Javascript function undefined error when using ajax inside the function body

k.value is equal to checkbox.value; but the value of checkbox is 1, so k.value is undefined;

you can use:

if ($("#myCheckbox"+k).is(":checked")) {  

As I said in my comments, do not use inline event handlers as they are a thing of the past and is a bad practice. Since you are using jQuery, make as much use of it as possible. Do not mix plain JS with jQuery unless you really have a need to do so. So, back to your question; your code may be re-written as the following.

The line "size": typeof vsize1 !== "undefined" && vsize1 || 0 is to ensure that the variable is defined and has a value or set it to 0 as a fallback if not.

From the doc: Note that synchronous requests may temporarily lock the browser, disabling any actions while the request is active. As of jQuery 1.8, the use of async: false with jqXHR ($.Deferred) is deprecated; you must use the success/error/complete callback options instead of the corresponding methods of the jqXHR object such as jqXHR.done() or the deprecated jqXHR.success().

As per the above quote, I have commented the async: false since it shouldn't be used unless there is really a need to do so. If async: false needs to be used, you must use success (like in your question) in place of .done!

$(function() {
    //change event is more appropriate than click for form elements.
    $(".mycheck").on("change", function() {
        var $elem = $(this); // event target (one of the che
        var dataID = $elem.data("id"); // 1, 2 0r 3
        // Please note that the below line would only trigger the ajax call for the first checkbox as the data-id is being checked and when the box is checked.
        if ($elem.is(":checked") && dataID == 1) {
            $.ajax({
                //async: false,
                url: 'getCDP.php', 
                data: {
                    "size": typeof vsize1 !== "undefined" && vsize1 || 0
                },
                dataType: 'json'
            }).done(function(data) {
                alert("ssdsd");
            }).fail(function() {
                alert ("fail");
            });
        } else {
           alert ("The check box is not checked."); 
        }
    });
});

The above would work for just one checkbox and also for multiple. The associated HTML for the above code would look like this:

<input type="checkbox" id="myCheckbox1" value="22" class="mycheck" data-id="1" />
<input type="checkbox" id="myCheckbox2" value="23" class="mycheck" data-id="2" />
<input type="checkbox" id="myCheckbox3" value="24" class="mycheck" data-id="3" />

Your alert is wrong. You opened it with '{' rather than '('.

Works fine for me here: jsfiddle.net/adamjld/8wf6f9m8

You have few typo mistakes in your code.. read it carefully before you post question here..

• missing ; after vsize1

• wrong bracket in alert{"ssdsd");

This is beacause you did't defined

'vsize1' 

that you are passing as data. try defining it or give a static value here.

Check this fiddle please (ajax working).

You can define your checkbox like this:

<input type="checkbox" onchange="OnChangeCheckbox (this,1)" id="myCheckbox1" value="22" />
<label for="myCheckbox">Sample check box</label> 

Send the checkbox (using this) as parameter to the function

Define your vsize1 variable when the function is called or send it as a parameter (if it is depending on the checkbox that is checked)