returns.

small change, big difference: you are not calling alert IN the succes function
success: function (data) {
_myNote = data.d;
alert(_myNote);
}