This little conflict error drove me mad for quite a little bit.

I run several websites, some personal, some for clients, quite a few of them run on WordPress. Its quick, it’s easy, and its fairly secure if configured properly. Even better, its updated and patched often, and has a whole bunch of free plugins that allow you to extend functionality easily. With all this freedom often comes a price. I paid such a price recently when I ran into a problem post WordPress 3.5 to 3.6 upgrade on one of my client’s sites. I always test updates in development environment so the actual client’s website was not affected, however I did spend a considerable amount of time chasing this down. The error was with jQuery and its helper files. It prevented a lot of the nice flashy JavaScript functionality that most clients like. Namely the issue was with anything that involved easing.

Fire Bug and Chrome Consoles were reporting the following:

jQuery.easing[jQuery.easing.def] is not a function

I went searching for what could be causing this. After a little bit of digging I was able to determine that after the DOM is fully loaded there is a plugin that’s dynamically appending an older version of jQuery to the end of the head section of the site. This explains the jQuery.easing.def not being a function.

To fix this issue I had to dig deep into the bowels of WordPress and de-register old versions of jQquery and add the most current version. I created a quick and dirty plugin that allows me to easily update the version of jQuery being added in the future. Here’s what I did:

1. Create the plugin directories:
• /wp-content/plugins/latest_jquery
• /wp-content/plugins/latest_jquery/js
2. Create the main plugin php file:
• /wp-content/plugins/latest_jquery.php
3. Add the main plugin code into /wp-content/plugins/latest_jquery.php:

/*
Plugin Name: Latest jQuery
Plugin URI: http://www.andretimokhin.com
Description: This plugin updates jQuery to the latest version.
Version: 1
Author: Andre Timokhin
Author URI: http://www.andretimokhin.com/
License: GPL3
*/

function latest_jquery_update() {
		//Remove any already registered jquery libraries
		wp_deregister_script('jquery'); 
		//Enqueue our own version of jQuery located in /wp-content/plugins/latest_jquery/js/*
		wp_enqueue_script('jquery', plugins_url('/js/jquery-2.0.3.min.js', __FILE__), false, '2.0.3');	
		// Also load the migrate library plugin
		wp_enqueue_script('jquery-migrate', plugins_url('/js/jquery-migrate-1.2.1.min.js', __FILE__), array('jquery'), '1.2.1'); // require jquery, as loaded above	
}

add_action('wp_enqueue_scripts', 'latest_jquery_update');

4. Add latest jQuery to /wp-content/plugins/latest_jquery/js/ folder. In my case I’m currently using 2.0.3
5. Add latest jQuery Migrate plugin to /wp-content/plugins/latest_jquery/js/ folder. In my case I’m currently using 1.2.1
6. Activate the plugin and enjoy restored jQuery functionality.

Hope this helps someone save a bunch of diagnosing time.