js性能---为什么将你的script标签置于</body>标签之前

最新推荐文章于 2024-04-14 21:21:06 发布

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本文详细解析了网页加载过程中浏览器如何从服务器获取文档，并解释了JavaScript脚本如何阻塞其他资源（如CSS、图片）的并行下载。重点强调了脚本在头部的位置对其下载顺序的影响。

The reason is because of how web pages load. When a browser hits a web page, it first sends a request to the server for the document at the URL. Then once that document arrives at the browser, the browser parses it and starts making additional requests to the server for external components like style sheets, images, and scripts. The browser downloads these components in parallel—up to two components at one time (according to the HTTP 1.1 specification).
But scripts block parallel downloads. This means that once a browser starts downloading a script, it won’t download anything else until the script has finished downloading. (js)资源下载会阻塞其他资源（如css，img）的并行下载
Since browsers read HTML documents from the top to the bottom, starting with the doctype and <html> opening tag, any scripts that are in the HEAD are going to be some of the first components downloaded. And until they are finished downloading the scripts, nothing else on the page will be downloaded.