Yielding with Timers  用定时器让出时间片
    Despite your best efforts, there will be times when a JavaScript task cannot be completed in 100 milliseconds or less because of its complexity. In these cases, it's ideal to yield control of the UI thread so that UI updates may occur. Yielding control means stopping JavaScript execution and giving the UI a chance to update itself before continuing to execute the JavaScript. This is where JavaScript timers come into the picture.
    尽管你尽了最大努力，还是有一些JavaScript任务因为复杂性原因不能在100毫秒或更少时间内完成。这种情况下，理想方法是让出对UI线程的控制，使UI更新可以进行。让出控制意味着停止JavaScript运行，给UI线程机会进行更新，然后再继续运行JavaScript。于是JavaScript定时器进入了我们的视野。
Timer Basics  定时器基础
    Timers are created in JavaScript using either setTimeout() or setInterval(), and both accept the same arguments: a function to execute and the amount of time to ......

Splitting Up Tasks  分解任务
    What we typically think of as one task can often be broken down into a series of subtasks. If a single function is taking too long to execute, check to see whether it can be broken down into a series of smaller functions that complete in smaller amounts of time. This is often as simple as considering a single line of code as an atomic task, even though multiple lines of code typically can be grouped together into a single task. Some functions are already easily broken down based on the other functions they call. For example:
    我们通常将一个任务分解成一系列子任务。如果一个函数运行时间太长，那么查看它是否可以分解成一系列能够短时间完成的较小的函数。可将一行代码简单地看作一个原子任务，多行代码组合在一起构成一个独立任务。某些函数可基于函数调用进行拆分。例如：
function saveDocument(id){
  //save the document
  openDocument(id)
  writeText(id);
  closeDocument(id);
  //update the UI to indi ......

第七章  Ajax  异步JavaScript和XML
    Ajax is a cornerstone of high-performance JavaScript. It can be used to make a page load faster by delaying the download of large resources. It can prevent page loads altogether by allowing for data to be transferred between the client and the server asynchronously. It can even be used to fetch all of a page's resources in one HTTP request. By choosing the correct transmission technique and the most efficient data format, you can significantly improve how your users interact with your site.
    Ajax是高性能JavaScript的基石。它可以通过延迟下载大量资源使页面加载更快。它通过在客户端和服务器之间异步传送数据，避免页面集体加载。它还用于在一次HTTP请求中获取整个页面的资源。通过选择正确的传输技术和最有效的数据格式，你可以显著改善用户与网站之间的互动。
    This chapter examines the fastest techniques for sending data to and receiving it from the server, as well as the most efficient formats for encodi ......

Data Formats  数据格式
    When considering data transmission techniques, you must take into account several factors: feature set, compatibility, performance, and direction (to or from the server). When considering data formats, the only scale you need for comparison is speed.
    在考虑数据传输技术时，你必须考虑这些因素：功能集，兼容性，性能，和方向（发给服务器或者从服务器接收）。在考虑数据格式时，唯一需要比较的尺度的就是速度。
    There isn't one data format that will always be better than the others. Depending on what data is being transferred and its intended use on the page, one might be faster to download, while another might be faster to parse. In this section, we create a widget for searching among users and implement it using each of the four major categories of data formats. This will require us to format a list of users on the server, pass it back to the browser, parse that list into a native JavaScript data structure, and ......

Data Format Conclusions  数据格式总结
    Favor lightweight formats in general; the best are JSON and a character-delimited custom format. If the data set is large and parse time becomes an issue, use one of these two techniques:
    总的来说越轻量级的格式越好，最好是JSON和字符分隔的自定义格式。如果数据集很大或者解析时间成问题，那么就使用这两种格式之一：
• JSON-P data, fetched using dynamic script tag insertion. This treats the data as executable JavaScript, not a string, and allows for extremely fast parsing. This can be used across domains, but shouldn't be used with sensitive data.
  JSON-P数据，用动态脚本标签插入法获取。它将数据视为可运行的JavaScript而不是字符串，解析速度极快。它能够跨域使用，但不应涉及敏感数据。
• A character-delimited custom format, fetched using either XHR or dynamic script tag insertion and parsed using split(). This technique parses extremely large datasets slightly faster than the JSON-P technique, and ge ......

第八章  Programming Practices  编程实践
    Every programming language has pain points and inefficient patterns that develop over time. The appearance of these traits occurs as people migrate to the language and start pushing its boundaries. Since 2005, when the term "Ajax" emerged, web developers have pushed JavaScript and the browser further than it was ever pushed before. As a result, some very specific patterns emerged, both as best practices and as suboptimal ones. These patterns arise because of the very nature of JavaScript on the Web.
    每种编程语言都有痛点，而且低效模式随着时间的推移不断发展。其原因在于，越来越多的人们开始使用这种语言，不断扩种它的边界。自2005年以来，当术语“Ajax”出现时，网页开发者对JavaScript和浏览器的推动作用远超过以往。其结果是出现了一些非常具体的模式，即有优秀的做法也有糟糕的做法。这些模式的出现，是因为网络上JavaScript的性质决定的。
Avoid Double Evaluation  避免二次评估
    JavaSc ......