Question: We have had issues with Mootools not being very backward compatible specifically in the area of drag and drop functionality. I was wondering if anyone has had any similar problems with jQuery not being backward compatible. We are starting to use it quite heavily and are thinking about upgrading to a newer version to start using several plugins that require it. Will we have any issues if we get rid of the older version?

Answer: 
*My experience* is that when upgrading, I sometimes find that stuff that used to work breaks. For instance, when upgrading from 1.7.2 to 1.8.3 I found some drag-and-drop features stopped working. Some problems may be due to deprecated jQuery functions being dropped, still in use by incompatible versions of [jQuery UI](http://jqueryui.com/).

Here is a [quote from another user](http://sympmarc.com/2011/06/27/jquery-and-backward-compatibility): 

> 
> Backward compatibility may not be the jQuery team’s strong suit, but
>  as with most things, it’s a work in progress. I’m not meaning to
>  disparage the great work they do, but it points out the fact that you
>  simply *must* always test your scripts with new versions of jQuery. 
> 
> 
>
In my experience, jQuery 1.x has been very backwards-compatible until version 1.9.

> 
> The jQuery 1.x line had major changes as of jQuery 1.9.0. We strongly recommend that you also use the jQuery Migrate plugin if you are upgrading from pre-1.9 versions of jQuery or need to use plugins that haven't yet been updated. Read the [jQuery 1.9 Upgrade Guide](http://jquery.com/upgrade-guide/1.9/) and the [jQuery 1.9 release blog post](http://blog.jquery.com/2013/01/15/jquery-1-9-final-jquery-2-0-beta-migrate-final-released/) for more information.
> 
> 
> 

And don't upgrade to 2.0 until you're sure that this isn't an issue for you:

> 
> jQuery 2.x has the same API as jQuery 1.x, but *does not support Internet Explorer 6, 7, or 8.*
> 
> 
>