Add async attribute to enqueued WordPress script

<?php

namespace NickDavis\AsyncExample;

const ASYNC_EXAMPLE = 'nd-async-example';

add_action( 'wp_enqueue_scripts', __NAMESPACE__ . '\enqueue_script' );
/**
 * Enqueues the async example script.
 *
 * @since v1.0.0
 */
function enqueue_script() {
	wp_enqueue_script( ASYNC_EXAMPLE, '//async.example.com', [], null, true );
}

add_filter( 'script_loader_tag', __NAMESPACE__ . '\add_async_attribute', 10, 2 );
/**
 * Add an aysnc attribute to an enqueued script.
 *
 * @url https://wordpress.stackexchange.com/a/198372
 *
 * @since v1.0.0
 *
 * @param string $tag Tag for the enqueued script.
 * @param string $handle The script's registered handle.
 * @return string Script tag for the enqueued script
 */
function add_async_attribute( $tag, $handle ) {
    // Just return the tag normally if this isn't one we want to async.
    if ( ASYNC_EXAMPLE !== $handle ) {
        return $tag;
    }

    return str_replace( ' src', ' async src', $tag );
}