最佳实践：非阻塞脚本载入_html/css_WEB-ITnose
当今Web页面和脚本已经越来越复杂，页面性能有时会变得很差。 这常常被误解为网络或浏览器的问题，其实前端技术对页面性能有着非常显著的影响。 前端对页面性能的影响也是多方面的：
- 脚本/样式文件的划分会影响浏览器缓存效率。
- 频繁的DOM操作会占用大量CPU资源。
- 资源链接的先后顺序也会影响页面渲染速度。
- 同步的网络请求会使页面停止响应。
本文将探讨不同的脚本载入方式对页面性能的影响， 包括『浏览器忙提示』、『页面停止渲染』、『光标停止响应』等。 最终给出无阻塞脚本载入的最佳实践。
什么是脚本阻塞
本文的主题是无阻塞脚本载入技术，那么什么是脚本阻塞呢？
在 Web性能优化：prefetch, prerender 一文中提到， 浏览器在获取资源时会经过DNS解析、建立连接、下载文件、渲染页面等过程。 浏览器只会同时下载3-5个脚本，然后顺序执行它们。 并且在下载和执行过程中，页面会停止渲染和响应（也就是我们说的页面卡死）。
性能最差的情形是某个脚本下载超时，后面的脚本和样式都会被阻塞很长时间， 浏览器的图标会一直处于忙的状态。 如果这个脚本出现在HTML HEAD标签中，阻塞过程中整个页面都将是空白，用户体验极差。
为了防止脚本阻塞，我们来探讨非阻塞的脚本载入方式。 采用这种脚本在如方式时，浏览器（以Chrome为例）不会显示忙指示器图标， 页面也不会停止响应。
XHR
缺点：不能跨域，容易被劫持
我们知道 XHR 可以用来执行异步的网络请求，XHR Eval方法的原理便是通过XHR下载整个脚本，通过 eval() 函数来执行这个脚本。
$.get('/path/to/sth.js') .done(function(src){ eval(src); });因为 XHR 的下载过程是异步的，所以这个过程中浏览器图标不会显示『忙提示』。 JS的执行时间很短暂，可以认为页面始终不会停止响应。 XHR 有跨域问题，因此该方法只适用于资源位于同一域名的情况（或者开启CORS响应头字段）。
我们知道 eval() 方法是不安全的，除了使用 eval() 方法，我们还可以创建一个
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