[翻译]High Performance JavaScript(013)
Conditionals 条件表达式
Similar in nature to loops, conditionals determine how execution flows through JavaScript. The traditional argument of whether to use if-else statements or a switch statement applies to JavaScript just as it does to other languages. Since different browsers have implemented different flow control optimizations, it is not always clear which technique to use.
与循环相似，条件表达式决定JavaScript运行流的走向。其他语言使用if-else或者switch表达式的传统观点也适用于JavaScript。由于不同的浏览器针对流程控制进行了不同的优化，使用哪种技术并不总是很清楚。
if-else Versus switch if-else与switch比较
The prevailing theory on using if-else versus switch is based on the number of conditions being tested: the larger the number of conditions, the more inclined you are to use a switch instead of if-else. This typically comes down to which code is easier to read. The argument is that if-else is easier to read when there are fewer conditions and switch is easier to read when the number of conditions is large. Consider the following:
使用if-else或者switch的流行理论是基于测试条件的数量：条件数量较大，倾向于使用switch而不是if-else。这通常归结到代码的易读性。这种观点认为，如果条件较少时，if-else容易阅读，而条件较多时switch更容易阅读。考虑下面几点：
if (found){
//do something
} else {
//do something else
}
switch(found){
case true:
//do something
break;
default:
//do something else
}
Though both pieces of code perform the same task, many would argue that the if-else statement is much easier to read than the switch. Increasing the number of conditions, however, usually reverses that opinion:
虽然两个代码块实现同样任务，很多人会认为if-else表达式比witch表达式更容易阅读。如果增加条件体的数量，通常会扭转这种观点：
if (color == "red"){
//do
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2、原型方式写类，原型方式继承
core js自身的对象系统就是采用原型方式(prototype based)继承的。或者说core
js没有采用常见的类继承(class
based)系统，而是使用原型继承来实现自己的对象系统。工作中我们也可以用原型方式来实现继承，代码复用以构建自己的功能模块。
/**
* 父类Polygon:多边形
*
*/
functio ......
typeof 是JavaScript的运算符
——返回一个用来表示表达式的数据类型的字符串
typeof[(] expression [)] //typeof语法中的圆括号是可选项
typeof 运算符@import url(../html-vss/msdnie4a.css);
typeof
运算符把类型信息当作字符串返回。typeof
返回值有� ......
使用javascript创建Microsoft XML DOM,就可以完成这一工作.
// 装入数据.
var source = new ActiveXObject("Microsoft.XMLDOM");
source.async = false
source.load("history.xml");
// 装入样式表.
var stylesheet = new ActiveXObject("Microsoft.XMLDOM");
stylesheet.async = false
stylesheet.load( ......
Repaints and Reflows 重绘和重排版
Once the browser has downloaded all the components of a page—HTML markup, JavaScript, CSS, images—it parses through the files and creates two internal data structures:
当浏览器下载完所有页面HTML标记，JavaScri ......
