Post Gallery AJAX Loading Wordpress

functions.php

<?php
/**
 * Custom AJAX Action
 */
function load_more_post_gallery_image_callback() {
    $url     = wp_get_referer();
    $post_id = url_to_postid( $url );

    $imageInitialCount = intval( $_POST['imageInitialCount'] );
    $imageLoadingCount = intval( $_POST['imageLoadingCount'] );

    $postGalleryImageIdsString = get_post_meta($post_id, 'post_gallery', true);
    $postGalleryImageIds = explode(',', $postGalleryImageIdsString);

    $postGalleryImageLoadingIds = array_slice($postGalleryImageIds, $imageInitialCount, $imageLoadingCount);

    //print_r($postGalleryImageLoadingIds);

    foreach ($postGalleryImageLoadingIds as $postGalleryImageLoadingId) : ?>
        <img src="<?= wp_get_attachment_image_url($postGalleryImageLoadingId, 'full')?>">
    <?php endforeach;

    // exit for not return 0 in response
    wp_die();
}
add_action('wp_ajax_load_more_post_gallery_image', 'load_more_post_gallery_image_callback');
add_action('wp_ajax_nopriv_load_more_post_gallery_image', 'load_more_post_gallery_image_callback');

script.js

(function( $ ) {
    "use strict";

	var SITE =  {};
    /*---------------------------------------------*/
    /*	Load more post Gallery
    /*---------------------------------------------*/

    SITE.postGalleryLoadMoreBtnDom = $('.post-gallery-load-more-btn');

    SITE.loadMorePostGallery = function() {
        var imageTotalCount = SITE.postGalleryLoadMoreBtnDom.data('total-count'),
            imageLoadingCount = SITE.postGalleryLoadMoreBtnDom.data('loading-count');
        SITE.postGalleryLoadMoreBtnDom.click( function(e) {
            e.preventDefault();
        	if (!$(this).hasClass('post-gallery-loading')) {
                var imageInitialCount = parseInt($(this).attr('data-initial-count')),
                    imageLoadedCount = imageInitialCount + imageLoadingCount,
                    data = {
                        action: 'load_more_post_gallery_image',
                        imageInitialCount: imageInitialCount,
                        imageLoadingCount: imageLoadingCount
                    };

                $.ajax({
                    type: "POST",
                    url: ajaxurl,
                    data: data,
                    beforeSend: function() {
                        SITE.postGalleryLoadMoreBtnDom.addClass('post-gallery-loading');
                    },
                    complete: function() {
                        SITE.postGalleryLoadMoreBtnDom.removeClass('post-gallery-loading');
                    },
                    success: function(postGalleryLoadedImagesHtml) {
                        $('.post-gallery').append(postGalleryLoadedImagesHtml);
                        //console.log(imageInitialCount, imageLoadingCount,imageLoadedCount);

                        if (imageTotalCount <= imageLoadedCount) {
                            SITE.postGalleryLoadMoreBtnDom.hide();
                        } else {
                            SITE.postGalleryLoadMoreBtnDom.attr('data-initial-count', imageLoadedCount);
                        }
                    }
                });
			}
        });
    };
  
  
	/*---------------------------------------------*/
	/* Init Functions
	/*---------------------------------------------*/

	$( document ).ready( function() {
        if ( SITE.postGalleryLoadMoreBtnDom.length !== 0 ) {
            SITE.loadMorePostGallery();
        }
	});
})( jQuery );

single.php

    <div class="post-gallery">
        <?php
        $postGalleryImageIdsString = get_post_meta(get_the_ID(), 'post_gallery', true);
        $postGalleryImageIds = explode(',', $postGalleryImageIdsString);

        $postGalleryImageInitialCount = 2;
        $postGalleryImageLoadingCount = 3;
        $postGalleryImageTotalCount = count($postGalleryImageIds);

        $postGalleryImageIndex = 0;
        foreach ($postGalleryImageIds as $postGalleryImageId) : if ($postGalleryImageIndex == $postGalleryImageInitialCount) break; ?>
            <img src="<?= wp_get_attachment_image_url($postGalleryImageId, 'full')?>">
        <?php $postGalleryImageIndex++; endforeach; ?>
    </div>

    <?php if ($postGalleryImageTotalCount > $postGalleryImageInitialCount) : ?>
        <div class="post-gallery-load-more-wrap">
            <a class="post-gallery-load-more-btn" href="#"
               data-initial-count="<?= $postGalleryImageInitialCount; ?>"
               data-loading-count="<?= $postGalleryImageLoadingCount; ?>"
               data-total-count="<?= $postGalleryImageTotalCount; ?>"></a>
        </div>
    <?php endif; ?>

style.css

/*
-----Post Gallery----
*/
.post-gallery img {
  display: block;
  width: 100%;
  height: auto;
  margin-top: 10px;
  border: solid 1px #e6e6e6;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
}
.post-gallery img:first-of-type {
  margin-top: 0;
}
.post-gallery img:first-of-type {
  margin-top: 0;
}
.post-gallery-load-more-btn {
  display: block;
  width: 88px;
  height: 100px;
  margin: 155px auto;
  background: url("images/post-gallery-loading.png") no-repeat center;
  background-size: contain;
}
.post-gallery-load-more-btn:not(.post-gallery-loading):hover {
  -webkit-transform: rotate(-90deg);
  -moz-transform: rotate(-90deg);
  -ms-transform: rotate(-90deg);
  -o-transform: rotate(-90deg);
  transform: rotate(-90deg);
}
.post-gallery-loading {
  -webkit-animation: rotation 2s infinite linear;
}

@-webkit-keyframes rotation {
  from {
    -webkit-transform: rotate(-90deg);
  }
  to {
    -webkit-transform: rotate(-449deg);
  }
}