Adding DEFER or ASYNC in JS scripts via functions.php

gistfile1.txt

// Single script

function make_script_defer( $tag, $handle, $src ){
    if ( 'wp_register_script-name-1' != $handle ) {
        return $tag;
    }
   return str_replace( '<script', '<script defer', $tag );
 }
add_filter( 'script_loader_tag', 'make_script_defer', 10, 3 );
 
// Multiple scripts (Array) 

function add_defer_attribute($tag, $handle) {
   // add script handles to the array below
   $scripts_to_defer = array('wp_register_script-name-1', ‘wp_register_script-name-2’);
   
   foreach($scripts_to_defer as $defer_script) {
      if ($defer_script === $handle) {
         return str_replace(' src', ' defer="defer" src', $tag);
      }
   }
   return $tag;
}
add_filter('script_loader_tag', 'add_defer_attribute', 10, 2);

// Pre-rquisite: wp_register_script and wp_enqueue_scripts. Scripts must be registered prior

function registering_scripts() {

	wp_register_script( 'wp_register_script-name-1', content_url().'/path-to-file/js/owl.carousel.min.js', array(), '1.0.0', true );
	wp_register_script( 'wp_register_script-name-2', content_url().'path_to_js_file, array(), '1.0.0', true );

}

add_action( 'wp_enqueue_scripts', 'registering_scripts' );