Go to your WordPress Dashboard > Appearance > Theme Editor
Select/open functions.php file (of child theme) from theme files.
Paste the code snippet given below at the end of functions.php file.
You can specify JS files to exclude from defer in the array (‘jquery.js’).
Finally, click Update File to save changes. That’s all.

// Defer Parsing of JavaScript in WordPress via functions.php file
// Learn more at https://technumero.com/defer-parsing-of-javascript/

function defer_parsing_js($url) {
//Add the files to exclude from defer. Add jquery.js by default
$exclude_files = array(‘jquery.js’);
//Bypass JS defer for logged in users
if (!is_user_logged_in()) {
if (false === strpos($url, ‘.js’)) {
return $url;
}

foreach ($exclude_files as $file) {
if (strpos($url, $file)) {
return $url;
}
}
} else {
return $url;
}
return “$url’ defer=’defer”;

}
add_filter(‘clean_url’, ‘defer_parsing_js’, 11, 1);