从类型的角度来看，NaN是一个数字，但不是像1、2或329131这样的普通数字。“不是一个数字”的名字指的是表示的值是特殊的，是关于IEEE格式规范领域，而不是javascript语言领域。

NaN != NaN，因为它们不是相同的非数字。因此……很有道理。 浮点数+0.00和-0.00不一样的原因。四舍五入可以做到它们实际上不为零。

至于typeof，这取决于语言。大多数语言会说NaN是浮点数、双精度数或数字，这取决于他们如何对它进行分类……据我所知，没有一种语言会说这是未知类型或null。

它的意思是不是一个数字。这不是javascript的特性，而是常见的计算机科学原理。

从http://en.wikipedia.org/wiki/NaN:

There are three kinds of operation which return NaN: Operations with a NaN as at least one operand Indeterminate forms The divisions 0/0, ∞/∞, ∞/−∞, −∞/∞, and −∞/−∞ The multiplications 0×∞ and 0×−∞ The power 1^∞ The additions ∞ + (−∞), (−∞) + ∞ and equivalent subtractions. Real operations with complex results: The square root of a negative number The logarithm of a negative number The tangent of an odd multiple of 90 degrees (or π/2 radians) The inverse sine or cosine of a number which is less than −1 or greater than +1.

所有这些值可能都不相同。NaN的一个简单测试是测试value == value为false。

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。

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.

考虑NAN的最好方法是它不是一个已知的数字。这就是为什么NAN != NAN，因为每个NAN值代表一个唯一的未知数字。nan是必要的，因为浮点数的值范围有限。在某些情况下，舍去小数位会丢失，导致出现像1.0/11*11 != 1.0这样的无意义的结果。真正大的值是nan，无穷大就是一个很好的例子。

鉴于我们只有十个手指，任何显示大于10的值的尝试都是不可能的，这意味着这些值必须是nan，因为我们已经丢失了这个大于10的值的真实值。浮点值也是如此，其值超过了浮点数所能容纳的限制。