defer_async_script.js

// ==UserScript==
// @name           defer/async for Opera
// @namespace      http://d.hatena.ne.jp/edvakf/
// @include        *
// @license        Public Domain
// ==/UserScript==

(function() {
  var loaded = false;
  window.addEventListener('load', function() {
    loaded = true;
  }, false);

  // support defer
  window.opera.addEventListener('BeforeExternalScript', function(e) {
    var script = e.element;
    if (!script.hasAttribute('defer') ||
        script.hasAttribute('async') ||
        script.hasAttribute('data-defer') ||
        loaded) {
      return;
    }
    var load_script = function() {
      var script2 = script.cloneNode(true);
      script2.setAttribute('data-defer', 'yes');
      script.parentNode.insertBefore(script2, script);
      script.parentNode.removeChild(script);
    }
    window.addEventListener('load', load_script, false);
    return e.preventDefault();
  }, false);

  // support async
  window.opera.addEventListener('BeforeExternalScript', function(e) {
    var script = e.element;
    console.log(script.src);
    if (!script.hasAttribute('async') ||
        script.hasAttribute('data-async')) {
      return;
    }
    var load_script = function() {
      var script2 = script.cloneNode(true);
      script2.setAttribute('data-async', 'yes');
      script.parentNode.insertBefore(script2, script);
      script.parentNode.removeChild(script);
    }
    setTimeout(load_script, 10);
    return e.preventDefault();
  }, false);
}());