也是无意间看见的,以前想过这个问题,但是没多想,今天看到这段代码豁然开朗
(function() {
var dynamicScript = document.createElement('script');
dynamicScript.type = 'text/javascript';
dynamicScript.async = true;
dynamicScript.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.xxx.com/dynamicScript.js';
var firstScript = document.getElementsByTagName('script')[0];
s.parentNode.insertBefore(dynamicScript, firstScript);
})();