jQuery has deprecated synchronous XMLHTTPRequest

Solution 1:

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.

Solution 2:

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.