WordPress `async` and `defer` tags for registered or enqueued scripts.

script_loader_tag_add_attributes.php

<?php
/**
 * Use in WordPress themes or plugins in order to add
 * `async` and `defer` attributes to script tags when
 * using `wp_enqueue_script` or `wp_register_script`.
 */

if ( ! function_exists( 'tag_add_attribute_async' ) ) :
    /**
     * Add Async Attribute
     *
     * By adding '+async' to the $handle when registering or enqueueing the `script`
     * tag will add the attribute `async`.
     *
     * @param string $tag    The <script> tag for the enqueued script.
     * @param string $handle The script's registered handle.
     *
     * @return string
     */
    function tag_add_attribute_async(string $tag, string $handle): string {
        if (strpos($handle, '+async') !== false) {
            $tag = str_replace( '<script ', '<script async ', $tag );
        }
        
        return $tag;
    }

    add_filter('script_loader_tag', 'tag_add_attribute_async', 10, 2);
endif; // function_exists( 'tag_add_attribute_async' )

if ( ! function_exists( 'tag_add_attribute_defer' ) ) :
    /**
     * Add Defer Attribute
     *
     * By adding '+defer' to the $handle when registering or enqueueing the `script`
     * tag will add the attribute `defer`.
     *
     * @param string $tag    The <script> tag for the enqueued script.
     * @param string $handle The script's registered handle.
     *
     * @return string
     */
    function tag_add_attribute_defer(string $tag, string $handle): string {
        if (strpos($handle, '+defer') !== false) {
            $tag = str_replace( '<script ', '<script defer ', $tag );
        }
        
        return $tag;
    }

    add_filter('script_loader_tag', 'tag_add_attribute_defer', 10, 2);
endif; // function_exists( 'tag_add_attribute_defer' )