问题描述
我注意到 jQuery 和相关的主题插件，如 jQuery.UI 传递 undefined 作为模块定义中使用的匿名函数的参数，如下所示：
I have noticed jQuery and related keynote plugins like jQuery.UI pass undefined as a parameter into anonymous functions used in their module definitions, like so:
(function($, undefined) { ... })(jQuery);
或者，我注意到jQuery和/或其他人推荐的其他插件不会将undefined传递为参数。
Alternatively, I have noticed other plugins recommended by jQuery and/or others do NOT pass undefined in as a parameter.
这可能是一个愚蠢的问题，但是......
This is probably a silly question, but...
不应该总是有空无论如何？为什么传递它？这里有其他目的或技巧吗？
Shouldn't it always be available anyway? Why pass it in? Is there some sort of other purpose or trick going on here?
推荐答案
这有两个原因：
1）如果 undefined 是函数作用域中的变量而不是全局对象属性，则minifiers可以将其减少为单个字母实现更好的压缩率。
1) If undefined is a variable in the function scope rather than a global object property, minifiers can reduce it to a single letter thus achieving a better compression rate.
2）在ES5 *之前， undefined 是全局对象的属性，没有写入-保护。所以它可能被覆盖，导致奇怪和意外的行为。你可以这样做：
2) Before ES5*, undefined was a property of the global object without write-protection. So it could be overwritten, resulting in strange and unexpected behavior. You could do something like this:
var test = 123;
undefined = 123;
if (test === undefined){
// this will actually be true, since undefined now equals 123
}
通过函数参数 undefined （名称实际上无关紧要），你没有将参数传递给，你可以确保你有一个真正 undefined 的变量，这样你就可以对这个变量测试undefinedness。
By having an function argument undefined (the name actually does not matter) which you don't pass a parameter to, you could make sure you have a variable which really is undefined so you can test "undefinedness" against this variable.
顺便说一下。测试undefined的最安全的方法是： typeof（var）==='undefined'
Btw. the safest method to test for undefined is: typeof ( var ) === 'undefined'
（*）使用EcmaScript 5，全局属性未定义， NaN 和无穷大变成了只读。因此，它在所有现代浏览器中得到普遍采用 - 当然除了IE 9之外 - 不再可能覆盖这些值。
(*) With EcmaScript 5, the global properties undefined, NaN and Infinity became readonly. So with its general adoption in all modern browsers - of course with the exception of IE 9 - overwriting those values was not possible anymore.
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