On my wordpress page, I have the following loop, which successfully outputs an attachment's caption.

<?php 
$the_query = new WP_Query(array( 'post_type' => 'attachment')); 
while ( $the_query->have_posts() ) :  $the_query->the_post();

$attachment_data = wp_prepare_attachment_for_js();
echo '<h2>'.$attachment_data['caption'].'</h2>'
;?>

Now, I also need to call the attachment caption outside of the loop. This is what I'm trying in functions.php:

function custom_info()
     {  
         global $wp_query;
         $query_id = $wp_query->post->ID;
         $attachment_data_query = wp_prepare_attachment_for_js( $query_id );

         wp_register_script( 'custom_data_info');
         $info = array(
             'data_query'   => $attachment_data_query['caption']
         );
         wp_enqueue_script( 'custom_data_info' );
         wp_localize_script('custom_data_info', 'info', $info);
    }
     if ( !is_admin() ) add_action( "wp_enqueue_scripts", "custom_info", 10 );

And then it would be called in the external javascript file like so:

return info.data_query

I know the custom_info function and javascript file are talking with each other correctly. The problem is with how I am defining $query_id

EDIT: Almost fixed. The problem was that I didn't include a foreach statement, and so the first item in the loop was being repeated over and over again.

I've adjusted below, but for some reason, now the array is empty upon output of info.data_query...

function custom_info(){  
     global $wp_query;
     wp_register_script( 'custom_data_info');

     $info = array('data_query' => array());

     foreach ( $wp_query->posts as $query_id) {
        $attachment_data_query = wp_prepare_attachment_for_js( $query_id );
        $info['data_query'][$query_id] = $attachment_data_query['caption'];

      }