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
typeof NaN返回'number'，因为:
ECMAScript spec says the Number type includes NaN: 4.3.20 Number type set of all possible Number values including the special “Not-a-Number” (NaN) values, positive infinity, and negative infinity So typeof returns accordingly: 11.4.3 The typeof Operator The production UnaryExpression : typeof UnaryExpression is evaluated as follows: Let val be the result of evaluating UnaryExpression. If Type(val) is Reference, then If IsUnresolvableReference(val) is true, return "undefined". Let val be GetValue(val). Return a String determined by Type(val) according to Table 20. Table 20 — typeof Operator Results ================================================================== | Type of val | Result | ================================================================== | Undefined | "undefined" | |----------------------------------------------------------------| | Null | "object" | |----------------------------------------------------------------| | Boolean | "boolean" | |----------------------------------------------------------------| | Number | "number" | |----------------------------------------------------------------| | String | "string" | |----------------------------------------------------------------| | Object (native and does | "object" | | not implement [[Call]]) | | |----------------------------------------------------------------| | Object (native or host and | "function" | | does implement [[Call]]) | | |----------------------------------------------------------------| | Object (host and does not | Implementation-defined except may | | implement [[Call]]) | not be "undefined", "boolean", | | | "number", or "string". | ------------------------------------------------------------------
此行为符合IEEE浮点运算标准(IEEE 754):
4.3.19 Number value primitive value corresponding to a double-precision 64-bit binary format IEEE 754 value 4.3.23 NaN number value that is a IEEE 754 “Not-a-Number” value 8.5 The Number Type The Number type has exactly 18437736874454810627 (that is, 253−264+3) values, representing the double-precision 64-bit format IEEE 754 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9007199254740990 (that is, 253−2) distinct “Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a single special NaN value. (Note that the NaN value is produced by the program expression NaN.)
我们可以认为NaN是一个特殊情况对象。在本例中，NaN的对象表示一个没有数学意义的数字。数学中还有一些特殊情况的对象，比如INFINITE等等。
你仍然可以用它做一些计算，但这会产生奇怪的行为。
更多信息在这里:http://www.concentric.net/~ttwang/tech/javafloat.htm(基于java，不是javascript)
NaN仍然是数字类型，但它表示不能表示有效数字的值。
Javascript只有一种数字数据类型，即标准的64位双精度浮点数。所有东西都是双的。NaN是double的特殊值，但它仍然是double。
所有parseInt所做的是将字符串“转换”为数值数据类型，因此结果总是“number”;只有当原始字符串不可解析时，它的值才会是NaN。
它是Number类型的特殊值，如POSITIVE_INFINITY
为什么?通过设计
