【WordPress】もっと見るボタン＋都度読み込み

register_rest_route( $namespace, $route, $args, $override )

add_action( 'rest_api_init', function() {
  register_rest_route(
    //ネームスペース
    'sample',
    //ベースURL
    '/post-data',
    //オプション
    array(
      'methods'  =>  WP_REST_Server::READABLE,
      'callback' => 'fetch_post_data'
    )
  );
} );

function fetch_post_data( $param ) {
  return rest_response( 'fetch-post-data', $param );
}

function rest_response( $file_name, $param = null ) {
  $api_file = locate_template( "api/${file_name}.php" );
  $res      = !empty( $api_file ) ? include_once $api_file : [];
  $response = new WP_REST_Response( $res );
  $response->set_status( 200 );
  return $response;
}

$args = [
	'post_type'      => 'post',
	'posts_per_page' => -1,
	'orderby'        => 'date',
	'order'          => 'DESC'
];

$info = [];
$query = new WP_Query( $args );
if( $query->have_posts() ) {
	while ( $query->have_posts() ) {
		$query->the_post();
		global $post;

		$title = esc_html(get_the_title());
		$category = get_the_category();
		$date = get_the_date();
		$result = [
			'title'    => $title,
			'category' => $category[0]->name,
			'date'     => $date
		];

		$info[] = $result;
	}
}

wp_reset_postdata();

return $info;

https://kumatech-lab.com/wp-json/sample/post-data

<section class="con">
		<div class="back-title">
			<h1><span class="h1-border">投稿一覧</span></h1>
		</div>
		<div id="resXhronready">
			<table>
				<tbody id="list-parent">
					
				</tbody>
			</table>
		</div>
		<div class="btn-wrap">
			<input type="button" class="btn" value="もっと見る" onclick="xhrJsonOnReady();">
		</div>
	</section>

function xhrJsonOnReady() {
			const url = 'https://kumatech-lab.com/wp-json/sample/post-data'
			const request = new XMLHttpRequest();

			request.onreadystatechange = function() {
				if (request.readyState === XMLHttpRequest.DONE) {
					if(request.status === 200){
						let result = request.response;
						let parElem = document.getElementById("list-parent");
						let try_num =document.getElementById('list-parent').childElementCount;
						let loopCount = result.length - try_num ;
						if( loopCount >= 0 ) {
							if( loopCount > 5 ) loopCount = 5;
						} 
						for( let i = try_num ; i < try_num + loopCount ; i++ ) {
							//要素作成
						}
					} 
				}
			};

			request.open("GET" , url);
			request.responseType = "json";
			request.send();
		}