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In my plugin, I want to test to see if jQuery or Prototype (or both) are going to be loaded by another plugin. So, have wp_enqueue_script('jquery') or wp_enqueue_script('prototype') has already been called.

I have code appropriate to my plugin in files plugin.prototype.js and plugin.jquery.js and if Prototype is queued, my plugin will use plugin.prototype.js. This way I avoid loading more than necessary into the site. If neither has loaded, I will queue up whichever is smaller.

How can I test to see what has been queued up? How do I make sure my code runs last?

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up vote 11 down vote accepted

Use wp_script_is() to check if a script is in the queue.

function add_my_scripts() {
    $doing_jquery = wp_script_is('jquery', 'queue');
    $doing_prototype = wp_script_is('prototype', 'queue');

    var_dump($doing_jquery, $doing_prototype);
add_action('wp_print_scripts', 'add_my_scripts');
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So does the wp_print_scripts hook get executed right before, during or right after the scripts have printed? – artlung Aug 12 '10 at 14:10
It prints the scripts, so... during. :) You should use the add_action() priority argument to make your function run later. – Annika Backstrom Aug 12 '10 at 14:17
Sorry, I misunderstood your question. The wp_print_scripts action runs (allowing you to enqueue your scripts), and the scripts are printed after all the wp_print_script actions are done. – Annika Backstrom Aug 12 '10 at 14:19
I was unaware of wp_script_is - seems to be exactly what I was looking for. Thanks so much. – artlung Aug 12 '10 at 14:24
Related to this, can you tell me how to handle plugin script collisions like jQuery Apprise or other popular component? See question here: wordpress.stackexchange.com/q/49020/13 – Volomike Apr 14 '12 at 16:51

To make sure your code runs after jQuery or Prototype is loaded, use the $deps parameter to wp_enqueue_script, and pass either array('jquery') or array('prototype'). If you want to know whether a script is in the queue, you can use the query() method of WP_Dependencies (which is the superclass of WP_Scripts). So something like this should work:

global $wp_scripts;
$jQueryIsLoaded = (bool) $wp_scripts->query('jquery');
$prototypeIsLoaded = (bool) $wp_scripts->query('prototype');

Of course, plugins could enqueue them after you decided on one, so try to make sure you execute this check at the last possible moment.

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How do I execute at the last possible moment? – artlung Aug 12 '10 at 14:08
Silly me, I didn't see that wp_script_is does this for you, so Adam's answer is the way to go, including the use of the wp_print_scripts hook (which is indeed called just before the scripts are printed, so you can add new scripts there). – Jan Fabry Aug 12 '10 at 14:09

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