async_load.js

async_load.js

(function() {
    function async_load(){
      var s = document.createElement('script');
      s.type = 'text/javascript';
      s.async = true;
      s.src = 'http://yourdomain.com/script.js';
      var x = document.getElementsByTagName('script')[0];
      x.parentNode.insertBefore(s, x);
    }
    if (window.attachEvent) {
      window.attachEvent('onload', async_load);
    } else {
      window.addEventListener('load', async_load, false);
    }
})();