[翻译]High Performance JavaScript(025)

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
    JavaScript, like many scripting languages, allows you to take a string containing code and execute it from within running code. There are four standard ways to accomplish this: eval(), the Function() constructor, setTimeout(), and setInterval(). Each of these functions allows you to pass in a string of JavaScript code and have it executed. Some examples:
    JavaScript与许多脚本语言一样，允许你在程序中获取一个包含代码的字符串然后运行它。有四种标准方法可以实现：eval()，Function()构造器，setTimeout()和setInterval()。每个函数允许你传入一串JavaScript代码，然后运行它。例如：
var num1 = 5,
num2 = 6,
//eval() evaluating a string of code
result = eval("num1 + num2"),
//Function() evaluating strings of code
sum = new Function("arg1", "arg2", "return arg1 + arg2");
//setTimeout() evaluating a string of code
setTimeout("sum = num1 + num2", 100);
//setInterval() evaluating a string of code
setInterval("sum = num1 + num2", 100);
    Whenever you're evaluating Jav