Use async/defer in a backwards compatible manner

wp_enqueue_non_blocking_script.php

<?php

/**
 * Register scripts with a `defer` or `async` strategy in a backwards compatible manner.
 *
 * From WordPress 6.3 onwards, the `wp_register_script` function accepts an `$args` array that
 * can include a `strategy` key with a value of either `async` or `defer`.
 *
 * This helper function handles the backwards compatibility for older versions of WordPress. When a
 * `strategy` key is present in the `$args` array (and is either `defer` or `async`), the
 * `script_loader_tag` filter is used to add the attribute to the script tag. Note that
 * for older versions of WordPress, dependency is not managed and the attribute is added unconditionally.
 *
 * When support for WP<6.3 is no longer required, simply replace all instances of this function with
 * `wp_register_script()`.
 *
 * @see wp_register_script()
 *
 * @param string           $handle    Name of the script. Should be unique.
 * @param string|false     $src       Full URL of the script, or path of the script relative to the WordPress root directory.
 *                                    If source is set to false, script is an alias of other scripts it depends on.
 * @param string[]         $deps      Optional. An array of registered script handles this script depends on. Default empty array.
 * @param string|bool|null $ver       Optional. String specifying script version number, if it has one, which is added to the URL
 *                                    as a query string for cache busting purposes. If version is set to false, a version
 *                                    number is automatically added equal to current installed WordPress version.
 *                                    If set to null, no version is added.
 * @param array|bool       $args     {
 *      Optional. An array of additional script loading strategies. Default empty array.
 *      Otherwise, it may be a boolean in which case it determines whether the script is printed in the footer. Default false.
 *
 *      @type string    $strategy     Optional. If provided, may be either 'defer' or 'async'.
 *      @type bool      $in_footer    Optional. Whether to print the script in the footer. Default 'false'.
 * }
 * @return bool Whether the script has been registered. True on success, false on failure.
 */
function wpnext_register_script( $handle, $src, $deps, $ver, $args ) {

	// If >= 6.3, re-use wrapper function signature.
	if ( version_compare( strtok( get_bloginfo( 'version' ), '-' ), '6.3', '>=' ) ) {
		wp_register_script(
			$handle,
			$src,
			$deps,
			$ver,
			$args
		);
	} else {
		wp_register_script(
			$handle,
			$src,
			$deps,
			$ver,
			isset( $args['in_footer'] ) ? $args['in_footer'] : false
		);

		if ( isset( $args['strategy'] ) ) {
			wp_script_add_data( $handle, 'strategy', $args['strategy'] );
		}
	}
}

/**
 * Enqueue scripts with a `defer` or `async` strategy in a backwards compatible manner.
 *
 * From WordPress 6.3 onwards, the `wp_enqueue_script` function accepts an `$args` array that
 * can include a `strategy` key with a value of either `async` or `defer`.
 *
 * This helper function handles the backwards compatibility for older versions of WordPress. When a
 * `strategy` key is present in the `$args` array (and is either `defer` or `async`), the
 * `script_loader_tag` filter is used to add the attribute to the script tag. Note that
 * for older versions of WordPress, dependency is not managed and the attribute is added unconditionally.
 *
 * When support for WP<6.3 is no longer required, simply replace all instances of this function with
 * `wp_enqueue_script()`.
 *
 * @see wp_enqueue_script()
 *
 * @param string           $handle    Name of the script. Should be unique.
 * @param string           $src       Full URL of the script, or path of the script relative to the WordPress root directory.
 *                                    Default empty.
 * @param string[]         $deps      Optional. An array of registered script handles this script depends on. Default empty array.
 * @param string|bool|null $ver       Optional. String specifying script version number, if it has one, which is added to the URL
 *                                    as a query string for cache busting purposes. If version is set to false, a version
 *                                    number is automatically added equal to current installed WordPress version.
 *                                    If set to null, no version is added.
 * @param array|bool       $args     {
 *      Optional. An array of additional script loading strategies. Default empty array.
 *      Otherwise, it may be a boolean in which case it determines whether the script is printed in the footer. Default false.
 *
 *      @type string    $strategy     Optional. If provided, may be either 'defer' or 'async'.
 *      @type bool      $in_footer    Optional. Whether to print the script in the footer. Default 'false'.
 * }
 */
function wpnext_enqueue_script( $handle, $src, $deps, $ver, $args ) {
	wpnext_register_script( $handle, $src, $deps, $ver, $args );
	wp_enqueue_script( $handle );
}

if ( version_compare( get_bloginfo( 'version' ), '6.3', '<' ) ) {
	add_filter(
		'script_loader_tag',
		static function( $tag, $handle ) {
			$strategy = wp_scripts()->get_data( $handle, 'strategy' );
			if ( in_array( $strategy, array( 'async', 'defer' ), true ) ) {
				$p = new WP_HTML_Tag_Processor( $tag );
				if ( $p->next_tag( 'script' ) ) {
					$p->set_attribute( $strategy, true );
					$tag = $p->get_updated_html();
				}
			}
			return $tag;
		},
		10,
		2
	);
}