Defer parsing of JavaScript

Hello,
on my GTmetrix my webiste has got 0 in Defer parsing of JavaScript.
So, I used this code:

function defer_parsing_of_js ( $url ) {
if ( FALSE === strpos( $url, '.js' ) ) return $url;
if ( strpos( $url, 'jquery.js' ) ) return $url;
return "$url' defer ";
}
add_filter( 'clean_url', 'defer_parsing_of_js', 11, 1 );

but nothing happened.
How can I improve the grade?