terms-filter.php

<?php


/**
 * Setup query
 */
// Move this on top
$args = [
    'paged'          => $page,
    'post_type'      => 'post',
    'post_status'    => 'publish',
    'posts_per_page' => $qty,
];

/**
 * Check if term exists
 */
// Check if there are any terms specified, if not don't add any
if ( $term && $tax ) :

    if ( ! term_exists( $term, $tax) && $term != 'all-terms' ) :
        $response = [
            'status'  => 501,
            'message' => 'Term doesn\'t exist',
            'content' => 0
        ];
        die(json_encode($response));
    endif;
    /**
     * Tax query
     */
    if ($term == 'all-terms') : 
        $tax_qry[] = [
            'taxonomy' => $tax,
            'field'    => 'slug',
            'terms'    => $term,
            'operator' => 'NOT IN'
        ];
    else :
        $tax_qry[] = [
            'taxonomy' => $tax,
            'field'    => 'slug',
            'terms'    => $term,
        ];
    endif;

    $args['tax_query'] => $tax_qry;

endif;