JavaScript：undefined！== undefined？ [英] JavaScript: undefined !== undefined?

问题描述

注意：根据 ECMAScript5.1，第15.1.1.3节，window.undefined是只读的。

NOTE: As per ECMAScript5.1, section 15.1.1.3, window.undefined is read-only.

• 现代浏览器实现这一点正确。例如：Safari 5.1，Firefox 7，Chrome 20等


• Undefined仍然可以改变：Chrome 14，...

• Modern browsers implement this correctly. for example: Safari 5.1, Firefox 7, Chrome 20, etc.
• Undefined is still changeable in: Chrome 14, ...

当我最近整合 Facebook Connect 与 Tersus ，我最初收到错误消息无效枚举尝试调用Facebook API函数时，值和处理程序已存在。

When I recently integrated Facebook Connect with Tersus, I initially received the error messages Invalid Enumeration Value and Handler already exists when trying to call Facebook API functions.

它已转问题的原因是

object.x === undefined

当'object'中没有属性'x'时返回false。

returning false when there is no property 'x' in 'object'.

我工作了通过在两个Facebook函数中用常规相等替换严格相等来解决问题：

I worked around the problem by replacing strict equality with regular equality in two Facebook functions:

FB.Sys.isUndefined = function(o) { return o == undefined;};
FB.Sys.containsKey = function(d, key) { return d[key] != undefined;};

这使得一切都适合我，但似乎暗示Facebook的JavaScript代码和我自己的。

This made things work for me, but seems to hint at some sort of collision between Facebook's JavaScript code and my own.

可能导致这种情况的原因是什么？

What could cause this?

提示：有充分证明 undefined == null while undefined！== null 。这不是问题。问题是我们怎么得到 undefined！== undefined 。

Hint: It is well documented that undefined == null while undefined !== null. This is not the issue here. The question is how comes we get undefined !== undefined.

推荐答案

事实证明你可以将window.undefined设置为你想要的任何东西，所以当object.x是 real <时，得到 object.x！== undefined / em>未定义。在我的情况下，我无意中将undefined设置为null。

It turns out that you can set window.undefined to whatever you want, and so get object.x !== undefined when object.x is the real undefined. In my case I inadvertently set undefined to null.

最简单的方法是：

window.undefined = null;
alert(window.xyzw === undefined); // shows false

当然，这不太可能发生。在我的情况下，错误更微妙，相当于以下情况。

Of course, this is not likely to happen. In my case the bug was a little more subtle, and was equivalent to the following scenario.

var n = window.someName; // someName expected to be set but is actually undefined
window[n]=null; // I thought I was clearing the old value but was actually changing window.undefined to null
alert(window.xyzw === undefined); // shows false

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