js延遲加載

爲了提高頁面響應速度，我們需要讓一些不必一開始就加載的頁面延遲加載，把下面代碼放到body的最後一行：

 <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>