jQuery has deprecated synchronous XMLHTTPRequest

To avoid this warning, do not use:

async: false

in any of your $.ajax() calls. This is the only feature of XMLHttpRequest that's deprecated.

The default is async: true, so if you never use this option at all, your code should be safe if the feature is ever really removed.

However, it probably won't be -- it may be removed from the standards, but I'll bet browsers will continue to support it for many years. So if you really need synchronous AJAX for some reason, you can use async: false and just ignore the warnings. But there are good reasons why synchronous AJAX is considered poor style, so you should probably try to find a way to avoid it. And the people who wrote Flash applications probably never thought it would go away, either, but it's in the process of being phased out now.

Notice that the Fetch API that's replacing XMLHttpRequest does not even offer a synchronous option.

My workabout: I use asynchronous requests dumping the code to a buffer. I have a loop checking the buffer every second. When the dump has arrived to the buffer I execute the code. I also use a timeout. For the end user the page works as if synchronous requests would be used.

This happened to me by having a link to external js outside the head just before the end of the body section. You know, one of these:

<script src="http://somesite.net/js/somefile.js">

It did not have anything to do with JQuery.

You would probably see the same doing something like this:

var script = $("<script></script>");
script.attr("src", basepath + "someotherfile.js");
$(document.body).append(script);

But I haven't tested that idea.

The accepted answer is correct, but I found another cause if you're developing under ASP.NET with Visual Studio 2013 or higher and are sure you didn't make any synchronous ajax requests or define any scripts in the wrong place.

The solution is to disable the "Browser Link" feature by unchecking "Enable Browser Link" in the VS toolbar dropdown indicated by the little refresh icon pointing clockwise. As soon as you do this and reload the page, the warnings should stop!

This should only happen while debugging locally, but it's still nice to know the cause of the warnings.

No one of the previous answers (which all are correct) was suited to my situation: I don't use the async parameter in jQuery.ajax() and I don't include a script tag as part of the content that was being returned like:

<div>
SOME CONTENT HERE
</div>
<script src="/scripts/script.js"></script>

My situation is that I am calling two AJAX requests consecutively with the aim to update two divs at the same time:

function f1() {
$.ajax(...); // XMLHTTP request to url_1 and append result to div_1
}


function f2() {
$.ajax(...); // XMLHTTP request to url_2 and append result to div_2
}


function anchor_f1(){
$('a.anchor1').click(function(){
f1();
})
}


function anchor_f2(){
$('a.anchor2').click(function(){
f2();
});
}


// the listener of anchor 3 the source of problem
function anchor_problem(){
$('a.anchor3').click(function(){
f1();
f2();
});
}


anchor_f1();
anchor_f2();
anchor_problem();

When I click on a.anchor3, it raises the warning flag.I resolved the issue by replacing f2 invoking by click() function:

function anchor_problem(){
$('a.anchor_problem').click(function(){
f1();
$('a.anchor_f2').click();
});
}

It was mentioned as a comment by @henri-chan, but I think it deserves some more attention:

When you update the content of an element with new html using jQuery/javascript, and this new html contains <script> tags, those are executed synchronously and thus triggering this error. Same goes for stylesheets.

You know this is happening when you see (multiple) scripts or stylesheets being loaded as XHR in the console window. (firefox).