“把脚本放在底部”是正确的吗？-Is“PutScriptsattheBottom”Correct?

28  

When a user requests a page from your site, the page HTML starts streaming to the browser. As soon as a browser encounters a tag for an external image, script, CSS file, etc., it will start downloading that file simultaneously.

当用户从您的站点请求页面时，页面HTML将开始流式传输到浏览器。一旦浏览器遇到外部图像，脚本，CSS文件等的标记，它将立即开始同时下载该文件。

If you put your scripts at the bottom of a page, they'll be loaded last. In other words, the HTML content/structure/css/images of youe page/app will be loaded first, and something can show up in the browser faster; Your users don't have to wait for a script to finish downloading before they see something in your application.

如果您将脚本放在页面底部，它们将最后加载。换句话说，将首先加载您的页面/应用程序的HTML内容/结构/ css /图像，并且可以更快地在浏览器中显示某些内容;您的用户在看到应用程序中的某些内容之前不必等待脚本完成下载。

14  

The reason why pages that have scripts at the top of the page still score an 'A' is because this is not as important as other performance improvements that could be made.

页面顶部具有脚本的页面仍然得分为“A”的原因是因为这不像其他可以进行的性能改进那么重要。

Each rule is weighted, so some rules affect the ySlow grade more than others.

每个规则都是加权的，因此有些规则比其他规则更能影响ySlow等级。

I always put scripts at the bottom. There are very few reasons for needing scripts at the top of your page. The only reason I can think of is you need your Javascript to execute immediately before anything else in the page, which is quite rare.

我总是把脚本放在底部。在页面顶部需要脚本的原因很少。我能想到的唯一原因是你需要你的Javascript在页面中的任何其他地方之前立即执行，这是非常罕见的。

8  

Sometimes, you have no choice but to put scripts at a specific location in page. However, if you put scripts at the bottom of the page, the browser won't interrupt its rendering to load JS engine to process your script (which might be costly, if you have loops running for large number of times) and will probably display an early view of the page sooner.

有时，您别无选择，只能将脚本放在页面中的特定位置。但是，如果您将脚本放在页面底部，浏览器将不会中断其渲染以加载JS引擎来处理您的脚本（如果您有多次循环运行，这可能会很昂贵）并且可能会显示早一点查看页面。

6  

I'm not sure it's a performance issue (could be), but I was at a presentation given by Microsoft, and the presenter said that web spiders sometimes only read the first x amount of characters on a web page to index it. So, if you have a large amount of scripts, your content may not be reached and indexed properly (and your site's page rank won't be as high).

我不确定这是一个性能问题（可能），但是我在微软的演示文稿中，并且主持人说网络蜘蛛有时只读取网页上的第一个x字符数量来索引它。因此，如果您有大量脚本，则可能无法正确访问和索引您的内容（并且您网站的网页排名不会那么高）。

6  

The code may reference DOM objects that haven't been instantiated yet is the most obvious reason I can think of.

代码可能引用尚未实例化的DOM对象，但这是我能想到的最明显的原因。