Installation is available on NPM and as a PHP Composer package.

Install via NPM

npm install --save


Install via PHP Composer

composer require styletools/async



$async has been designed for above-the-fold optimization. The script is compiled into individual modules via Google Closure Compiler that does not just compress javascript but also optimizes it for speed. The individual modules can be concatenated without the use of a module loader.

IIFE Generator

An IIFE is a pre-concatenated selection of $async modules that is additionally optimized using Google Closure Compiler with mode Advanced Optimizations. IIFE or Immediately-invoked Function Expressions is a coding pattern for loading a script. An IIFE can be used in the browser safely.

For more documentation, see IIFE Generator

Manual concatenation

The Google Closure Compiler module architecture enables to concatenated a selection of modules without the use of a module loader. You can simply wrap the module source text in a IIFE and include it safely in the HTML document.

Example concatenation

// include a selection of $async modules inline
echo "<script>(function(){"; // wrap in IIFE
readfile('vendor/styletools/async/dist/cache.js'); // cache module
readfile('vendor/styletools/async/dist/localstorage.js'); // localStorage module
echo "})();</script>";

// CSS Loader config
$async_config = array(
'load' => array( 'sheet.css' ),
'load_options' => array( 'load_timing' => 'requestAnimationFrame' )
// discover modules via config
$modules = StyleTools\AsyncCSS::discover_modules($async_config);
// manually add some modules
$modules[] = 'api'; // public API and events
$modules[] = 'attr-config'; // data-c HTML attribute config
// generate IIFE
$iife_options = array(
'cache' => true,
'compress' => true,
'mode' => 'unary'
$module_iife = StyleTools\IIFE::generate($modules, $iife_options);
// compress JSON config
$compressed_config = StyleTools\AsyncCSS::compress($async_css_config, null, false, true);
// inline $async script
echo '<script data-c=\''.(($compressed_config) ? $compressed_config : '').'\'>'.$module_iife.'</script>';