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《高性能 JavaScript 编程》

原文 2010 年 3 月 23 日首发，刘新翻译，欢迎指正！

http://blog.sina.com.cn/situdesign

第一章 Loading and Execution 加载和运行 ........................................................................................................ 2

第二章 Data Access 数据访问 ............................................................................................................................ 26

第三章 DOM Scripting DOM 编程 .................................................................................................................... 56

第四章 Algorithms and Flow Control 算法和流程控制 .................................................................................. 104

第五章 Strings and Regular Expressions 字符串和正则表达式...................................................................... 139

第六章 Responsive Interfaces 响应接口 .......................................................................................................... 184

第七章 Ajax 异步 JavaScript 和 XML ............................................................................................................. 216

第八章 Programming Practices 编程实践........................................................................................................ 261

第九章 Building and Deploying High-Performance JavaScript Applications ..................................................... 279

第十章 Tools 工具 ............................................................................................................................................ 306

[原版] [译文]

第一章 Loading and Execution 加载和运行

JavaScript performance in the browser is arguably the most important usability issue facing developers. The

problem is complex because of the blocking nature of JavaScript, which is to say that nothing else can happen

while JavaScript code is being executed. In fact, most browsers use a single process for both user interface (UI)

updates and JavaScript execution, so only one can happen at any given moment in time. The longer JavaScript

takes to execute, the longer it takes before the browser is free to respond to user input.

JavaScript 在浏览器中的性能，可认为是开发者所要面对的最重要的可用性问题。此问题因 JavaScript

的阻塞特征而复杂，也就是说，当 JavaScript 运行时其他的事情不能被浏览器处理。事实上，大多数浏览

器使用单进程处理 UI 更新和 JavaScript 运行等多个任务，而同一时间只能有一个任务被执行。JavaScript

运行了多长时间，那么在浏览器空闲下来响应用户输入之前的等待时间就有多长。

On a basic level, this means that the very presence of a <script> tag is enough to make the page wait for the

script to be parsed and executed. Whether the actual JavaScript code is inline with the tag or included in an

external file is irrelevant; the page download and rendering must stop and wait for the script to complete before

proceeding. This is a necessary part of the page’s life cycle because the script may cause changes to the page

while executing. The typical example is using document.write() in the middle of a page (as often used by

advertisements). For example:

从基本层面说，这意味着<script>标签的出现使整个页面因脚本解析、运行而出现等待。不论实际的

JavaScript 代码是内联的还是包含在一个不相干的外部文件中，页面下载和解析过程必须停下，等待脚本

完成这些处理，然后才能继续。这是页面生命周期必不可少的部分，因为脚本可能在运行过程中修改页面

内容。典型的例子是 document.write()函数，例如：

<html>

<head>

<title>Script Example</title>

</head>

<body>

<p>

document.write("The date is " + (new Date()).toDateString());

</script>

</p>

</body>

</html>

When the browser encounters a <script> tag, as in this HTML page, there is no way of knowing whether the

JavaScript will insert content into the <p>, introduce additional elements, or perhaps even close the tag. Therefore,

the browser stops processing the page as it comes in, executes the JavaScript code, then continues parsing and

rendering the page. The same takes place for JavaScript loaded using the src attribute; the browser must first

download the code from the external file, which takes time, and then parse and execute the code. Page rendering

and user interaction are completely blocked during this time.

当浏览器遇到一个<script>标签时，正如上面 HTML 页面中那样，无法预知 JavaScript 是否在<p>标签中

添加内容。因此，浏览器停下来，运行此 JavaScript 代码，然后再继续解析、翻译页面。同样的事情发生

在使用 src 属性加载 JavaScript 的过程中。浏览器必须首先下载外部文件的代码，这要占用一些时间，然后

解析并运行此代码。此过程中，页面解析和用户交互是被完全阻塞的。

Script Positioning 脚本位置

The HTML 4 specification indicates that a <script> tag may be placed inside of a <head> or <body> tag in an

HTML document and may appear any number of times within each. Traditionally, script> tags that are used to

load external JavaScript files have appeared in the <head>, along with <link> tags to load external CSS files and

other metainformation about the page. The theory was that it's best to keep as many style and behavior

dependencies together, loading them first so that the page will come in looking and behaving correctly. For

example:

HTML 4 文档指出，一个<script>标签可以放在 HTML 文档的<head>或<body>标签中，可以在其中多次

出现。传统上，<script>标签用于加载外部 JavaScript 文件。<head>部分除此类代码外，还包含<link>标签

用于加载外部 CSS 文件和其他页面中间件。也就是说，最好把风格和行为所依赖的部分放在一起，首先加

载他们，使得页面可以得到正确的外观和行为。例如：

<html>

<head>

<title>Script Example</title>

<-- Example of inefficient script positioning -->

</head>

<body>

<p>Hello world!</p>

</body>

</html>

Though this code seems innocuous, it actually has a severe performance issue: there are three JavaScript files

being loaded in the <head>. Since each <script> tag blocks the page from continuing to render until it has fully

downloaded and executed the JavaScript code, the perceived performance of this page will suffer. Keep in mind

that browsers don't start rendering anything on the page until the opening <body> tag is encountered. Putting

scripts at the top of the page in this way typically leads to a noticeable delay, often in the form of a blank white

page, before the user can even begin reading or otherwise interacting with the page. To get a good understanding

of how this occurs, it's useful to look at a waterfall diagram showing when each resource is downloaded. Figure

1-1 shows when each script and the stylesheet file get downloaded as the page is loading.

虽然这些代码看起来是无害的，但它们确实存在性能问题：在<head>部分加载了三个 JavaScript 文件。

因为每个<script>标签阻塞了页面的解析过程，直到它完整地下载并运行了外部 JavaScript 代码之后，页面

处理才能继续进行。用户必须忍受这种可以察觉的延迟。请记住，浏览器在遇到<body>标签之前，不会渲

染页面的任何部分。用这种方法把脚本放在页面的顶端，将导致一个可以察觉的延迟，通常表现为：页面

打开时，首先显示为一幅空白的页面，而此时用户即不能阅读，也不能与页面进行交互操作。为了更好地

理解此过程，我们使用瀑布图来描绘每个资源的下载过程。图 1-1 显示出页面加载过程中，每个脚本文件

和样式表文件下载的过程。

Figure 1-1. JavaScript code execution blocks other file downloads

图 1-1，JavaScript 代码运行过程阻塞其他文件下载

Figure 1-1 shows an interesting pattern. The first JavaScript file begins to download and blocks any of the

other files from downloading in the meantime. Further, there is a delay between the time at which file1.js is

completely downloaded and the time at which file2.js begins to download. That space is the time it takes for the

code contained in file1.js to fully execute. Each file must wait until the previous one has been downloaded and

executed before the next download can begin. In the meantime, the user is met with a blank screen as the files are

being downloaded one at a time. This is the behavior of most major browsers today.

图 1-1 是一个令人感兴趣的模板。第一个 JavaScript 文件开始下载，并阻塞了其他文件的下载过程。进

一步，在 file1.js 下载完之后和 file2.js 开始下载之前有一个延时，这是 file1.js 完全运行所需的时间。每个

文件必须等待前一个文件下载完成并运行完之后，才能开始自己的下载过程。当这些文件下载时，用户面

对一个空白的屏幕。这就是今天大多数浏览器的行为模式。

Internet Explorer 8, Firefox 3.5, Safari 4, and Chrome 2 all allow parallel downloads of JavaScript files. This is

good news because the <script> tags don’t necessarily block other <script> tags from downloading external

resources. Unfortunately, JavaScript downloads still block downloading of other resources, such as images. And

even though downloading a script doesn’t block other scripts from downloading, the page must still wait for the

JavaScript code to be downloaded and executed before continuing. So while the latest browsers have improved

performance by allowing parallel downloads, the problem hasn’t been completely solved. Script blocking still

remains a problem.

Internet Explorer 8, Firefox 3.5, Safari 4, 和 Chrome 2 允许并行下载 JavaScript 文件。这个好消息表明，当

一个<script>标签正在下载外部资源时，不必阻塞其他<script>标签。不幸的是，JavaScript 的下载仍然要阻

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