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make an empty ajax request

I have a function that makes an ajax request to write some data to a database.
Yet, when for some reason, the function is called more than once on the page (which sometimes seems to happen), I check via javascript how often the function was called and the ajax request is not executed if it was called more than once to avoid adding data twice.
However, since the ajax request is never executed in this case, the .done function does not work anymore, which breaks the functionality of the script further down the line.
My workaround is, if the function was already called, to make an 'empty' ajax request in which I do not transfer any data to the PHP script on the server-side.
This way, I can still use the .done function and execute the rest of script normally (without adding any duplicate data).
Is that a sensible way to deal with such a case?
It seems to me that it is a waste of server resources, since there is no need to communicate with the server in this case.
Is there a better way to make an "empty" ajax request, such that the .done function can still be executed?

Jquery async call not working on user event

In my webpage many on-load ajax call those works fine.
Action takes time as per processing time. Means if any action that has been complete will send response and will not wait for first to finish.
But if same I am trying to do with on-lick or any user event. All ajax call works synchronously. I want these should not wait to finish the execution of first running action. All should start and complete independently.
I am using jquery 1.8 where default async= true;
Please help me here to resolve this.

Issue may be due to session lock.
more detail you can find it here
http://php.net/manual/en/function.session-write-close.php
call session_write_close function if there no session write activity in your script or function.
Such issues are observed in many concurrent ajax call and previous call has some session write activity. In this case session will be locked until it completed its execution.

If you set async to true, they will all fire at the same time.
make explicitly declaration of async = true option in each ajax call .
Useful jsfiddle link
http://jsfiddle.net/jquerybyexample/tS349/

how to check if AJAX request has been completed?

I need to add a sibling to certain HTML elements in the Vaadin application. I don't want to do this on the server side, because it's far too complicated in my case.
I wrote a javascript, that does the magic and it is executed after the page loads the first time. But then, when I click a certain button, some of the elements are loaded from backend using AJAX by Vaadin.
I subscribe for this button "click" event in the javascript. In the listener function I want to wait until Vaadin completes the request and makes the changes in the DOM. Then, I would run my function again.
Question is - how to detect when Vaadin completes its request? Setting timeout is not an answer for me.

Most AJAX library/routine provides you a callback when the AJAX is done. So basically you can set a flag before you start AJAX callback, and then call that flag when your callback is called. At other places you can then check that flag to see if your AJAX call has come back yet.
Some other AJAX library already does that for you and has something like "isInProgress()" that you can simply check anytime to see if it is has come back yet.

Sync AJAX for specific case

I have 3 buttons on page.
Each one makes AJAX request by clicking on it.
These all requests should makes in async mode.
But clicking on any button in second time should:
1. Stop current request which was made clicking on this button at first time.
OR
Do Nothing.
Option#1 is prefer.
I know about abort(), but my question - how to detect that Ajax request (from certain button) is still not finished ? Here the main point is - request from certain button. I do not want stop all ajax requests. I want stop only request which was made by clicking in the same button at first time.
In my project i'm using jQuery
is this http://api.jquery.com/category/deferred-object/ can help me ? if yes can you provide any suitable example ?
Thanks

I don't believe deferred objects will do what you want. They're designed more for doing promises and aggregate callbacks. You can look into the state of an ajax call by keeping a reference to the jqXHR object returned by $.ajax though.
var ajax;
function onClick() {
if (!ajax || ajax.state() === "resolved") {
ajax = $.ajax(url);
}
}

you need to have a variable associated with each possible ajax process, which tracks whether that process is currently running. When an ajax call starts, set that variable to indicate it's running. When that ajax call completes, set the variable to indicate that the process has stopped. Then, when the user clicks the button, you can examine the variable to decide whether you need to abort() the ajax call or not.

JavaScript flow or execution

If I have a JS function as follows;
function testFn()
{
x.ajaxMethod(param1,JScallBackFunction); //Please do not worry about the syntax..this just indicates an external method call
alert("Line after ajaxMethod");
}
The ajaxMethod(), lets say is some kind of method defined in an external Java file (so it can be through DWR or anything) which returns some data...Point is it takes some time to execute this line of code...
Now my question is when will the alert on next line get fired (i.e. alert("Line after ajaxMethod");)
Will it wait for these 2 things to complete (ajaxMethod execution as well as JScallBackFunction)
OR
It will be fired immediately without waiting for any of the above 2 things to complete ?
Also if you could guide in general about the JavaScript method flow execution, that will be great.

It depends. Ajax calls are usually asynchronous which means the execution of code will not be paused until the asynchronous function returns. Therefore the alert will be executed immediately.
Asynchronous functions in javascript are usually to do with Ajax and loading something from a remote server. If you do wish to force JavaScript to wait while loading that content then you can set a flag for the XMLHTTPRequest object.
this is a good question to read: When is JavaScript synchronous?

it will fire immediately after the ajax call. if you want it to wait put it in the callback function.
edit: a method that defines a callback is essentially this:
function(param1, callback) {
// do stuff
callback(); // execute callback
}

First, when you say
The ajaxMethod(), lets say is some
kind of method defined in an external
Java file
I suppose you really mean external JavaScript file.
When you send an Ajax request, you ask the browser to send a request to the server for you.
This request on the server may take sometime and you don't want to "wait" on it. (This is the whole idea of Async requests - stuff in the background).
So you tell the browser, here send this request to the server. Don't bother me unless the server responds, and once the server responds (we have a "response"), call this method. This is called callback. The method is called at a later point, when the response comes.
So the statement
x.ajaxMethod(param1,JScallBackFunction);
(assuming that it does gets a XmlHttpRequest, initializes it and calls the send method on it*) actually does two things:
Sends the Ajax request
Registers a call back function that will be called when the server responds (when we have an response). JScallBackFunction will be called when there is an response from the server.
But since this is an asynchronous request, the browser does not "wait" instead it continues to the next statement (if there is one) after the Ajax call and executes it.
So, alert("Line after ajaxMethod"); will be executed immediately.
*If this does not make any sense for you, this is how an Ajax request is actually "created" and "sent". This article may help you understand.