【原】浅谈Firefox下的js、css、图片阻塞现象（一）

2019-11-14 16:37:25

字体：大中小

来源：转载

供稿：网友

外部js文件阻塞body中的图片

以如下html为例：

 1 <!DOCTYPE HTML> 2 <html> 3 <head> 4 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> 5 <title> 外部JS文件阻塞图片 </title> 6 <script src="https://95598.gd.csg.cn/javascript/jquery-1.7.2.min.js"  type="text/Javascript"></script> 7 </head> 8 <body> 9 <img src="https://95598.gd.csg.cn/images/secImages/logo.gif" alt="" />10 <img src="https://95598.gd.csg.cn/images/secImages/header_city_turn.png" width="50" height="21" alt="切换城市" />11 <img src="https://95598.gd.csg.cn/images/secImages/customer_service.jpg" width="150" height="150" />12 <img src="https://95598.gd.csg.cn/activity/images/wel_act.jpg" alt="" />13 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_2.png" width="48" height="48"/> 14 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_3.png" width="48" height="48"/>15 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_top.png" width="48" height="48"/>16 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t1.png" width="169" height="53" alt="" />17 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t2.png" width="169" height="53" alt="" />18 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t3.png" width="169" height="53" alt="" />19 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a1.png" width="64" height="64" alt="" />20 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a2.png" width="64" height="64" alt="" />21 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a3.png" width="60" height="60" alt=""/>22 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a4.png" width="64" height="64" alt="" />23 </body>24 </html>

script 在<head>中

经过firebug网络分析：

图 1-1

观察图1-1可以得出如下两个结论：

1.浏览器针对请求（如图片）是并发处理的，在IP、端口号相同时，Firefox的默认并发请求数是6（HTTP1.1和HTTP1.0的并发请求数因浏览器版本的不同而数量不同），可以看到6张图片是同时从服务器下载

2.js文件加载完后，中间约有0.1s的空闲时间，这段时间浏览器没有进行任何操作，这一现象可以称作阻塞，外部js文件阻塞了body中的img请求（在IE下不存在阻塞）

如果将<script>块置于img中间，也会产生阻塞（在IE下不存在阻塞）

 1 <!DOCTYPE HTML> 2 <html> 3 <head> 4 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> 5 <title> 外部JS文件阻塞图片 </title> 6 </head> 7 <body> 8 <img src="https://95598.gd.csg.cn/images/secImages/logo.gif" alt="" /> 9 <img src="https://95598.gd.csg.cn/images/secImages/header_city_turn.png" width="50" height="21" alt="切换城市" />10 <script src="https://95598.gd.csg.cn/javascript/jquery-1.7.2.min.js" type="text/javascript"></script>11 <img src="https://95598.gd.csg.cn/images/secImages/customer_service.jpg" width="150" height="150" />12 <img src="https://95598.gd.csg.cn/activity/images/wel_act.jpg" alt="" />13 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_2.png" width="48" height="48"/> 14 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_3.png" width="48" height="48"/>15 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_top.png" width="48" height="48"/>16 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t1.png" width="169" height="53" alt="" />17 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t2.png" width="169" height="53" alt="" />18 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t3.png" width="169" height="53" alt="" />19 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a1.png" width="64" height="64" alt="" />20 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a2.png" width="64" height="64" alt="" />21 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a3.png" width="60" height="60" alt=""/>22 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a4.png" width="64" height="64" alt="" />23 </body>24 </html>

scipt置于中间

图 1-2

如果将外部js文件置于</body>前，则可以消除阻塞现象

 1 <!DOCTYPE HTML> 2 <html> 3 <head> 4 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> 5 <title> 外部JS文件阻塞图片 </title> 6 </head> 7 <body> 8 <img src="https://95598.gd.csg.cn/images/secImages/logo.gif" alt="" /> 9 <img src="https://95598.gd.csg.cn/images/secImages/header_city_turn.png" width="50" height="21" alt="切换城市" />10 <img src="https://95598.gd.csg.cn/images/secImages/customer_service.jpg" width="150" height="150" />11 <img src="https://95598.gd.csg.cn/activity/images/wel_act.jpg" alt="" />12 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_2.png" width="48" height="48"/> 13 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_3.png" width="48" height="48"/>14 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_top.png" width="48" height="48"/>15 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t1.png" width="169" height="53" alt="" />16 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t2.png" width="169" height="53" alt="" />17 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t3.png" width="169" height="53" alt="" />18 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a1.png" width="64" height="64" alt="" />19 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a2.png" width="64" height="64" alt="" />20 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a3.png" width="60" height="60" alt=""/>21 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a4.png" width="64" height="64" alt="" />22 <script src="https://95598.gd.csg.cn/javascript/jquery-1.7.2.min.js"  type="text/javascript"></script>23 </body>24 </html>