Using jQuery .attr or .prop to set attribute value not working

I am not exactly sure of the reason why the original code isn't working, but the $this seems to be the cause for some reason. Try the below and it seems to work. Fiddle is here.

I will try to update the answer with the reason as soon as I find it.

var loaded = $(".preview-button").attr('data-loaded');
if (loaded === "no") {
    $.ajax({
        success: function (result) {
            $(".preview-button").attr('data-loaded', 'yes');
            alert($(".preview-button").attr('data-loaded'));
        }
    });
} else {
    alert("data loaded");
}

Refer this thread and this seems to be the reason why the $this doesnt seem to work from inside the AJAX call.

reading the question ..

so that the ajax is not ran again if the user clicks on the button more than once.

i think you need one(), it allows the event to run just once.. no need of changing the attributes and properties

example

 $(".preview-button").one('click',function(){
//your ajax stuff   
    alert('clicked!!!!');
 });