How do I defer or async this WordPress javascript snippet to load lastly for faster page load times?

Question

I have various javascripts that are necessary plugins in one of my WordPress domains, and I know where in the php file it\'s called from.

I\'m taking every measure I

Answer 1

I believe it's bad practice to defer/async WordPress jQuery. Better solution would be to exclude jQuery from the filter:

if (!is_admin()) {
    add_filter( 'script_loader_tag', function ( $tag, $handle ) {    
        if ( strpos( $tag, "jquery.js" ) || strpos( $tag, "jquery-migrate.min.js") ) {
            return $tag;
        }
        return str_replace( ' src', ' async src', $tag );
    }, 10, 2 );
}

You can use defer instead of async

Answer 2

Very little modification code Mike Kormendy, which allows you to add 2 attributes at once:

// Async load
function add_asyncdefer_attribute($tag, $handle)
{
    $param = '';
    if ( strpos($handle, 'async') !== false ) $param = 'async ';
    if ( strpos($handle, 'defer') !== false ) $param .= 'defer ';
    if ( $param )
        return str_replace('<script ', '<script ' . $param, $tag);
    else
        return $tag;
}

Result:

<script async defer type='text/javascript' src='#URL'></script>

Answer 3

This blog post links to two plugins of interest:

Asynchronous Javascript
Improve page load performance by asynchronously loading javascript using head.js

WP Deferred Javascripts
Defer the loading of all javascripts added with wp_enqueue_scripts, using LABJS (an asynchronous javascript library).

Haven't tested them but checked the code and they do pretty fancy stuff with WordPress scripts enqueuing process.

But then WPSE comes to rescue:

// Adapted from https://gist.github.com/toscho/1584783
add_filter( 'clean_url', function( $url )
{
    if ( FALSE === strpos( $url, '.js' ) )
    { // not our file
        return $url;
    }
    // Must be a ', not "!
    return "$url' defer='defer";
}, 11, 1 );

Answer 4

Trying to keep things somewhat modular and all encompassing, the following approach dynamically chooses how to embed the tag with the async or defer attributes by just appending a small identifier to the $handle name:

/**
* Add async or defer attributes to script enqueues
* @author Mike Kormendy
* @param  String  $tag     The original enqueued <script src="...> tag
* @param  String  $handle  The registered unique name of the script
* @return String  $tag     The modified <script async|defer src="...> tag
*/
// only on the front-end
if(!is_admin()) {
    function add_asyncdefer_attribute($tag, $handle) {
        // if the unique handle/name of the registered script has 'async' in it
        if (strpos($handle, 'async') !== false) {
            // return the tag with the async attribute
            return str_replace( '<script ', '<script async ', $tag );
        }
        // if the unique handle/name of the registered script has 'defer' in it
        else if (strpos($handle, 'defer') !== false) {
            // return the tag with the defer attribute
            return str_replace( '<script ', '<script defer ', $tag );
        }
        // otherwise skip
        else {
            return $tag;
        }
    }
    add_filter('script_loader_tag', 'add_asyncdefer_attribute', 10, 2);
}

Example usage:

function enqueue_my_scripts() {

    // script to load asynchronously
    wp_register_script('firstscript-async', '//www.domain.com/somescript.js', '', 2, false);
    wp_enqueue_script('firstscript-async');

    // script to be deferred
    wp_register_script('secondscript-defer', '//www.domain.com/otherscript.js', '', 2, false);
    wp_enqueue_script('secondscript-defer');


    // standard script embed
    wp_register_script('thirdscript', '//www.domain.com/anotherscript.js', '', 2, false);
    wp_enqueue_script('thirdscript');
}
add_action('wp_enqueue_scripts', 'enqueue_my_scripts', 9999);

Outputs:

<script async type='text/javascript' src='//www.domain.com/somescript.js'></script>
<script defer type='text/javascript' src='//www.domain.com/otherscript.js'></script>
<script type='text/javascript' src='//www.domain.com/anothercript.js'></script>

Thanks to @MattKeys @crissoca for inspiring my answer here.