Add a script as a dependency to a registered script

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wp_register_script() (see codex) allows you to specify dependencies: scripts that must be loaded before the one being registered is loaded (if it is loaded).

But suppose the script is a third-party one (WordPress or another plug-in) so that you are not the one calling wp_register_script(). How can you inject a script as a dependency for a pre-registered script?

Remarks: In my particular use-case the dependency is not strict – the registered script does not require this second script, but the latter alters the original script.

A similar question could be asked for styles, but I suspect the answers would be nearly identical.

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Digging through https://github.com/WordPress/WordPress/blob/3.5.1/wp-includes/class.wp-dependencies.php all registered scripts are stored in the global $wp_scripts.

You can access them directly through that, but I prefer to use the API when it exists. In this case, $wp_scripts->query() returns a particular registered script (a _WP_Dependency object – see source).

A _WP_Dependency object stores the dependencies as an array of handles, which you can access directly, and insert a dependency. The following function does that:

/**
 * Add $dep (script handle) to the array of dependencies for $handle
 * @see http://wordpress.stackexchange.com/questions/100709/add-a-script-as-a-dependency-to-a-registered-script
 * @param string $handle Script handle for which you want to add a dependency
 * @param string $dep Script handle - the dependency you wish to add
 */
function wpse100709_append_dependency( $handle, $dep ){
    global $wp_scripts;

    $script = $wp_scripts->query( $handle, 'registered' );
    if( !$script )
        return false;

    if( !in_array( $dep, $script->deps ) ){
        $script->deps[] = $dep;
    }

    return true;
}

Obviously you must add this somewhere between the original script ($handle) being registered and it being enqueued.

Example usage

Suppose script_a has been registered on the init hook with priority 10, and you want to add script_b as a dependency:

add_action( 'init', 'wpse100709_register_script_b', 11 );
function wpse100709_register_script_b(){

   //Register script b
   wp_register_script( 'script_b', ... );

   //Now add script b as being a pre-requisite for script a
   wpse100709_append_dependency( 'script_a', 'script_b' )

   //If script a is enqueued, script b is enqueued before it.

}

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