[翻译]High Performance JavaScript(008)

Nested Members  嵌套成员
    Since object members may contain other members, it's not uncommon to see patterns such as window.location.href in JavaScript code. These nested members cause the JavaScript engine to go through the object member resolution process each time a dot is encountered. Figure 2-12 shows the relationship between object member depth and time to access.
    由于对象成员可能包含其它成员，例如不太常见的写法window.location.href这种模式。每遇到一个点号，JavaScript引擎就要在对象成员上执行一次解析过程。图2-12显示出对象成员深度与访问时间的关系。
Figure 2-12. Access time related to property depth
图2-12  访问时间与属性深度的关系
    It should come as no surprise, then, that the deeper the nested member, the slower the data is accessed. Evaluating location.href is always faster than window.location.href, which is faster than window.location.href.toString(). If these properties aren't on the object instances, then member resolution will take longer as the prototype chain is searched at each point.
    结果并不奇怪，成员嵌套越深，访问速度越慢。location.href总是快于window.location.href，而后者也要比window.location.href.toString()更快。如果这些属性不是对象的实例属性，那么成员解析还要在每个点上搜索原形链，这将需要更长时间。
Caching Object Member Values  缓存对象成员的值
    With all of the performance issues related to object members, it's easy to believe that they should be avoided whenever possible. To be more accurate, you should be careful to use object member only when necessary. For instance, there's no reason to read the value of an object member more than once in a single function:
    由于所有这些性能问题与对象成员有关，所以如果可能的话请避免使用它们。更确切地说，你应当小心地，只在必要情况下使用对象成员。例如，没有理由在一个函数中多次读取同一个对象成员的值：
function hasEitherClass(element, className1, className2){
  return element.c