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JavaScript 的性能优化：加载和执行

概览无论当前JavaScript代码是内嵌还是在外链文件中，页面的下载和渲染都必须停下来等待脚本执行完成。JavaScript执行过程耗时越久，浏览器等待响应用户输入的时间就越长。浏览器在下载和执行脚本时出现阻塞的原因在于，脚本可能会改变页面或JavaScript的命名空间，它们对后面页面内容造成影响。一个典型的例子就是在页面中使用document.write()。例如清单1清单1JavaScript代码内嵌示例Sou</span> <p class="auth-span"> <span class="muted"><i class="fa fa-user"></i><span class="zj-icon-text">系统</span></span> <span class="muted"><i class="fa fa-clock-o"></i><span class="zj-icon-text">2019-08-29 22:50:45</span></span> <span class="muted"><i class="fa fa-eye"></i><span class="zj-icon-text">3596</span></span>  </p> </article><article class="excerpt"> <header> <a title="PHP" class="label label-important zj-del-target" href="/category/64">PHP<i class="label-arrow"></i></a> <h2> <a class="zj-titles zj-del-target" href="/article/16939" title="用php读取和编写xml dom(使用 DOM 库、SAX 解析" target="_blank" onclick="return recordId('',)">用php读取和编写xml dom(使用 DOM 库、SAX 解析 </a> </h2> </header> <div class="focus"> <a class="zj-del-target" href="/article/16939" target="_blank"><img style="width: 200px; height: 123px;" class="thumb" src="/Public/static/images/article/13.jpg" alt="" title=""></a> </div> <span class="note" onclick="if (jq.browser.versions.mobile) {window.location.href='/article/16939';}else{window.open('/article/16939');}" style="cursor: pointer;">有许多技术可用于用PHP读取和编写XML。本文提供了三种方法读取XML：使用DOM库、使用SAX解析器和使用正则表达式。还介绍了使用DOM和PHP文本模板编写XML。 </p> </article><article class="excerpt"> <header> <a title="C#" class="label label-important zj-del-target" href="/category/43">C#<i class="label-arrow"></i></a> <h2> <a class="zj-titles zj-del-target" href="/article/13005" title="C#里巧用DateTime预设一些可选的日期范围(如本" target="_blank" onclick="return recordId('',)">C#里巧用DateTime预设一些可选的日期范围(如本 </a> </h2> </header> <div class="focus"> <a class="zj-del-target" href="/article/13005" target="_blank"><img style="width: 200px; height: 123px;" class="thumb" src="/Public/static/images/article/20.jpg" alt="" title=""></a> </div> <span class="note" onclick="if (jq.browser.versions.mobile) {window.location.href='/article/13005';}else{window.open('/article/13005');}" style="cursor: pointer;">C#里巧用DateTime预设一些可选的日期范围(如本年度、本季度、本月等)//大家在做报表或查询的时候都会有给用户预设一些可选的日期范围(如上图)//如本年度销售额、本季度利润、本月新增客户//C#里内置的DateTime基本上都可以实现这些功能，巧用DateTime会使你处理这些事来变轻松多了//今天DateTime.Now.Date.ToShortDateString();//昨天，就是今天的日期减一DateTime.Now.AddDays(-1).</span> <p class="auth-span"> <span class="muted"><i class="fa fa-user"></i><span class="zj-icon-text">系统</span></span> <span class="muted"><i class="fa fa-clock-o"></i><span class="zj-icon-text">2019-08-29 21:57:54</span></span> <span class="muted"><i class="fa fa-eye"></i><span class="zj-icon-text">3596</span></span>  </p> </article><article class="excerpt"> <header> <a title="SqlServer" class="label label-important zj-del-target" href="/category/46">SqlServer<i class="label-arrow"></i></a> <h2> <a class="zj-titles zj-del-target" href="/article/7521" title="SQL SERVER 2012 第三章 T-SQL 基本语句 having" target="_blank" onclick="return recordId('',)">SQL SERVER 2012 第三章 T-SQL 基本语句 having </a> </h2> </header> <div class="focus"> <a class="zj-del-target" href="/article/7521" target="_blank"><img style="width: 200px; height: 123px;" class="thumb" src="/Public/static/images/article/4.jpg" alt="" title=""></a> </div> <span class="note" onclick="if (jq.browser.versions.mobile) {window.location.href='/article/7521';}else{window.open('/article/7521');}" style="cursor: pointer;">SELECTManagerIDASManager,COUNT(*)ASReportsFROMHuman.Resources.Employee2WHEREEmployeeID!=5GROUPBYManagerIDHAVINGCOUNT(*)>3;having是聚合以后算的，很棒。where是聚合以前算的。用FORXML子句可以输出XML格式。ALLDISTINCT的用法。SQLSERVER2012第三章T-SQL基本语句having子句</span> <p class="auth-span"> <span class="muted"><i class="fa fa-user"></i><span class="zj-icon-text">系统</span></span> <span class="muted"><i class="fa fa-clock-o"></i><span class="zj-icon-text">2019-08-12 01:54:14</span></span> <span class="muted"><i class="fa fa-eye"></i><span class="zj-icon-text">3596</span></span>  </p> </article><article class="excerpt"> <header> <a title="编程技术" class="label label-important zj-del-target" href="/category/30">编程技术<i class="label-arrow"></i></a> <h2> <a class="zj-titles zj-del-target" href="/article/22160" title="轻量级B/S应用开发引擎：NoahWeb" target="_blank" onclick="return recordId('',)">轻量级B/S应用开发引擎：NoahWeb </a> </h2> </header> <div class="focus"> <a class="zj-del-target" href="/article/22160" target="_blank"><img style="width: 200px; height: 123px;" class="thumb" src="/Public/static/images/article/12.jpg" alt="" title=""></a> </div> <span class="note" onclick="if (jq.browser.versions.mobile) {window.location.href='/article/22160';}else{window.open('/article/22160');}" style="cursor: pointer;">同事介绍，说一款BS开发引擎比较强大，处理表单之类的比较不错，今天找来看看。网址：http://www.noahweb.net/帮助：http://docs.noahweb.net/看看他们网站上怎么介绍的：//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////</span> <p class="auth-span"> <span class="muted"><i class="fa fa-user"></i><span 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