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: 
jQuery is so serious about backwards compatibility that they produce a "backwards compatibility" plugin for each release: <http://docs.jquery.com/Release:jQuery_1.2#jQuery_1.1_Compatibility_Plugin>. It let people who don't need backwards compatibility save on page weight.
jQuery has produced backwards compatibility plugins since version 1.0. 

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> Web development has changed a lot over the years, and jQuery has changed along with it. Through all of this time, the team has tried to walk the line between maintaining compatibility with code from the past versus supporting the best web development practices of the present.
> 
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The project has the following backwards compatibility repos:

* [jquery-compat-1.0(Compatibility plugin for those upgrading to jQuery 1.1+ from jQuery 1.0)](https://github.com/jquery-archive/jquery-compat-1.0)
* [jquery-compat-1.1(Compatibility plugin for those upgrading to jQuery 1.2+ from jQuery 1.1)](https://github.com/jquery-archive/jquery-compat-1.1)
* [jquery-xpath(Implement XPath functionality for those upgrading to jQuery 1.2 from jQuery 1.1)](https://github.com/jquery-archive/jquery-xpath)
* [jquery-compat-1.3(Backwards compatibility plugin for jQuery 1.3 (to be used with jQuery 1.4))](https://github.com/jquery-archive/jquery-compat-1.3)
* [jquery-migrate-1.x-stable(Migrate older jQuery code to jQuery 1.9+)](https://github.com/jquery/jquery-migrate/tree/1.x-stable)
* [jquery-migrate(Migrate older jQuery code to jQuery 3.0+)](https://github.com/jquery/jquery-migrate)

**References**

* [jQuery Blog](http://blog.jquery.com/category/jquery/)
* [jQuery Wiki: Won't Fix](https://github.com/jquery/jquery/wiki/Won't-Fix)
* [jQuery Core Upgrade Guides](http://jquery.com/upgrade-guide/)
* [jQuery Migrate Plugin - Warning Messages (2.0)](https://github.com/jquery/jquery-migrate/blob/1.x-stable/warnings.md)
* [jQuery Migrate Plugin - Warning Messages (3.0)](https://github.com/jquery/jquery-migrate/blob/master/warnings.md)
* [jqLint](https://github.com/jokeyrhyme/jqlint)