Synchronous XMLHttpRequest warning and <script>

UPDATE: This has been fixed in jQuery 3.x. If you have no possibility to upgrade to any version above 3.0, you could use following snippet BUT be aware that now you will lose sync behaviour of script loading in the targeted content.

You could fix it, setting explicitly async option of xhr request to true:

$.ajaxPrefilter(function( options, original_Options, jqXHR ) {
    options.async = true;
});

Browsers now warn for the use of synchronous XHR. MDN says this was implemented recently:

Starting with Gecko 30.0 (Firefox 30.0 / Thunderbird 30.0 / SeaMonkey 2.27)

https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/Synchronous_and_Asynchronous_Requests#Synchronous_request

Here's how the change got implemented in Firefox and Chromium:

• https://bugzilla.mozilla.org/show_bug.cgi?id=969671
• http://src.chromium.org/viewvc/blink?view=revision&revision=184788

As for Chrome people report this started happening somewhere around version 39. I'm not sure how to link a revision/changeset to a particular version of Chrome.

Yes, it happens when jQuery appends markup to the page including script tags that load external js files. You can reproduce it with something like this:

$('body').append('<script src="foo.js"></script>');

I guess jQuery will fix this in some future version. Until then we can either ignore it or use A. Wolff's suggestion above.