FacetWP hide posts if unfiltered

facetwp_hide_posts_if_unfiltered.php

<?php
// using FacetWP to filter posts but hide certain category if no facet is selected

add_filter( 'facetwp_filtered_post_ids', function( $post_ids ) {
    $facets = FWP()->facet->facets;

    // get post IDs to hide by category
    $hide_post_ids = get_posts(array(
        'numberposts'   => -1,
        'tax_query'     => array(
            array(
                'taxonomy'  => 'category',
      	 	'field' => 'slug',
		'terms' => 'hidden-category',
            ),
        ),
        'fields'        => 'ids',
    ));

    // check whether the only facet isn't selected (unfiltered status)
    if ( empty( $facets['only_facet']['selected_values'] ) ) {
        return array_diff( $post_ids, $hide_post_ids );
    }

    // otherwise return filtered posts including those of the category above
    return $post_ids;
}, 9 );
?>