preload.js

test_script_elem = document.createElement("script"),
explicit_preloading = typeof test_script_elem.preload == "boolean",
real_preloading = explicit_preloading || (test_script_elem.readyState && test_script_elem.readyState == "uninitialized"),



script = document.createElement('script')
if (real_preloading) {
  registry_item.elem = script;
  if (explicit_preloading) {
    script.preload = true;
    script.onpreload = onload;
  }
  else {
    script.onreadystatechange = function(){
      if (script.readyState == "loaded") onload();
      script.onreadystatechange = null;
    };
  }
  script.src = src; // now preloaded. appending will execute it
}

else {
  script.type = "text/cache-script";
  create_script_load_listener(script,registry_item,"ready", function() {
    // doesn't get here
    append_to.removeChild(script);
    onload();
  })
  script.src = src;
  append_to.insertBefore(script,append_to.firstChild);
}