最近因工作的需要主要做web前段的优化，其中之一就是js的优化，在项目中的js代码相对比较规范的，都是$(function(){})这种格 式。但是大家都知道js的下载比较耗时的，它的解析和执行是阻塞式的。我们是否可以先让js下载下来，在windows的load事件中在去解析和执行 了。答案是肯定的，其中ControlJS .js就实现了这种方式。个人觉得它比LAB的延迟加载要优秀的一点，减少了js的加载时间。但是该js功能相对比较强大，同时也没有解决js依赖关系的加载。个人自己写了一个小插件 来实现js异步加载延迟执行。
代码如下：
(function () { var document = window.document; var getAttribute = function (elem, name) { var attrs = elem.attributes; var len = attrs.length; for (var i = 0; i < len; i++) { var attr = attrs[i]; if (name === attr.nodeName) { return attr.nodeValue; } } return undefined; }; var scriptsData; var orders = []; var findScripts = function () { var aScripts = document.getElementsByTagName('script'); var len = aScripts.length; var scriptsArr = []; var scriptsSrc = []; for (var i = 0; i < len; i++) { var script = aScripts[i]; if ("text/asynjs" === getAttribute(script, "type") && "undefined" === typeof (script.founded)) { script.founded = true; var order = getAttribute(script, "order") || getAttribute(script, "data-order") || 0; script.order = order; if (orders.indexOf(order) < 0) { orders.push(order); } var src = getAttribute(script, "asynsrc") || getAttribute(script, "data-asynsrc"); script.src = src; if (src && scriptsSrc.indexOf(src) < 0) { scriptsArr.push(script); } } } orders.sort(function compare(a, b) { return a - b; }); return scriptsData = scriptsArr; }; var downloadScripts = function (scriptArr) { var scripts = scriptArr || findScripts(); var len = scripts.length; for (var i = 0; i < len; i++) { var img = new Image(); img.src = scripts[i].src; img.style.display = "none"; } }; var processScripts = function (scriptArr) { var scripts = (scriptArr && scriptArr.constructor === Array) ? scriptArr : scriptsData; var len = scripts.length; var failscripts = []; var wdata = []; for (var i = orders.length - 1; i >= 0; i--) { var order = orders[i]; var urls = []; var fns = []; for (var j = 0, len = scripts.length; j < len; j++) { var script = scripts[j]; if (script.order == order) { urls.push(script.src); } } if (i == orders.length - 1) { wdata[order] = processScript(urls) } else { var nextorder = orders[i + 1]; fns.push(wdata[nextorder]); wdata[order] = processScript(urls, fns); } } var exorder = orders[0] wdata[orders[0]](); }; var processScript = function (urls, fn) { return function () { for (var i = 0; i < urls.length; i++) { var se = document.createElement('script'); if (fn && fn[i]) { se.onload = se.onreadystatechange = fn[i]; } se.src = urls[i]; var s1 = document.getElementsByTagName('script')[0]; s1.parentNode.insertBefore(se, s1); } } } if (document.addEventListener) { document.addEventListener("DOMContentLoaded", function () { downloadScripts(); }, false); window.addEventListener("load", function () { setTimeout(processScripts, 0); }, false); } else if (document.attachEvent) { document.attachEvent("onreadystatechange", function () { downloadScripts(); }); window.attachEvent("onload", function () { setTimeout(processScripts, 0); }); } })();
调用代码：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>test</title> <script type="text/javascript" src="Scripts/asynload.js"></script> </head> <body> <div id="container"> <img src="images/bmw_m1_hood.jpg" width="765" height="574" alt="BMW M1 Hood"> <script type="text/asynjs" order="3" asynsrc="test.js"></script> <script type="text/asynjs" order="1" asynsrc="Scripts/jquery-1.7.2.js"></script> <script type="text/asynjs" order="2" asynsrc="Scripts/jquery.lazyload.js"></script> <img src="images/bmw_m1_side.jpg" width="765" height="574" alt="BMW M1 Side" /> <img data-original="images/viper_1.jpg" width="765" height="574" alt="Viper 1" /> <img data-original="images/viper_corner.jpg" width="765" height="574" alt="Viper Corner" /> <img data-original="images/bmw_m3_gt.jpg" width="765" height="574" alt="BMW M3 GT" /> <img data-original="images/corvette_pitstop.jpg" width="765" height="574" alt="Corvette Pitstop" /> </div> </body> </html>
IE9下执行结果：
firefox的执行结果：
其中jquery.lazyload.js是图片延迟加载的jquery插件。test.js的代码$(function () {$("img[data-original]").lazyload();});。