js延迟加载

为了提高页面响应速度，我们需要让一些不必一开始就加载的页面延迟加载，把下面代码放到body的最后一行：

 <script type="text/javascript">function downloadJSAtOnload() {var element = document.createElement("script");element.src = "index.js";document.body.appendChild(element);}if (window.addEventListener)   window.addEventListener("load", downloadJSAtOnload, false);else if (window.attachEvent)   window.attachEvent("onload", downloadJSAtOnload);else window.onload = downloadJSAtOnload;</script>