Defer loading of stats.wp.com script

[tillkruss]

They way the stats.wp.com script is currently loaded is a bad practice and slows down the rendering of the page.

Please implement a non render-blocking loading of the statistics script like Google Analytics or HTML5 Boilerplate does.

[tillkruss]

In order to achieve that, the script hosted on stats.wp.com needs to be adjusted as well, because it's currently using document.write in the called st_go() function. Which is also a bad practice, because if it runs after a page finishes loading, it overwrites the entire document with the content it outputs.

[tillkruss]

The code below uses the template_redirect action to override Jetpack's $stats_footer. The JavaScript code can easily be compressed into a 1-3 liner.

add_action( 'template_redirect', function() {

    global $stats_footer;

    if ( preg_match( '~/e-(\d+).js"~', $stats_footer, $matches ) && isset( $matches[1] ) ) {
        $week = $matches[1];
    }

    if ( preg_match( '~st_go\((.+)\);~', $stats_footer, $matches ) && isset( $matches[1] ) ) {
        $data = $matches[1];
    }

    if ( preg_match( '~linktracker_init\(.+\);~', $stats_footer, $matches ) && isset( $matches[0] ) ) {
        $linktracker = $matches[0];
    }

    if ( ! isset( $week, $data, $linktracker ) ) {
        return;
    }

    $stats_footer = <<<END
<script type="text/javascript">
    (function(i,z,x){
        r = i.createElement(z);
        d = i.getElementsByTagName(z)[0];
        r.src='//stats.wp.com/e-$week.js';
        r.async=true;
        r.onload = r.onreadystatechange = function() {
            a = this.readyState;
            if(a&&a!="complete"&&a!="loaded") return;

            u = '//pixel.wp.com/g.gif?host='+escape(i.location.host)+'&rand='+Math.random();
            for(v in x){u=u+'&'+v+'='+escape(x[v]);}
            u=u+'&ref='+escape(i.referrer);

            b = i.createElement('img');
            b.src = u;
            b.id = 'wpstats';

            i.body.appendChild(b);

            $linktracker

        };
        d.parentNode.insertBefore(r,d);
    }(document, 'script', $data ));
</script>
END;

} );

[jeherve]

Closing as duplicate of #566