只是出于好奇。
typeof NaN是number似乎不太符合逻辑。就像NaN === NaN或NaN == NaN返回false。这是JavaScript的特性之一吗，或者有什么原因吗?
编辑:谢谢你的回答。然而，让人明白这不是一件容易的事情。阅读答案和维基我明白了更多，但仍然，一句话像
与NaN的比较总是返回无序结果，即使是与NaN本身比较也是如此。比较谓词是信号或非信号，信号版本表示此类比较的无效异常。等式和不等式谓词是非信号，因此x = x返回false可用于测试x是否为静态NaN。
让我头晕目眩。如果有人能把它翻译成人类(而不是数学家)可读的语言，我会很感激。
当前回答
Javascript uses NaN to represent anything it encounters that can't be represented any other way by its specifications. It does not mean it is not a number. It's just the easiest way to describe the encounter. NaN means that it or an object that refers to it could not be represented in any other way by javascript. For all practical purposes, it is 'unknown'. Being 'unknown' it cannot tell you what it is nor even if it is itself. It is not even the object it is assigned to. It can only tell you what it is not, and not-ness or nothingness can only be described mathematically in a programming language. Since mathematics is about numbers, javascript represents nothingness as NaN. That doesn't mean it's not a number. It means we can't read it any other way that makes sense. That's why it can't even equal itself. Because it doesn't.
其他回答
因为NaN是数值型数据类型。
它是Number类型的特殊值，如POSITIVE_INFINITY
为什么?通过设计
一个例子
假设我们正在将一个字符串转换为一个数字:
Number("string"); // returns NaN
我们将数据类型更改为数字，但它的值不是数字!
NaN仍然是数字类型，但它表示不能表示有效数字的值。
NaN的一个更好的名称应该是一个数值例外，它更准确地描述了它的含义，而且不那么容易混淆。它实际上是另一种伪装成具有原始类型的异常对象(通过语言设计)，同时它在错误的自我比较中不被视为原始类型。这就是困惑。只要语言“不下定决心”在适当的例外对象和原始数之间进行选择，这种混乱就会存在。
The infamous non-equality of NaN to itself, both == and === is a manifestation of the confusing design forcing this exception object into being a primitive type. This breaks the fundamental principle that a primitive is uniquely determined by its value. If NaN is preferred to be seen as exception (of which there can be different kinds), then it should not be "sold" as primitive. And if it is wanted to be primitive, that principle must hold. As long as it is broken, as we have in JavaScript, and we can't really decide between the two, the confusion leading to unnecessary cognitive load for everyone involved will remain. Which, however, is really easy to fix by simply making the choice between the two:
要么使NaN成为一个特殊的异常对象，其中包含有关异常如何产生的有用信息，而不是将当前实现的信息丢弃，导致更难调试的代码; 或者让NaN成为一个基本类型为number的实体(称为“numeric”不那么容易混淆)，在这种情况下，它应该等于自身，不能包含任何其他信息;后者显然是一个较差的选择。
将NaN强制转换为数字类型的唯一可以想象的好处是能够将其扔回任何数值表达式中。然而，这使得它成为脆弱的选择，因为任何包含NaN的数值表达式的结果都将是NaN，或者导致不可预测的结果，例如NaN < 0求值为false，即返回布尔值而不是保持异常。
即使“事情就是这样”，也没有什么能阻止我们为自己做出明确的区分，以帮助我们的代码更可预测，更容易调试。在实践中，这意味着识别这些异常并将其作为异常处理。不幸的是，这意味着更多的代码，但希望通过Flowtype的TypeScript等工具来缓解。
然后我们有混乱的安静和嘈杂，即信号NaN的区别。这实际上是关于如何处理异常，而不是异常本身，与其他异常没有什么不同。
类似地，Infinity和+Infinity是出现在实数线扩展中的数字类型元素，但它们不是实数。在数学上，它们可以用收敛到正无穷或负无穷的实数序列来表示。
