archive.php

<?php if ( ! is_wp_error( $topics ) && ! empty( $topics ) ) : ?>
  <div class="cats" data-filter="cat">
    <h4><?php pll_e( 'Valitse aihe' ) ?></h4>
    <ul>
      <li><a href="<?php echo get_term_link( $term_id ); ?>"<?php echo ( ! isset( $_GET['cat'] ) ) ? ' class="selected"' : ''; ?>><?php _e( 'Kaikki' ) ?></a></li>
      <?php foreach ( $topics as $topic ) : ?>
        <li>
          <a href="<?php echo yeahboy_make_filter_url( $base_url_term_id, 'cat', $topic->term_id ) ?>" data-id="<?php echo $topic->term_id ?>" <?php echo ( $_GET['cat'] == $topic->term_id ) ? ' class="selected"' : ''; ?>>
            <?php echo $topic->name ?>
          </a>
        </li>
      <?php endforeach; ?>
    </ul>
  </div>
<?php endif;

...

<?php global $wp_query;
  if ( $wp_query->found_posts != $wp_query->post_count ) :
    $wp_query->query['paged'] = 1; ?>
    <div class="button-container">
      <p><a href="#" class="button button-ghost load-more" data-use-query="load_more_query"><?php pll_e( 'Lataa lisää' ) ?></a></p>
      <script>
        var base_category_name = <?php echo json_encode( $wp_query->query['category_name'] ) ?>;
        var load_more_query = <?php echo json_encode( $wp_query->query ) ?>;
      </script>
    </div>
  <?php endif; ?>

loadmore.js

$('.cats a').on('click', function(e) {
  		e.preventDefault();

  		var filter_type = $(this).closest('.cats').attr('data-filter')
  		var query_name = $('.slide-articles a.load-more').attr('data-use-query');
			var query = window[query_name];

			$(this).closest('.cats').find('a').removeClass('selected');
			$(this).addClass('selected');

			// Reset paged for new query.
			query.paged = 0;

			// Maybe set child category to query.
			if ( 'cat' === filter_type ) {
				if ( $(this).attr('data-id') == null ) {
					delete query.cat;
					query.category_name = base_category_name;
				} else {
					delete query.category_name;
					query.cat = $(this).attr('data-id');
				}
			}

			// Hide load more.
			$('.slide-articles .button-container').hide();

			// Remove old articles.
			$('.slide-articles .col').remove();

			// Get new articles.
			yeahboy_do_ajax_load();
  	} );

    // Load more stuff
    // Vue construct
    var blog = new Vue({
      el: '.slide-articles .cols',
      data: {
        posts: []
      }
    });

    // Load more ajax call
    jQuery('.slide-articles a.load-more').on( 'click', function(e) {
    	e.preventDefault();
    	yeahboy_do_ajax_load();
    } );

    function yeahboy_do_ajax_load() {
    	var button_container = $('.slide-articles a.load-more').closest('.button-container');
    	var query_name = $('.slide-articles a.load-more').attr('data-use-query');
			var query = window[query_name];

			// Alter query
	    query.paged = query.paged+1;
	    query._embed = true;

	    // Do query
	    $.ajax({
			  url: '/wp-json/wp_query/args/?' + jQuery.param( query ),
			}).done(function( response ) {
			  if( response.length !== 0 && response !== false ) {
          jQuery.each( response, function() {
            var self = this;
            blog.posts.push(this);
          } );

          if( response.length < dudeuserrating.posts_per_page ) {
            button_container.hide();
          } else {
          	button_container.show();
          }
        } else if( response == false ) {
          button_container.hide();
        }
			});
    }