您是否曾仔细查看过jQuery 1.4个源代码，并注意到它是如何以以下方式封装的:
(function( window, undefined ) {
//All the JQuery code here
...
})(window);
我读过一篇关于JavaScript Namespacing的文章和另一篇叫做"An Important Pair of Parens"的文章，所以我对这里的情况有所了解.
但我以前从未见过这种特殊的语法.那100人在那里干什么？为什么101需要通过，然后再次出现在末尾？
undefined是一个正态变量，只需使用undefined = "new value";即可更改.因此jQuery创建了一个真正未定义的局部"未定义"变量.
The window variable is made local for performance reasons. Because when JavaScript looks up a variable, it first goes through the local variables until it finds the variable name. When it's not found, JavaScript goes through the next scope etc. until it filters through the global variables. So if the window variable is made local, JavaScript can look it up quicker. Further information: Speed Up Your JavaScript - Nicholas C. Zakas