Wordpress - How to correctly include jquery-ui effects on wordpress

While WordPress does include the jQuery UI libraries, it does not include the UI/Effects library. That library is separate and standalone. You'll need to include a copy of the effects.core.js file and enqueue it separately.

Note that you should name it jquery-effects-core when en-queuing it, for naming consistency.

You can include it like this:

wp_enqueue_script("jquery-effects-core",'http://example.com/whatever/effects.core.js', array('jquery'), '1.8.8');

Edit: This answer was written before WordPress 3.3, which now includes the various effects libraries as part of core. You can simply enqueue the pieces of the effects library that you need to use now.

The list of slugs for these files can be found in wp-includes/script-loader.php, but the core's slug is jquery-effects-core.

wp_enqueue_script("jquery-effects-core");

@dabito,

You're not loading your scripts right ... Don't call wp_enqueue_script() inside your theme template file (this looks like it's header.php). You need to call this function from a separate hook.

In your theme's functions.php file, place the following code:

function my_add_frontend_scripts() {
        wp_enqueue_script('jquery');
        wp_enqueue_script('jquery-ui-core');
}
add_action('wp_enqueue_scripts', 'my_add_frontend_scripts');

If both scripts are properly registered, this should load them just fine (by adding the appropriate <script /> tags in the header. Then your other JavaScript code should work.

If you want to add scripts to the admin side of things, add your action to admin_enqueue_scripts instead.

You can also enqueue the whole jQuery UI directly from Google. This is how I do it:

wp_enqueue_script('jquery-ui', 'http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.6/jquery-ui.min.js', array('jquery'), '1.8.6');

And since jQuery is listed as a dependency for jQuery UI, you don't need to manually enqueue it. WordPress will do it automatically for you.