Add tota11y to WP Theme using WP_DEBUG

functions.php

<?php
/**
 * Theme functions.php file.
 *
 * This file adds functions to the {theme}.
 *
 * @package nls
 * @author  NicktheGeek
 * @license GPL-2.0+
 * @link    http://designsbynickthegeek.com/
 */

// Any code required to setup the theme like calling parent theme functions.php file in Genesis.

/**
 * Enqueue Scripts and Styles.
 */
public function theme_prefix_enqueue_scripts() {
  // This checks to see if WP_DEBUG is defined and is true before enqueueing the script.
  if ( defined( 'WP_DEBUG' ) && true === WP_DEBUG ) {
    /**
     * Copy tota11y.min.js from the latest release at:
     * https://github.com/Khan/tota11y/
     *
     * It goes in the {theme}/assets/js/ folder,
     * or edit the enqueue directory to match the theme JS folder location.
     *
     * I'm not sure it requires jQuery. The docs mention jQuery though, so I include it as required just to be sure.
     */
    wp_enqueue_script( 'nls-a11y-testing', get_stylesheet_directory_uri() . '/assets/js/tota11y.min.js', [ 'jquery' ], '1.0', true );
  }

  // Add any other scripts and styles the theme needs to enqueue.
}
add_action( 'wp_enqueue_scripts', 'theme_prefix_enqueue_scripts' );

// Whatever else goes in your theme functions.php file.