Add async or defer to enqueue WordPress scripts

functions.php

// Add async, defer, or other attributes to enqueue WordPress scripts
 * Add async attributes to enqueued scripts where needed.
 * The ability to filter script tags was added in WordPress 4.1 for this purpose.
 */
function my_async_scripts( $tag, $handle, $src ) {
    // the handles of the enqueued scripts we want to async
    $async_scripts = array( 'some-script', 'another-script' );

    if ( in_array( $handle, $async_scripts ) ) {
        return '<script type="text/javascript" src="' . $src . '" async="async"></script>' . "\n";
    }

    return $tag;
}
add_filter( 'script_loader_tag', 'my_async_scripts', 10, 3 );