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<metahttp-equiv="Content-Type"content="text/html; charset=UTF-8"/>5<title>外部JS文件阻塞图片</title>6<scriptsrc="https://95598.gd.csg.cn/javascript/jquery-1.7.2.min.js"type="text/Javascript"></script>7</head>8<body>9<imgsrc="https://95598.gd.csg.cn/images/secImages/logo.gif"alt=""/>10<imgsrc="https://95598.gd.csg.cn/images/secImages/header_city_turn.png"width="50"height="21"alt="切换城市"/>11<imgsrc="https://95598.gd.csg.cn/images/secImages/customer_service.jpg"width="150"height="150"/>12<imgsrc="https://95598.gd.csg.cn/activity/images/wel_act.jpg"alt=""/>13<imgsrc="https://95598.gd.csg.cn/images/secImages/sidebar_2.png"width="48"height="48"/>14<imgsrc="https://95598.gd.csg.cn/images/secImages/sidebar_3.png"width="48"height="48"/>15<imgsrc="https://95598.gd.csg.cn/images/secImages/sidebar_top.png"width="48"height="48"/>16<imgsrc="https://95598.gd.csg.cn/images/secImages/electricity_t1.png"width="169"height="53"alt=""/>17<imgsrc="https://95598.gd.csg.cn/images/secImages/electricity_t2.png"width="169"height="53"alt=""/>18<imgsrc="https://95598.gd.csg.cn/images/secImages/electricity_t3.png"width="169"height="53"alt=""/>19<imgsrc="https://95598.gd.csg.cn/images/secImages/icon_big_a1.png"width="64"height="64"alt=""/>20<imgsrc="https://95598.gd.csg.cn/images/secImages/icon_big_a2.png"width="64"height="64"alt=""/>21<imgsrc="https://95598.gd.csg.cn/images/secImages/icon_big_a3.png"width="60"height="60"alt=""/>22<imgsrc="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<metahttp-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<imgsrc="https://95598.gd.csg.cn/images/secImages/logo.gif"alt=""/>9<imgsrc="https://95598.gd.csg.cn/images/secImages/header_city_turn.png"width="50"height="21"alt="切换城市"/>10<scriptsrc="https://95598.gd.csg.cn/javascript/jquery-1.7.2.min.js"type="text/javascript"></script>11<imgsrc="https://95598.gd.csg.cn/images/secImages/customer_service.jpg"width="150"height="150"/>12<imgsrc="https://95598.gd.csg.cn/activity/images/wel_act.jpg"alt=""/>13<imgsrc="https://95598.gd.csg.cn/images/secImages/sidebar_2.png"width="48"height="48"/>14<imgsrc="https://95598.gd.csg.cn/images/secImages/sidebar_3.png"width="48"height="48"/>15<imgsrc="https://95598.gd.csg.cn/images/secImages/sidebar_top.png"width="48"height="48"/>16<imgsrc="https://95598.gd.csg.cn/images/secImages/electricity_t1.png"width="169"height="53"alt=""/>17<imgsrc="https://95598.gd.csg.cn/images/secImages/electricity_t2.png"width="169"height="53"alt=""/>18<imgsrc="https://95598.gd.csg.cn/images/secImages/electricity_t3.png"width="169"height="53"alt=""/>19<imgsrc="https://95598.gd.csg.cn/images/secImages/icon_big_a1.png"width="64"height="64"alt=""/>20<imgsrc="https://95598.gd.csg.cn/images/secImages/icon_big_a2.png"width="64"height="64"alt=""/>21<imgsrc="https://95598.gd.csg.cn/images/secImages/icon_big_a3.png"width="60"height="60"alt=""/>22<imgsrc="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<metahttp-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<imgsrc="https://95598.gd.csg.cn/images/secImages/logo.gif"alt=""/>9<imgsrc="https://95598.gd.csg.cn/images/secImages/header_city_turn.png"width="50"height="21"alt="切换城市"/>10<imgsrc="https://95598.gd.csg.cn/images/secImages/customer_service.jpg"width="150"height="150"/>11<imgsrc="https://95598.gd.csg.cn/activity/images/wel_act.jpg"alt=""/>12<imgsrc="https://95598.gd.csg.cn/images/secImages/sidebar_2.png"width="48"height="48"/>13<imgsrc="https://95598.gd.csg.cn/images/secImages/sidebar_3.png"width="48"height="48"/>14<imgsrc="https://95598.gd.csg.cn/images/secImages/sidebar_top.png"width="48"height="48"/>15<imgsrc="https://95598.gd.csg.cn/images/secImages/electricity_t1.png"width="169"height="53"alt=""/>16<imgsrc="https://95598.gd.csg.cn/images/secImages/electricity_t2.png"width="169"height="53"alt=""/>17<imgsrc="https://95598.gd.csg.cn/images/secImages/electricity_t3.png"width="169"height="53"alt=""/>18<imgsrc="https://95598.gd.csg.cn/images/secImages/icon_big_a1.png"width="64"height="64"alt=""/>19<imgsrc="https://95598.gd.csg.cn/images/secImages/icon_big_a2.png"width="64"height="64"alt=""/>20<imgsrc="https://95598.gd.csg.cn/images/secImages/icon_big_a3.png"width="60"height="60"alt=""/>21<imgsrc="https://95598.gd.csg.cn/images/secImages/icon_big_a4.png"width="64"height="64"alt=""/>22<scriptsrc="https://95598.gd.csg.cn/javascript/jquery-1.7.2.min.js"type="text/javascript"></script>23</body>24</html>
调整script位置到末尾
图 1-3
对比图1-1、图1-2、图1-3，同样的文件加载，只是顺序上的不同，图1-3的加载时间比图1-1、图1-2节省了约0.1s(测试结果有一定随机性)