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How to check if jQuery library exist in head tags?

<script type="javascript/text" src="http://code.jquery.com/jquery-1.6.2.js"></script>

and if not exist how do I load in head tags the jquery library, I'm doing a plugins and I want to load my plugins script in jQuery and also do able to check if jQuery library exist so my jQuery script will run

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up vote 5 down vote accepted

scripts and styles should never be embedded directly in themes or templates because of potential conflicts between plugins and themes.

To use jQuery in a plugin or theme it should be enqueued with wp enqueue script. This will make sure it's added only once, and any scripts that define it as a dependency will load after.

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so you mean wp ennqueue script does the checking and if its already exist it will not load the jQuery library in wp plugins? – idontknowhow Aug 8 '11 at 4:56
yes, enqueue jQuery and enqueue whatever scripts your plugin uses with jQuery set as a dependency, and WP will handle it. – Milo Aug 8 '11 at 5:01

You can check whether the jQuery library already exist/loaded in the site and load it if not using the following code.

function load_jquery() {
    if ( ! wp_script_is( 'jquery', 'enqueued' )) {

        wp_enqueue_script( 'jquery' );

add_action( 'wp_enqueue_scripts', 'load_jquery' );
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@Milo already pointed this out, just enqueue your script with jQuery listed as a dependency. When WordPress loads your script, it will see the dependency and load jQuery for you automatically. For example:

function enqueue_my_scripts(){
    if( ! is_admin() )
        wp_enqueue_script( 'stacked-script', get_bloginfo('stylesheet_directory') . '/scripts/stacked.js', array('jquery'), '1.0.1' );
add_action( 'wp_enqueue_scripts', 'enqueue_my_scripts' );

Will put the following in the header of your site:

<script type='text/javascript' src='http://site-url.com/wp-includes/js/jquery/jquery.js?ver=1.6.1'></script> 
<script type='text/javascript' src='http://site-url.com/wp-content/themes/stacked-theme/scripts/stacked.js?ver=1.0.1'></script> 

jQuery will be loaded first, followed by your script.

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Use this way to check jquery class loaded or not loaded.

if (jQuery) {  
    alert('Jquery Loaded');
} else {
    alert('Jquery Not Loaded');
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