Example how to defer js in Wordpress with function.php file

defer example.php

<?php

if (!is_admin()) {
    add_filter( 'script_loader_tag', function ( $tag, $handle ) {    
        if ( strpos( $tag, "jquery.js" ) || 
             strpos( $tag, "leaflet.js") /*|| 
             strpos( $tag, "add any script you want" )*/ ) {
            return $tag;
        }
	return str_replace( ' src', ' defer="defer" src', $tag );
    }, 10, 2 );
}